BCA Rational is at Ion of Flat Slab Reinforcement

\

RATIONALISATION ~ ~~

OF FLAT SLAB

REINFORCEMENT

A report on research to investigate the balance between the costs of materials, labour and time in reinforced concrete flat slab construction and potential benefits.

C H Goodchild BSc, CEng, MCIOB, MlStructE

1 jETR REINFORCED CONCRETE

ENVIRONMEN r TRANSPORT -d REGIONS

Foreword This publication is one of the main outcomes from a DETR PI1 Research Project Rationalisation offlat slab reinforcement (ref 39/3/284 cc 0807). The project was jointly funded by the Department of the Environment, Transport and the Regions under the Partners in Innovation scheme, the Reinforced Concrete Council (RCC) and by industry. The project was managed and this publication was produced by the RCC, which was set up to promote better knowledge and understanding of reinforced concrete design and building technology.

The Council's members are ASW, the major supplier of reinforcing steel in the UK, and the British Cement Association, representing the major manufacturers of Portland cement in the UK.

Charles Goodchld, author of this publication, is Associate Director for the Reinforced Concrete Council.

Acknowledgements The RCC thanks the DETR and acknowledges the funding given to the project by the Department and the support given by many individuals and companies, especially researchers and participants at the project's Advisory Group meetings. These are listed below. Pal Chana John Clapson Consultant Richard Day Concrete Society Charles Fowler Michael Flynn Colin Gray University of Reading Alan McDonack ROM Dick Pankhurst Consultant George Poncia Byrne Brothers John Theophilus Bjorn Watson Anthony Hunt Associates

BRE Ltd (formerly of the Concrete Research and Innovation Centre)

Formerly University of Reading Formerly Reinforced Concrete Council

Lorien (formerly Allied Bar Coaters)

Thanks are also due to:

Ian Beveridge Andy Butler Michael Carter David Crick Andrea Croft Les Dobinson Mike Fuller Leon Furness Tony Franks David Hart Frank Hodgson David Johnson

ROMLJKSA S m h ~ (foxmerly of John Doyle C o m o n ) Balfoiir Beatty David Crick Associates Concrete Society Ove Arup & Partners BRC Buro Happold ASW Gardiner & Theobald Frank Hodgson Associates Nottingham Trent University

Gerard Kennedy Powell Tolner Assocs Steve Lillie and all at Byrne Bros Charles McBeath Whitby & Bird Martin McGovern Whitby & Bird Tomas Moesch AncoTech bv David Moore BRE Ltd David Pope Peter Brett Associates Martin Saunders DEHA Geny Shaw Curtins David Smith BRC David Wagstaff Norwich Union Robert Vollum Imperial College

Thanks are also due to The Concrete Society and University of Reading RPEG for permission to use previously unpublished material. Thanks too to Words & Pages.

97.376 First published 2000 ISBN 0 7210 1576 X Price group L

8 British Cement Association 2000

Published by the British Cement Association on behalf of the industry sponsors of the Reinforced Concrete Council. British Cement Association Century House, Telford Avenue Crowthome, Berkshire RGI 1 6YS Telephone (01344) 762676 Fax (01344) 761214 w.bca .0rg .uk

All advice or information from the British Cement Association is intended for those who will evaluate the significance and limitations of its contents and take responsibility for its use and application. No liability (including that for negligence) for any loss resulting from such advice or information is accepted. Readers should note that all BCA publications are subject to revision from time to time and should therefore ensure that they are in possession of the latest version.

Rationalisation of flat slab reinforcement Contents

Executive summary Introduction Methodology Background studies ,

4.1 Introduction 4.2 Literature review 4.3 Further cost model studies 4.4 The cost of time 4.5 Concrete Society: Rationalisation of reinforcement 4.6 Background studies: Conclusions European Concrete Building Project at Cardington 5.1 Introduction 5.2 Design 5.3 Design and detailing 5.4 Pre-construction 5.5 Commentary on reinforcement systems used 5.6 Design and detailing records 5.7 Process issues 5.8 Construction 5.9 Other research on reinforcement Cardington: Structural performance 6.1 Ultimate strength in flexure 6.2 Serviceability Cardington: Analysis of construction process data 7.1 Introduction 7.2 Site measurement 7.3 Results and analysis 7.4 Variability and uncertainty 7.5 Conclusions and recommendations Discussion 8.1 Introduction 8.2 Background studies 8.3 Cardington 8.4 Integration of results 8.5 Best value 8.6 Concluding remarks Conclusions

10 Recommendations 11 References

Appendices Appendix I Appendix I1 Appendix I11 Appendix IV Appendix V Appendix VI Appendix VI1 Appendix VI11

Details of reinforcement Summary of interviews for Concrete Society research Supplements to Chapter 7 :Analysis of construction process data Data used for costing: materials and labour Data used for costing: cost of time Reinforcement detail drawings Bending strength evaluation of the ECBP floor slabs Charts showing potential savings for flexural reinforcement and punching shear systems

1 5 7

. 11 11 12 17 22 30 33

35 37 38 39 41 46 47 52 55

57 5 8

67 68 69

84

85 86 92 99

117 119

35

57

67

si

85

121 127 131 135

135 138 140 148 150 156 171 199

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1 Executive summary

Background Within the concrete construction industry there are many different views about what constitutes the best way of reinforcing concrete for the most economic construction. This is especially true of reinforced concrete flat slabs where strict adherence to the current British Standard can result in up to 60 different reinforcement arrangements within a single slab with consequent inefficiencies in detailing, manufacturing, handling and fixing of reinforcement.

In line with the objectives of the Egan Repod'), the primary objective of this project was to reduce the overall costs of reinforced concrete flat slab construction by disseminating practical guidance on the rationalisation (i.e. the elimination of redundant variation) of reinforcement to contractors and designers. Improved rationalisation should improve the competitiveness of flat slabs and indeed other forms of concrete construction.

The research The project aimed to evaluate the timekost benefits of various generic methods of reinforcing flat slabs. Following on from literature searches and background studies, comparative studies were undertaken on six suspended slabs in the in-situ building of the European Concrete Building Project (ECBP) at BRE Cardington.

Several different generic arrangements of loose bar and fabric were used for the flexural reinforcement. Many different types of punching shear reinforcement were used. The chosen configurations followed on from much discussion and the designs for the flexural reinforcement were based on three different types of analysis (elastic, finite element and yield line). The structural performance of the various arrangements of reinforcement was checked and found to be satisfactory.

Construction process data was recorded, analysed and is reported upon in this report.

Research defining the cost of time was undertaken. The results were used to integrate critical time costs into the overall economics of the various configurations and speculate on the implications.

Findings The research indicated that:

Reinforcement Different reinforcement arrangements can have significant impact on overall (material plus labour) costs. In the systems investigated up to 30% of overall costs can be saved on flexural reinforcement and 50% on punching shear reinforcement, excluding any benefit from reduced critical path time.

For flexural reinforcement, it was found that rationalised arrangements of traditional reinforcement produced best value in economic terms in all projects apart from large buildings (say substantially larger than 3 storeys andor 4500 m2) where two-way prefabricated mats offered most benefit.

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Rationalisation of flat slab reinforcement

0 For flexural reinforcement there appear to be great opportunities for the use of yield line design to determine reinforcement of flat slabs: yield line design leads to low reinforcement weight and highly rationalised layouts.

For punching shear reinforcement, the use of a proprietary system appears to be almost always worthwhile. The additional material cost is more than outweighed by savings in labour and time.

0

Time 0 Proprietary punching shear reinforcement systems are between three and ten times faster

to fix per column than traditional links.

Switching from traditional methods of detailing and fixing loose reinforcement to two- way prefabricated mats and proprietary punching shear reinforcement systems can save 50% of fixing time (measured in man hours).

Savings in critical path time on site cannot be obtained simply through rationalising at the detailing stage: rationalisation must embrace the whole design and construction process in order to obtain worthwhile benefits.

Undoubtedly there are time savings to be gained by using very highly rationalised configurations of reinforcement but their effects on overall productivity and critical time are hard to judge.

e

0

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costs 0 Time-related costs of time-based preliminaries and, notably, finance costs are exceedingly

important. This is especially true for clients, where, particularly in large buildings, these costs outweigh additional material costs from using innovative methods.

A cost structure based on reinforcement weight alone almost always penalises and, therefore, inhibits prefabrication and innovation. Appropriate time-related costs should also always be taken into account.

Information on costs, pricing policies and productivity rates needs to become more available. Access to straightforward costing and productivity data would encourage more economic design.

It should be recognised that to some parties to a contract it is material cost that is important and that to others, especially clients, it is time costs that are important. Thus, the relative importance of these costs varies between parties. Their motivation may therefore differ and may change during the procurement process.

0

0

Cardington data 0 Many of the findings in this report are based on data from Cardington that were gathered

under imperfect conditions, chiefly lack of repetition. However, these data gave strong indications that were substantiated by comparisons with commercial information. They were better than any previous research data and were held to be a sound basis for the comparisons made.

Recommendations Best practice

Current evidence suggests that traditionally designed rationalised flexural (or main) reinforcement should be used on all but the largest buildings (say buildings larger than 7- storeys and 10,000m2).

For shear reinforcement, the use of proprietary shear systems, specifically stud rails and shear ladders, appears to be almost always worthwhile, regardless of building size.

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,1 Executive summarv

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Recommendations for future best practice 0

The value of potential time savings on site must be'acknowledged by both the design and construction teams in order to make correct decisions on rationalision.

Yield line design appears to provide a great opportunity for more competitive concrete building structures. If the opportunity is to be grasped then the concrete frame industry must present designers and the wider construction industry with comprehensive design guidance and design aids to instil confidence in its use.

0 The concrete frame industry should strive towards greater vertical integration. This would produce many benefits. For instance, integrating computer design with computer detailing would reduce the risk of wrongly estimating the weight of reinforcement for a project. Likewise, the optimisation of flat slab construction would become more relevant if methods of costing and productivity data were more available and easily understood by designers.

Further studies Studies aimed at identifying and analysing in detail value chains -the transmission of value right through the supply chain - should be encouraged.

More data are required in order to determine optimum design for different structural arrangements. Notably, data are required for loose bar arrangements derived from yield line and finite element designs and to prove the benefits of yield line design and prefabrication. Given the demonstrated variability of design input and site practice, a practical way to obtain this data would be to organise industry-wide data gathering and benchmarking exercises.

The method of providing shear reinforcement known as the ACI stirrup system provides many benefits to the construction process. Research is required to develop and adapt design methods in order to demonstrate compliance with BS 81 10 and EC2. This applies particularly to the design of stirrups in close proximity to holes in the slab.

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Figure 1.1 The in-situ concrete frame building at Cardington.

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2 Introduction Current trends in building point towards more prefabrication, more efficiency, reduced site activity, and safer and faster construction. Clients are demanding lower costs and higher quality: The concrete industry, and many of its customers, believe that rationalisation, including rationalisation of reinforcement, can answer some'of these demands.

In common with The Concrete Society's Rationalisation of reinforcement, Technical Report 53(2), 'rationalisation' is taken to mean the elimination of unnecessary variation. Unnecessary variation can occur in many parts of the construction process, but this project considers just the provision of reinforcement in flat slabs.

Flat slabs are amongst the most popular forms of in-situ concrete frame construction, accounting for between 30% and 50% of the market share for concrete-framed multi-storey buildings. There are numerous views about what constitutes the most economic form of flat slab construction and the most economic way of reinforcing them. This manifests itself in many different arrangements of reinforcement to resist flexural bending, restrict deflection and resist punching shear at columns. In order to consider this apparent area of inefficiency, comparative studies were proposed to evaluate the benefits of rationalising reinforcement, by comparing traditional arrangements with different layouts of loose reinforcement and bespoke prefabricated mats (tailored fabrics) and by comparing different types of punching shear reinforcement.

The primary objective of the project was to reduce the costs of flat slab construction by disseminating practical guidance on the rationalisation of reinforcement for flat slabs. It was held that greater productivity would lead to lower construction costs and increased competitiveness. Any guidance produced was to be aimed at designers (engineers and architects), clients, their advisors and contractors (main contractors and specialists).

At the outset the intention was to: 0 Demonstrate on a reasonably large scale the benefits of optimum reinforcement

arrangements. Obtain objective quantitative data from which meaningful recommendations could be developed and disseminated. Look at the balance between cost of materials and saving in time.

0

0

This report was prepared following completion of construction and receipt of individual reports by the University of Reading(3), Nottingham Trent University"), Lorien plc(38) and ongoing research by Imperial College(8). The recommendations from this and the other work at Cardington are being published in the form of short easy-to-follow Best Practice Guides. The current (2000) list of Best Practice Guides is as follows:

Improving concrete frame constru~tion(~) Concreting for improved speed and efficiency(") Early age strength assessment of concrete on site(") Improving rebar information and supply('2) Early striking for efficient flat slab con~truction('~) Rationalisation of flat slab reinforcement(44) (a summary of this report)

The project was funded jointly by DETR under their Partners in Innovation programme (originally the Partners in Technology programme), by the RCC and by the concrete and

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reinforcement industries. Contribution of time, money and material proved invaluable and are gratefully acknowledged. The project was managed by the Reinforced Concrete Council.

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3 Methodology Outline During the protracted pre-construction phase, a Steering Group was set up and met many times to decide upon the arrangements of reinforcement to be used on the suspended floors of the in-situ building at the European Concrete Building Project (ECBP) at BRE's Cardington site in Bedfordshire. The final configurations of reinforcement used in the building, together with responsibilities for design and supply are shown in Appendix I and summarised in Table 1. The meetings were often held in conjunction with those of a parallel DETR PIT project, Rationalisation of reinforcement, undertaken by The Concrete Society(*) .

During this time background research and literature searches were undertaken. The University of Reading prepared their Cost of time report(3) for the project. This report looks at the value of saving time from the viewpoint of the main parties to a (building) construction contract. As construction at Cardington approached and the structural design by the ECBP's consultant engineer, Buro Happold, became available, so alternative designs, arrangements and types of reinforcement were arranged, prepared, checked and drawn up. The opportunity was taken for Nottingham Trent University to compare the theoretical structural capacities of the final designs.

This project, Rationalisation ofjlat slab reinforcement, formed a crucial part of the European Concrete Building Project, as it complemented the ECBP's overall objective of increasing the competitiveness of concrete building structures through business process re-engineering.

The many projects carried out on the ECBP were interdependent and called for many compromises from many quarters at many times, sometimes to the detriment of individual projects. The structure provided a large amount of process data, which is reported upon elsewhere(4* '). On the performance side, the-opportunity was taken to incorporate many facilities for research. Future testing of the structure will help clarify our understanding of the behaviour of real building structures(6). The main effects of the interaction of the other projects at Cardington on this project were the delayed start, interference with 'normal' reinforcement operations and the unavoidably artificial time pressures on site.

The in-situ building was constructed in early 1998 in the hanger at Cardington. During construction, data on the reinforcement process were gathered through observations, timesheets, time-lapse photography and video. Inevitably there were the problems, delays, and management compromises associated with construction on any site. In the case of this project these problems led to less-than-ideal data being collected, but nonetheless, Cardington provided a lot of information, which is presented in this report.

More details of the project are given in the following sections.

Background studies A literature search ,was undertaken to review previous work in the area to gain wider understanding of the subject and to be forewarned of potential difficulties with research on site. PrCcis of the references found are presented in Chapter 4, Background studies.

Following publication of their Cost model study, the Reinforced Concrete Council undertook a series of studies on the cost optimisation of flat slabs by looking at thinner slabs and different methods of providing reinforcement. Material quantity and cost differences were assessed and reported upon. However, it quickly became apparent that time-cost differences

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could potentially overwhelm material cost differences. The value of time needed to be defined so the following item of work was commissioned.

The Cost of time study(3) by the University of Reading’s Production Engineering Group was commissioned to help give an accurate prediction of the time related costs of construction in order to provide a measure of process improvement. The aim was to produce equations that would show the benefit of completion of a project ahead of programme. The particular intention was to show the contribution that improvements in concrete frame construction could make. The equations were aimed at making direct calculations possible. The report is presented in a summarised form, in section 4.4, together with additional material from the RCC.

The Concrete Society’s project, Rationalisation of reinforcement, was camed out concurrently with the early stages of this project. There was much cross-fertilisation of ideas and interchange of information.

Steering Group The project was overseen by a the Steering Group that included representatives of Construct, UK Steel Association (RMPG), Concrete Society, British Cement Association, ECBP, consulting engineers and other interested members of industry. This group met many times over the duration of the project. The Steering Group’s work was co-ordinated with that of The Concrete Society’s Rationalisation of reinforcement workmg group(*).

Liaison with the ECBP One of the major tasks undertaken by the Steering Group was the agreement of configurations of reinforcement to be investigated. Many issues were discussed, not least the impact on other areas of research at Cardington. In the long lead up to construction the final arrangement of reinforcement was decided and is shown in Table 5.1 on page 39. A more detailed breakdown including definitions and breakdown of responsibilities is given in Appendix I.

Once the many different configurations were finalised, arrangements were made for them to be designed, detailed, checked and procured, delivered and fixed on site. The above undertakings involved the RCC in a great deal of co-ordination including attendance at weekly site meetings during construction.

Structural analysis of proposals It was recognised that structural concrete productivity issues cannot be viewed in isolation. Changes to reinforcement configurations affect overall factors of safety. A report was commissioned from Nottingham Trent University to assess factors of safety against failure in bending of each configuration. An automated yield line analysis (a finite element analysis technique) was used. A summary of their report is presented in Section 6 .

Changes in reinforcement also (theoretically) affect serviceability. Although not funded by this project, A review of slab deflections in the in-situ concreteframe building by Dr R L Vollum is included in Section 6 with Dr Vollum’s permission. It should ‘be noted that Dr Vollum’s and other research work at Cardington continues (as at June 2000).

The results of research into serviceability (deflection, cracking, shrinkage, durability, etc), and ultimate performance (punching shear, fire, and explosion) may demand that the findings of this report must be reviewed at some future date. However, some reassurance may be taken

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3 Methodology

from the fact that Dr Vollum’s review includes measurements of deflection at up to 700 days and that further deflection is probably predictable.

Observation and analysis of construction data Operating under a sub-contract, Lorien plc observed and recorded the construction processes associated with the different arrangements and types of reinforcement used on site. The primary method of recording time of each reinforcement operation on site was through detailed timesheets kept by the steel fixers themselves. The content and detail of the timesheets was worked out and agreed with the fixers on site and initially the timesheets were checked through parallel recording by the researchers. Later the records were verified by observation, reference to video, time-lapse photographs, diaries and interviews. Towards the end of construction, the steel-fixing crew was debriefed. Records of time were broken down into delivery, storage, sorting, transport, fixing, etc. Lorien’s report analysis is presented later in Section 7. The nature of the data obtained meant that the analysis, recommendations and conclusions all have to be preceded by a caveat of the word ‘probably’. Nonetheless useful data were obtained and worthwhile conclusions made.

Discussion The various strands of the research were brought together and discussed. The aim was to identify the effects of the different configurations of reinforcement on costs to the various parties to a project. Detailed cost data were to be sought from industry and made available in order that overall time-cost differences could be assessed.

Report and disseminate findings This report forms part of the dissemination of the findings on this project. In common with other research projects at Cardington this report is to be summarised in a Best Practice Guide. This Guide‘44’ will give recommendations for the rationalisation of the whole reinforcement process to improve the efficiency and economy of remforced concrete flat slabs.

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4 Background studies

4.1 Introduction Although the cost of a concrete frame is generally a small proportion of the total project cost, the frame construction time may account for a large proportion of the programme. In a typical office building, for example, construction of the frame may represent only 10% of the cost but may take 50% of the time. For most of this time the structure is on the critical path for the completion of the building. For in-situ reinforced concrete structures, fixing of the reinforcement is on the critical path for at least part of the time. So far as reinforcement is concerned there are several ways to reduce critical time: 0 Simpler arrangements of reinforcement 0 Innovative methods of design 0 Proprietary systems

Prefabrication

All this adds up to rationalisation of reinforcement.

Savings in time in the frame construction process should therefore be reflected in savings in the total project cost. Potentially, added-value reinforcement systems that save time should be very attractive to the concrete frame industry’s clients. The problem has always been to define potential savings within an ever-changing market, and it has always been difficult to give practical guidance rather than anecdotal evidence.

Previous page is blank

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4.2 Literature review Optimum cost For the contractor, faster construction invariably reduces fixed costs. For the client, faster construction reduces the cost of finance (see Figure 4.1), and may increase profit from early trading. For both, optimum speed contributes to an improved return on capital, but as Khosrowshahi(’6) pointed out, as far as project costs and time are concerned, the priorities of the contractor and the client inevitably differ.

Finance Costfa, - /+ rinance Cost,,

x 0 0 S

i i !

i Time

Figure 4.1 Finance costs - fast versus slow construction

The constituent components that make up the overall cost-time curve are different for the different parties. Hence the optimum time for project duration will be different for the client and the contractor.

For contractors, indirect costs (e.g. overheads) tend to increase with time, while direct costs initially, at least, decrease with time (e.g. costs of labour). The contractor’s optimum time is different from the that of the client whose total cost is the sum of the contractor’s total cost, the contractor’s profit, the land cost and its related interest charges, interest payments on the capital-in-use (interim valuations), and the client’s other time-related costs such as professional fees, cost of land etc.

Finance costs Claps~n(’~) stated that in the UK, ‘first cost’ continues to exert a strong influence on choice of structural frame. However, he maintained, it is overall costs, including finance costs, that are, perhaps, the major concern of clients.

Finance charges are directly related to the period from site purchase to start of rental income or start of trading or disposal (occupation or sale). Hence, speed of construction should be, and is, a major factor in the choice of materials and methods - an argument that has been often, but unjustly, used to the detriment of using concrete frame construction. For simplicity finance charges are often expressed as

~~

b p r o x i m a t e finance cost = Construction c o d 2 x duration (months) x interest r a t e / l 2 I

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Speed trials In order to investigate speed a series of trials was undertaken on various arrangements of reinforcement(17). These trials showed that assembly times for beam and column cages could be halved, and times for fixing slab reinforcement could be significantly reduced. Using crane-handled two-directional mats could reduce the time for fixing reinforcement in slabs to 10% of that required for traditional loose bar reinforcement.

Table 4.1 Speed trials at Wexham Springs(17)

Activity % of time for loose bars Time

Flat slab

Single-directional mats 1 hr 30 rnin 26 Loose bars (not including breaks) 5 hr 45 rnin 100

Two-directional mats 36 min 10

Beam cages Loose bars Fabric links

20 min 100 10 min 50

Column cages

Single links - fabric links 4 min 40

10 min 50 Double links - fabric links

Single links - loose bars 10 min 100

Double links - loose bars 20 min 100

When using traditional loose bar reinforcement for the slab the work was very labour- intensive. A great deal of time was spent in marking out bar centres, opening up bundles and identifying bars and lifting them into position. Using hand-laid prefabricated welded fabric mats saved considerable time. The fabric mats were easily identified and their use improved the organisation and planning. Larger, crane-handled mats saved even more time, because there were significantly fewer items to position. It was clear that many of the traditional steel- fixing skills were not required. The results reinforced those from similar work carried out in Germany(I8) in recent years.

Evaluating the benefits Prefabrication generally speeds construction but may result in the use of additional reinforcement since this method of working usually depends upon adopting a degree of

. standardisation, which in turn implies using reinforcement maxima and a lower level of optimisation compared with using loose bars. This inevitably leads to questions such as “How much extra reinforcement will be required and will it be possible to recover the additional cost?” It is very difficult to generalise and it is precisely this trade-off that this project seeks to investigate.

By way of an example, Clapson(”) asked a leading firm of quantity surveyors to consider the effect of substituting prefabricated shear reinforcement for loose links in a typical six-storey concrete flat slab structure. All reinforcement was assumed to be loose bar except for the prefabricated shear reinforcement. Assuming shear reinforcement was on the critical path, it was calculated that the programme saving would amount to at least two weeks i.e. two days per floor with potential cost benefits of €260,700, as shown in Table 4.2.

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Table 4.2 Potential savings from using prefabricated shear reinforcment‘”)

e Extra costs

Prefabrication on-cost 13,700

Potential savings

1. Saving in site labour costs

2. Saving in fixed site costs (2 weeks)

3. Saving in finance cost (2 weeks at 12%)

5,800

20,600

48,000

2 0 0,o 0 0 4. Potential increase in rental income (2 weeks)

274,400

Net potential saving 260,700

Theoretically, rationalised reinforcement layouts should save time both in fixing and in the many operations leading up to fixing - design, detailing, cutting, bending, delivering, storing, locating and laying out. Provided the engineer and detailing staff have sufficient understanding of the materials and methods, benefits will immediately start to accrue.

In the drawing office both the drawings and schedules will be greatly simplified. 0 The detailed drawings are far easier to read. 0 There are fewer components to deal with, checking is improved, and reinforcement

losses much reduced.

The major benefit on site is the potential for reducing the time taken to fix the reinforcement. Compare, for example, the problems of checking shear reinforcement for a flat slab provided by some 20,000 or so loose links with only, say, 600 or so components required for prefabricated systems.

Production rates Not only is the number of components important but so also is the rate at which they can be fixed. Productivity is a complex science. Production rates are used in planning the resources required for an activity. They are tremendously important in determining estimates for tenders etc. and their accuracy can tip the balance between using one method and another or even one material against another.

Production rates used in planning are affected by many factors including the subjective andor objective bias of individual planning engineers. On site, a contractor’s organisational policy, choice of construction methods, and the individual’s experience and aptitude will influence the final productivity rates. It is not surprising that two planners are unlikely to derive identical productivity rates given the same operation. Traditionally, individual planners use their own individual rates, which are based on their or their company’s experience. The rates allow for, or are moderated for, unknown factors, site difficulties etc and their derivation, could be described as unscientific. Attempting to measure productivity of reinforced concrete operations is notoriously difficult.

As reported by Proverbs et aI(I9), planning engineers frequently maintain a library of basic productivity rates. These are adjusted for each project; taking into consideration specific site factors and conditions that they consider could impact on the productivity of construction operations.

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Christian and Hachey' analysed 15 responses to ' a questionnaire. The wide range of production rates given illustrated how difficult it is to estimate a particular production rate for any activity. Determining the value of these rates is complex because productivity is difficult to analyse. It is also difficult and time-consuming to interpret and evaluate. In practice, production ratzs were modified to reflect delay times and other time-consuming aspects of an activity. In most construction companies, production rates are usually established by a combination of experts' opinion and the use of handbooks that contain productivity data.

Significantly, Chst ian and Hachey found that there existed "substantial agreement" between the average productivity rates measured in the field and of those used by planning engineers. Where differences between productivity rates and actual output (using a number of sites for similar operations) were found, it was established that such differences were caused mainly by waiting and idle times (an impact of inadequate site supervision or management). However, Christian and Hachey reported that planners would very often modify their productivity rates for each estimate in order to reflect anticipated delay times.

Time measurements of production rates revealed that waiting time delays were an extremely significant part of reduced productivity. For a typical concrete worker 37% of time was non- productive (4% idle, 4% waiting for supervision and 29% waiting for materials). Management attention should be focused on these causes of inefficiency.

Table 4.3 presents descriptive statistics for the productivity rates provided by planning engineers from each of three countries. The researchers experienced difficulties in data interpretation and in the disparate number of responses. It can be observed that the coefficient of variation (C of V) values are exceptionally high (24% to 90%) despite the relatively small sample sizes. Assuming normal distribution, some 60 samples would have been required to ascribe 95% confidence limits

Planning engineers were requested to provide their productivity rates for the fixing of reinforcement to beams (in three different sizes), columns and floor slabs, as defined by model drawings. The model building utilised in-situ concrete flat-slab floors, and was seven storeys in height and contained 24 columns per floor.

Table 4.3 Productivity rates for fixing reinforcement to beams, columns and floor slabs (Proverbs et al(I9'

Element Country Number of Productivity rates respondents

Mean c of v# SD* (MPR)* %

Beams UK 32 25.4 43 10.9 France 13 31.9 41 13.1 Germany 10 18.0 28 5 .O

Columns UK 31 24.3 42 10.2 France 13 30.6 38 11.6 Germany 10 . 18.5 32 5.9

Floor slabs UK 31 11.4 90 10.3 France 13 20.3 30 6.1 Germany 10 15.0 24 3.6

*Operative-hours per tonne #?Coefficient of variation

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Floor slabs contained over 80% of the reinforcement in the model and the productivity rates ascribed to this operation would have a proportionally greater overall impact on the total operative hours required. While figures for slabs would suggest that for reinforcement operations overall, UK contractors are the most productive, previous research found UK productivity to be the lowest.

However, it is the RCC’s opinion that the Germans tend to use thicker slabs with lighter bars. In other words the lower production rate in Germany might reflect this difference in custom. Germany also uses more fabric(I5), and welded fabric reinforcement accounts for some 45% of the total reinforcement market, compared with just 15% in the UK. Thus uncomplicated and fast reinforcement might be accomplished by using mats in Germany and loose reinforcement in UK, thus skewing the figures. This illustrates the difficulty of making comparisons. It also reinforces the notion that planners sticking with what they know from personal experience works.

The Steel Reinforcement Promotion Group: Avoiding problems The Steel Reinforcement Promotion Group (SRPG) (*I) refers to steel fixing as being an art where ill-considered details can artificially create fixing problems. They suggest that four aspects should be considered to simplify fixing on site. 0 Relieving reinforcement congestion 0 Simplification without affecting design intent 0 Reducing the number of items to be fixed 0 Allowing for variations in site measurement to ensure cover


4.3 Further cost model studies

Introduction The RCC’s Cost model study(27) concluded that optimisation of design and different forms of construction were worthy of further investigation. As part of the build-up to the research at Cardington this optimisation was studied in more detail(28).

The study used the same three- and seven-storey concrete framed buildings as in the Cost model study. The slabs were to support imposed loads of 4.0 kN/m2 plus 1.0 kN/m2 for partitions, self weight and ‘services, and a finishes load of 1.5 kN/m2. The original choice of 300 mm thick reinforced concrete flat slabs for the 7.5 m square spans was made by the designers, YRM Anthony Hunt Associates, to avoid the need for shear links at columns. Further designs were commissioned, quantified and costed by Gardiner & Theobald. To maintain compatibility with the original Cost model study, reinforcement estimates and September 1993 rates were used for the budget costings. For simplicity, the effects of thinner slabs were considered first and then added to the effects of using different configurations of reinforcement.

The different methods of providing bending and shear reinforcement affect both programme and the amount of effort required by the engineer. The RCC had to estimate total programme differences from discussions with specialist contractors and by assuming time costs equated to the cost of the main contractor’s time-based preliminaries. Differences in engineers’ fees were determined by the designers, based on ACE Agreement 3 and complexity factors for each variation to reflect the design effort required.

Findings Thinner slabs Thinner slabs save money. Figure 4.2 shows savings up to €6.50/m2 gross floor area (10% of the original superstructure costs) could be saved by adopting 240 mm thick rather than 300 mm thick slabs. The savings come from less perimeter cladding, reduced volume of concrete in the superstructure, smaller foundations, and, in theory, less perimeter formwork and lower fees.

3 6.00

5 5.00

I, 4.00

3.00 b v) 2.00

w

U 0

In 0

5 0

0 C

rn 1.00

0.00 1 300 280 260 240 i Slab thickness - mm I

0 Engineers fees 0 Ext. Cladding

rn Formwork rn Concrete 61 Foundations

Figure 4.2 Average savings over the original Cost model study buildings due to using thinner slabs

17


Thinner slabs require more reinforcement and more design effort to make them work. This implies additional cost and time but there is a balance between these additional costs and the savings associated with thinner slabs.

However, the costs of reinforcing these thinner slabs needs to be considered!

Savings using traditional shear links Compared with the original Cost model study, where additional flexural reinforcement was used to enhance shear capacity so no links were required, the source of savings (or costs) is shown in Figure 4.3. Less flexural reinforcement is required at 300 mm thickness because shear links are used - a distinct saving. Links take time to fix and the penalty is in paying for contractors' time. At 300 mm there is a net saving. As the slab becomes thinner more reinforcement is required until the initial advantage is lost. There are other penalties - progressively more difficult design and more time on site for fixing links. Nonetheless adding these figures to those for thinner slabs (effectively adding together Figures 4.3 and 4.4) indicates (as shown in Figure 4.5) that the optimum solution would be 255 mm slabs, some €5.00/m2 cheaper or on average 8% less than that in the original Cost model study costs.

2.50

2.00

1.50 N g 1.00 w U) 0.50 UJ = 0.00 '5

-0.50 U g -1.00

-1.50

-2.00

Slab thickness - mm

0 Reinforcement =Preliminaries 6 Engineers fees

- TOTAL reinforcement. preliminaries and fees

Figure 4.3 Source of savings by using thinner slabs and traditional links at column positions (compared with original study where additional flexural reinforcement was used to enhance shear capacity) (averaged over the four original Cost model study buildings)

Design using no shear reinforcement This option requires considerably more tension reinforcement than other options in order to provide the required punching shear capacity, but demands less time on site. This solution was not feasible in all cases; for example, for a slab 240 mm thick on small columns in the M4C3 building, the punching shear stresses were too high.

The study showed that there was little advantage in going for this option at three storeys but at seven storeys (with 700mm square columns) savings approaching E6/m2 are possible.

18


1 .oo 0.00

-1 .oo -2.00

300 280 260

Slab thlckness - mm

3 In ul E

‘E U)

In 0 0

c 7

TOTAL reinforcement, prelims and engineers fees

0 TOTAL INCLUDING THICKNESS SAVINGS

Figure 4.4 Total savings from using thinner slabs and traditional links at column positions, including savings from using thinner slabs compared with the four original Cost model study buildings

Using shear hoops Traditional shear llnks are awkward to detail, fK and check on site, so have often been avoided by increasing the slab thickness or providing column heads. Such practices increase material costs and slow down construction. Shear hoops - prefabricated nests of links - offer a simple and practical way of providing shear reinforcement in flat slabs, with possible savings of around €4.30/m2.

Using shear heads Shear heads are prefabricated from structural steel sections into crosses (or ‘T’s or ‘L’s at perimeters) placed within the slab as a column head. They allow for large holes near columns but their use in this country has perhaps been restricted by the lack of a recognised and simple design method. Nonetheless they are common abroad and provide simplicity and speed. The results fiom the study are similar to those for shear hoops.

Using prefabricated reinforcement mats Prefabrication can lead to simplified reinforcement drawings that are cheaper to produce, simpler to check and less likely to be misinterpreted on site. It also results in a more uniform workload for the fvting crews and less congestion at the work face, allowing better access and shorter fixing times.

Comparisons Designs, details, quantities, costs and fees were worked up for the various configurations for the M4 buildings in the original Cost model study at 3 and 7 storeys and for the M62 buildings ‘at 3 and 7 storeys‘. The savings and costs were added together to give an indication of overall savings. Figures 4.5 and 4.6 show savings of up to €8/m2 (12%) of the original superstructure costs. While these figures were based on preliminary data, they show the potential of prefabricated mats, particularly two-way mats, a benefit that is widely recognised abroad. It would be interesting to combine the numbers for mats and shear hoops or shear heads.

’ The M4C3 building was assumed to be located in Reading, a three-storey concrete frame, square in plan with air conditioning and curtain walling. The M4C7 building is similar but seven storeys. The MG2C3 building was assumed to be located in Rochdale, a three-storey concrete frame with two rectangular wings, natural ventilation and traditional brick cladding. All buildings incorporated concrete flat slabs on a 7.5 m x 7.5 m grid. Floor plates had an area of 1 500m2 and based on a 199 1 survey the three-storey buildings were considered to be ‘average sized’ multi-storey offices.

19


’ $ p p 3 $ $ $ $ 3 Thlskness - mm

10.00

8.00

6.00

4.00

2.00

0.00 - reinforcement

68Two-way mats with traditional shear links

0 Loose main reinforcement and 1 1 NO shear reinforcement

OLoose main reinforcement and shearhoops

0 Loose main reinforcement and structural steel shear heads

BLoose main reinforcement and traditional shear links

Bone-way mats with traditional shear links

BDOne-way mats with NO shear reinforcement

mTwo-wav mats with NO shear

Figure 4.5 Overall savings for the M4C3 and M4C7 buildings respectively.

Figures 4.5 and 4.6 also show that it is not always feasible to use certain configurations. The option of using no shear links in the M4C3 building is not really possible unless unnecessarily large columns are used. At seven storeys the columns are of sufficient size to make this option viable.

10.00

2 8.00 w m

6.00 0 yl

5 4.00

2.00

0.00

~

OLoose main relnforcemenland NO shear reinforcement

OLoose main reinforcement and shearhoops

OLoose main relnforcementand structural steel shear heads

@Loose main reinforcement and traditional shear links

Bone-way mats wlth traditional shear links

m0ne-waymats with NO shear reinforcement

3Two-way mats wlth NO shear reinforcement

mlwo-way malS with traditional shear links

Figure 4.6 Overall savings for the M62C3 and M62C7 buildings respectively

Flat slabs, perhaps more so than any other form of concrete construction, require more understanding and design sophisfication to get the ‘best’ solution. Design tends to be governed by deflection and shear provisions rather than by thoughts of total construction cost.

In summary, shear systems and prefabricated reinforcement can greatly simplify work on site and so speed up construction and save clients money. The saving is directly related to the time cost. However, there is an initial price to pay in increased material and design costs.

Interim conclusions This preliminary study was able to look at theoretical savings. Significant cost savings can be made by optimising slab thicknesses and methods of reinforcement. The concrete construction industry should recognise that different methods and additional effort (and fees?) can give

20

. .


clients added value. In practice, however, this potential optimisation is often set against increasing risk - thinner slabs have less capacity to absorb late changes and have the potential for higher deflections.

The optimum thickness for the 7.5 m span flat slabs appeared to be 255 mm. The option of not using punching shear reinforcement is not viable with small columns. It was shown that optimising slab thickness saves money.

The proposed test facility at the European Concrete Building Project at Cardington was to a full-scale test bed to look more closely at the trade-off between material costs and productivity in practice.

21

. ..


4.4 The cost of time Summary The cost of time depends upon the perspective of the user.

For developers and owner/occupiers, early occupation brings early and extra revenue or rental income. However, this may be secondary to their core business of predicting and managing commercial risks and opportunities.

For contractors and specialist subcontractors early completion should mean less time-related overheads. However, overheads are normally costed as a single percentage addition, typically 10%. This is insensitive to the particular resource usage and thus does not reflect the often quite small changes in design that achieve production efficiencies.

Data for the calculations, particularly that for the contractors and specialist subcontractors are not readily available. Additional work would be required to build a cost database. Nonetheless financial equations and tables showing how time affects parties to a construction contract are presented to provide valuable measurement tools.

Introduction This section presents a prkcis of The cost of time repod3) by The University of Reading which was commissioned by the RCC for this project. Additions have been made by the RCC following discussions with members of the concrete frame industry.

One of the first issues to be addressed by the Rationalisation of Flat Slab Reinforcement Steering Group was the value of time. Initial investigations found that it was necessary to define costs and to differentiate between different members of the construction team. The Cost oftiine study was commissioned by the RCC to help give an accurate prediction of the time-related costs of construction in order to reflect construction process improvement. The aim was to produce equations that would show the benefit of completion of a project ahead of programme, particularly the contribution that improvements in concrete frame construction could make to the overall process. The equations are aimed at making direct calculations possible and to differentiate between the main parties to a contract.

Basis This study looked at the effect on costs of the operations taking slightly less or slightly more time than would be the case if traditional operations had been carried out in traditional ways in typical amounts of time. Traditional methods and typical times are of course impossible to define as methods and markets are constantly changing and particular projects have their own peculiarities. However this notion of typicality gives a basis for investigating the effects of time on costs. Using better methods should result in better times and costs. These in turn produce a yardstick of optimised contract times and values - against which future innovations can be judged.

It is assumed that the relatively small changes in time in constructing the frame do not affect other items such as supervision requirements; size of cranes, duration of other operations, etc.

22


The cost of time for a developer Property development: background The property development process can be divided into three phases: 0

0 T2 Constrixtion. 0

T, Acquisition -the time from acquisition of the land (waiting for planning permission and design plans) until construction starts.

T3 Disposal - the time from the end of construction until the building is let or sold.

These phases are associated with three main cost centres: 0

0 CD Cost of demolition, 0

CL Cost of land, (including acquisition costs, compensation, fees)

Cc Costs of construction. (including contract value, access roads, planning offsets, professional fees, etc. These ancillary costs can add 5 to 15% to the contract value).

I L 3 0 S

a a n- Dispoeal

Conetruction

.- t$ 8 2 Demolition

I

Time I Figure 4.7 Project costs and phases vs time

The property developer is looking for profit. With respect to a particular project this may be defined as being:

Profit = Property value - Total costs

where Property value = Net yearly rental income / Investor’s yield Total costs = Cost of land + Cost of demolition + Costs of construction + Cost of

finance

The main variables in this assessment of profit are: markets, finance (short-term borrowing costs) and construction costs.

All markets change with time. The construction, money (costs of borrowing, yields) and letting markets (rental income, opportunity) each have an indirect influence on profit. Construction time, on the other hand, has a direct influence on finance and construction costs of a project and is influenced by frame construction time. The calculations therefore concentrate on finance and cons@ction costs only.

23


Construction time Construction time affects: 0

0

Finance costs - as the money will have to be borrowed for a longer period. Construction costs - due to the preliminary costs, most of which are time dependent.

Finance costs Finance costs are a function of time i.e.

I Finance costs = f'n(costs, time) = costs x[(l+ interest rate)'ime-l] 1 where

interest rate is per unit of time.

In terms of a project, the finance costs are

i.e.

where F = finance costs j = interest rate per unit of time TI, T2, CL, CO etc as before

With respect to construction costs, Cc, it is normal practice for the contractor to be paid at monthly intervals for work completed. Finance is raised accordingly and interest will, therefore, occur in stages, although it may be 'rolled up' and repaid as a lump sum on completion. Early payments incur a compound rate over most of the contract period whereas payments made near the end of the building period will incur hardly any interest. This can be tracked using cash flow methods which are useful in complex schemes where, for example parts of the scheme are ledsold before others are completed, and in developments where greater information about the scheme is required. The cash flow approach can more easily accommodate additional refinements such as allowing for the usual S curve in construction expenditure, inflation and taxation.

However, it is usual practice in simple schemes to assume that the total amount of money for construction costs is borrowed for a proportion, usually half, of the building period. The interest on the construction cost will be paid for a period of T2/2 and then for another period of T3 until the disposal.

Construction costs Differences in construction costs theoretically should arise from:

Different costs of time-related preliminaries Different labour, plant and materials costs for doing the work to a different timescale.

Depending on the form of contract and admissibility of claims, these may not necessarily be passed on to the clienddeveloper. Where the construction period is increased or decreased through the endeavours of contractors they would probably not be passed on. On the other hand, if specification of method/ proprietary system is in the domain of the client's designers

24


within a traditional contract, then any saving (or cost) should reasonably be reflected in the contract sum i.e. the client would benefit (or pay).

The cost of time for an owner/occupier An owner/occupier’s prime objective is to procure a building where he can best carry out his business. Profit is taken indirectly in benefits derived from the occupation. Presuming fitting- out forms part of the works, the disposal (i.e. period to occupation) period is absent, (T3 = 0), but variations can still occur in acquisition and construction periods. The consequences of late delivery to an owner/occupier include: 0

0

0

0

0

The

Finance costs - as the money will have to be borrowed for a longer period. Construction costs - different costs of time-related preliminaries and different labour, plant and materials costs associated with doing the work to a different timescale. Lost additional profit - (clear additional profit from new facilities) x time Additional overheads - (e.g. extra rental for existing premises, storage, equipment ) x time Missed opportunities - estimated cost of loss or orders from future clients, damage to name and credibility in the market.

last three are all very hard to estimate. For the purposes of this exercise, lost additional profit plus additional overheads might be equated to loss in rental income as would be the case of developers with a pre-let building.

The cost of time for main contractors The consequences of late delivery to the contractor are differences in construction costs that include:

Different costs of time-related preliminaries Different labour, plant and materials - costs for doing the work to a different timescale.

Preliminaries vary depending on the type of project and the type of contract. They may account for between 5% and 15% of contract value. For the purposes of this research the time-based preliminaries that are concurrent with frame construction and relevant to costs can be divided into six main types.

Staff Scaffold access Temporary power Plant Accommodation Cleaning

In line with Table 4.4, these time-based preliminaries may be assumed to be approximately 5.5% of the contract value(*’) and for the purposes of comparison these costs may be considered pro-rata to contract duration.

Depending on the form of contract, differences in preliminaries and construction costs may or may not be passed on to the client. If savings are passed onto the client then there is little or even no incentive for the contractor to save costs or to save time. Conversely if savings are retained or at least shared with the client then there is incentive for the contractor to save time and costs. From this comes the current drives towards partnering and vertical integration.

25


Table 4.4 Costs of preliminaries from the RCC’s Cost’rnodel study(*’)

M4C3 M4C7 M62C3 M62C7 Ave. ~

Fixed preliminaries, € 50,000 75,000 60,000 90,000

Time-based preliminaries, 2 182,000 310,000 185,500 315,000

Contract value, € 3,357,638 7,397,120 2,799,430 6,148,393

Preliminaries, of whch 6.9% 5.2% 8.8% 6.8% 6.9%

Time-based preliminaries 5.4% 4.2% 6.6% 5.1% 5.5%

The cost of time for the specialist subcontractor The consequence of late delivery for the specialist subcontractor consists of differences in construction costs. These include: 0

0 Different Different costs of time-related preliminaries

- labour, - plant and - materials costs for doing the work to a different timescale.

Again, whether the specialist subcontractor has to bear additional costs or can reap savings is dependent on the form of subcontract. Nonetheless, specialist subcontractors do have time- related preliminaries (say 10% of subcontract value: according to the RCC’s Cost model study a concrete frame subcontract value is on average 10% of contract value). The subcontract may be ‘supply and fix’ or ‘fix only’. If ‘fix only’, the case for using costly materials to gain speed and overall advantage would have to be made upstream. Along with labour costs the subcontractors usually have the costs of specialist plant to bear. This is unlikely to change due to the nature of the different types of operations envisaged but may be required over a different timescale.

,

How time affects parties to a construction contract The following tables show the parties to construction contracts, the timescale of their involvement and which cost centres affect their costs. There are two tables as the beneficiaries appear to change when the innovation is post contract rather than pre-contract and part of the original design. It may be regarded as showing who gains from less contract time and under what heading. Equally, it may be thought of as showing who pays for extra time and how.

Time acceleration usually involves an innovation requiring either additional labour or material. Who pays for additional material is also shown.

26


Table 4.5 Who gains from time savings (innovatingpre traditional contract)

Basis: Traditional Contractors Clients contract, pre-contract Specialist trade Main Speculative Owner/ occupier innovation (labour only) developer (developer with pre-let)

Timescale of involvement Part of Tz

m

1 2 TI + T2 + T3

Who gains from less concrete frame contract time?

Construction costs:

Preliminaries 3 3 .I" d4 Labour 3 X2 .I" * .I" Plant 3 X2 d4

~ ~~

Rentayincome X

Finance costs .I Opportunity costs .I? .I?

Materials X ' 33 d4 .I" Who gains from less (and pays for additional) material?

Key .I Yes x No (no savings directly from on in-house costs but savings may be accrued from other parties.) a If the innovation over 'traditional methods' is part of a tender then any savings (or costs) would be expected to

be passed on, at least in part, in tenders from specialist subcontractor to main contractor to client. 1 Assuming 'labour only' subcontract otherwise passed on as above. 2 Some use of main contractor's labour and plant is inevitable, but items such as tower crane and banksmen

may be considered as included in preliminaries. Savings from subcontractors should be passed on. 3 'Labour only' and 'supply and fix' subcontracts likewise passed on as above. 4 Accrued from contractors

27


Table 4.6 Who gains from time savings (innovating pbst traditional contract)

Basis: Traditional Contractors Clients contract, post-contract innovation

Specialist trade Main Speculative Owner/ occupier (labour only) developer (developer with pre-let

Timescale of involvement Part of T2 T2 Tl + T2 + T3

Who gains from less concrete frame contract time?

Construction costs:

Preliminaries 9 3 d 3 x4 x4

x4, 5 x4, 5 Labour 4 3 X 2 3

x4. 5 x4. 5 Plant 4 3 X 2 3

RentaVincome X .I Finance costs d d Opportunity costs .I? d? Who gains from less (and pays for additional) material?

Materials X’ .I3- x4. 5 x4, 5

Key d Yes x a If the innovation is introduced post contract, then at least part of any savings (or costs) would be expected to

No, (no savings on in-house costs but some savings may accrue from savings from other parties)

be passed on from specialist subcontractor to main contractor to client. Assuming ‘labour only’ subcontract otherwise 4 The main contractor gains from savings taken from the subcontractor but not wholly passed on to the client. Some use of main contractor’s labour and plant is inevitable, but items such as tower crane and banksmen may be considered as included in preliminaries. Assuming ‘labour only’. If ‘supply and fix’ subcontract or innovation at clients behest then x. The client is not necessarily entitled to benefit from innovations introduced by specialist subcontractors but might gain from savings passed on by the main contractor, particularly if a contract variation is required to cover a variation to the specification. If innovation is at client’s request he may need to pay for it. For example, if acceleration is required he may be required to pay acceleration charges but may gain from early rental.

28


The following table aims to quantify the basis of cost of time for each cost centre relationship above.

Table 4.7 Who gains fiom less contract time and by how much: the cost of time ~ ~~

Contractors Clients

Specialist Main Speculative Owner/ occupier subcontractor developer (developer with

pre-let)

Timescale of involvement Part of T2 T2 T1+ T2 + T3

Construction costs

= a x x c c = b x &- x c c Preliminaries m m

Labour 4 T 2 x numbers x rates'

Plant 4 T 2 x numbers x rates'

Materials ~~

(Prime cost) Prime cost Prime cost

Lost rentalhncome X =GT2 x rate

Finance costs 6F = fn(GT2,T, C) 6F = fn(6T2,TY C) - see note 2 - see note 2

~~ ~

Opportunity costs

Approximately 1 %. Preliminaries for a concrete frame subcontract are, depending on the project, about 10% and the subcontract value is often approximately 10% of main contract value. Say 5% to 15%. Main contract preliminaries vary depending on the project. For the relatively very simple Cost model study(27' buildings it was 5.5%. Plant costs may be assumed to be included in preliminaries

These tables form the basis for further studies in 8.4 Integration ofresults

Further reading Byme, P and Cadman, D (1 984), Risks uncertainty and decision making in property development, E. & F.N. SPON, London.

Darlow, C (1988), Valuation and development appraisal, (Second Edition), Bath Press, Avon

Gray, C (1 984), The preliminary costs ofconstruction, MPhil dissertation, University of Reading.

29


4.5 Concrete Society: Rationalisation of reinforcement

Work on The Concrete Society’s project, Rationalisation of reinforcement(2), was carried out concurrently with the early stages of this project. There was much cross-fertilisation of ideas and interchange of information. Members of The Concrete Society’s Working Group gave their time to the RCC’s Steering Group and vice versa. The following provides a summary of The Concrete Society’s project with particular relevance to this project.

Objectives of Concrete Society research The motivation for the work came from a broad study undertaken by Allied Steel and Wire into the installed cost of reinforcement. As reported(22), ASW’s report concluded that the time costs associated with reinforcement was about the same as the cost of buying and fixing the reinforcement itself. A breakdown of the relative costs of all the processes and factors is illustrated in Figure 4.8.

Time costs 41 %

Figure 4.8 Cost per tonne of reinforcement in a structure (22)

The study also found that it was hard to quantify the cost of installation and other processes. Traditionally, all efforts to reduce costs have focused on the easily quantified cost per weight of reinforcement, especially as prices for fabrication and fixing are invariably quoted on a ’per tonne’ basis.

The advent of computer-aided design (CAD) has enabled designers to minimise more easily the weight of reinforcement used. Unfortunately, this generally results in increased complexity and greater time-related costs.

Rationalisation of reinforcement As reported by Clarke(23), within the context of The Concrete Society’s Technical Report 53(2), ‘rationalisation’ means the elimination of unnecessary variation. It does not simply mean ‘standardisation’, although this may be one aspect of the process. Ideally, rationalisation should be applied to the whole process of design and construction. Significant savings can be made at various stages of the process and decisions in one will affect those that follow. Good communication is essential. For maximum efficiency this communication must be both up

30

4 Background studies - I .

and down the supply chain as decisions on site or contractors preferences can also affect how work proceeds.

The traditional approach to designing concrete framed structures is to consider each element in turn and design the reinforcement accordingly. This tends to produce a solution that is highly efficient structurally but may be highly inefficient in the context of the business process. It is usually left to the reinforcement detailer to try to introduce some rationalisation, although at that late stage the possibilities may be very limited. The goal should be to reduce .costs by making all operations simpler thus making the total less costly. To achieve this The Concrete Society's report, Towards rationalising reinforcement for concrete structures (CSTR53)(2) recommends adopting all, or some, of the following procedures:

Reduce design time -by using standard or repetitive designs Reduce manufacturing time and costs - by using fewer sizes, more common shapes and more straight bars Reduce direct fixing time -by simplifying designs, reducing the number of loose bars and making greater use of prefabrication Reduce delays during fixing - by using simpler reinforcement schedules leading to fewer mistakes Reduce project delays - by reducing complexity, administration delays in information flow and the potential for mistakes Reduce project duration - by shortening fixing times and removing delays caused by complexity Reduce 'the learning curve' for fixers on a new site - by introducing standardisation of details for wider use throughout the concrete construction industry.

The report suggests that the design process needs to be modified so that the number of element types and variations in each type are kept to a minimum. Reinforcement should be 'typical' (or standard). Additional amounts of reinforcement should be necessary only for more heavily loaded members and this should be in the form of loose bars added to the typical reinforcement arrangement. This approach needs the input of an experienced engineer andor detailer at the outset.

At the detailing stage there is some scope to rationalise the reinforcement. Suggestions include: 0

0

Using straight bars in preference to bent bars Using standardised reinforcement units, detailing additional loose bars to meet any particular design requirement Aiming for reinforcement arrangements that use the fewest individual bar marks On long structures, arranging lap positions to suit the order of construction Using tailored fabric wherever possible (refer to the manufacturer for advice) Using standard reinforcement assemblies e.g. proprietary shear reinforcement systems for punching shear, and discussing their procurement and availability with the supplier early in the design process.

The full process of rationalising reinforcement is complex and involves many participants and viewpoints. Ideally, the design, detailing, purchase, supply and installation of the reinforcement should be in one vertically integrated process, so that all parties in the supply chain can contribute to and benefit from rationalisation and the potential savings. The goal of an integrated design and construction process should stimulate the concrete industry to change the current procurement process to enable the benefits of rationalisation of reinforcement to be fully realised.

31


The value chain Ideally, savings made at each stage in the construction process should be accumulated by passing the savings across a number of customer/supplier interfaces in the so-called 'value chain'. With reinforced concrete there is a large number of customerhpplier interfaces and several are between different types of business.

Within a value chain, ,value can 'leak out' of the system unless each customerhpplier interface is well managed. This is particularly true with passing on time savings, which requires planning and a substantial amount of dialogue between parties. For instance, unless a following operation can start immediately on completion of the operation in hand, much of the time benefit disappears. There is no advantage in finishing reinforcement early unless concreting can start earlier, a saving of one hour in fixing reinforcement may not allow earlier concreting because of the constraints of placing and finishing in the working day. For the contractor, it may be more economic to take the benefits of letting operatives be more efficient rather than taking less time overall.

On the broader scale, completing the frame early will bring no overall benefit to the time for completing a project unless the frame is on the critical path. In principle it is easier for integrated companies, rather than groups of subcontractors, to realise the benefits of time savings as they can organise follow-on trades to take advantage of rapid frame construction.

Concrete Society rationalisation of reinforcement: interviews Initial work on The Concrete Society's Rationalisation of reinforcement project included a report by the University of Reading(24) that made reference to a series of 12 interviews with members of the concrete industry (four engineers, five subcontractors, two fabricators and a steel mill)'25'. Whilst the interviews were unpublished, the notes provide interesting insights into some of the attitudes towards rationalisation and prefabrication. The points made in the 12 interviews are reproduced in Appendix 11. They include various representations on the effects of rationalisation and are discussed in Chapter 8.

University of Reading: A study of the procurement of reinforcement The main findings by the University of Reading (26) are given below.

Rationalisation of reinforcement must begin with designers understanding the issues and offering the benefits to their client. Initial considerations based simply upon the tonnage of steel give a misleading indication of the alternatives.

Prefabrication of reinforcement without proper justification can be more expensive in materials, manufacture and transportation than the use of loose bars, but there are large advantages in the installation process. Thus, a cost structure based on weight will almost always inhibit prefabrication.

In-situ concrete is primarily used for customised solutions to take advantage of its relatively low cost when compared with other structural materials.

Standardisation of reinforcing components, e.g. bar diameters and lengths, and cage construction, is very limited.

No one organisation has control of the complete value chain. The industry must decide where its best interests lie. If site work is to be significantly improved and reinforcement is to be rationalised, a vertically integrated solution must be found. This is most likely to be achieved by improved rnanagement of design and construction operations and by the use of information technology to transfer the elements of the value chain intact fiom one stage to the next.

The study found that relationships between processes in the industry are generally complex and there is a lack of hard data at a level of detail that would permit significant analysis. Even so, there appear to be opportunities to make significant cost savings.

32


4.6 Background studies: conclusions It may be concluded that the rationalisation of flat slab reinforcement is a complex subject. If rationalisation is to help to produce the optimum forms of flat slab construction then many factors have to be taken into account. The concept of time equals money is well known but is quite difficult to apply - and the value, or cost, of time is different to the various parties in the supply chain. Undoubtedly there are savings to be gained in fixing times by using more rationalised configurations of reinforcement but, as illustrated by fig.4.9, this usually means more weight of reinforcement. The effects of rationalisation on overall costs, productivity and critical time is hard to judge and hard evidence found appears to be anecdotal.

Nevertheless this background work provided the theoretical background to carry out studies on the full-scale building at Cardington.

High

Low

,.** Weight of .* reinforcement .+***

.e* ..*' *.a*

.*

.......... ...................... Prefabricate b

Manufactured meshes ____, b

Level of rationalisation Highly Usual Rationalised Highly detailed rationalised

Figure 4.9 Rationalisation vs complexity and weight

33

5 European Concrete Building Project at Cardington

5.1 Introduction The in-situ concrete building at Cardington was the first in a planned series of concrete buildings to be constructed at BRE 's Bedfordshire site under the auspices of the European Concrete Building Project (ECBP). This initiative created by BCA, BRE, CONSTRUCT and RCC was aimed at improving the performance of the concrete industry, and is the largest single research programme of its type in the world (29.30.31)

The construction of the in-situ building provided a unique opportunity to research a range of different ways of providing and detailing reinforcement. The building itself is a seven-storey in-situ concrete frame with flat slab floors. The floor plates consisted of four bays by three bays on a 7.5 m grid of columns. The structure was designed as though it were a contemporary office building in Bedford (Figure 5.1).

For the Cardington project as a whole, the over-riding aim was to examine the processes involved in constructing an in-situ framed building; obviously reinforcement was part of that research. One of the main areas of work was this project, Rationalisation of flat slab reinforcement. The construction of the in-situ building was part of a Partners in Technology project part-funded by the DETR that aimed at re-engineering the whole in-situ concrete frame construction process with the aim of reducing costs, increasing speed and improving quality.

The following process research studies were carried out on the in-situ frame whilst it was being built: Process re-engineering (Cranfield University) Research on formwork systems (Birmingham University) , Early striking and loading of concrete slabs (Leeds University) Process efficient concreting (Imperial College) Early acceptance procedures (Liverpool and Queens University, Belfast) Improved rebar information and supply (Loughborough University) Rationalisation of flat slab reinforcement (Reinforced Concrete Council) Health and safety (Birmingham University)

To aid the process research undertaken, extensive amounts of qualitative, quantitative and technical data were collected. Recommendations aimed at improving the overall business process of concrete frame construction are given in a series of Best Practice Guides (9.10, 11. 12. 13)


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Figure 5.1 Typical floor plan of the in-situ concrete building at Cardington

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5.2 Design Structural requirements for the seven storey in situ building aimed to provide an efficient reinforced concrete frame complying with Eurocode 2 (Em1 992)‘14’.

This building is modelled on an office block meeting the requirements of a contemporary developer on a 1200 m2 plot in central Bedford. The frame is three bays by four bays in plan and each bay is 7.5 m square. Floor-to-ceiling height is 3.5 m with the top floor employing a permanent concrete formwork system. To demonstrate that designing for imposed floor loads of 4 to 5 kNlm2 for today’s typical office building is mostly unnecessary, a standard load of 2.5 kN/m2 was specified.

Service core voids are located at each end of the building. The design assumed that external walls would be precast concrete. Stairs in the south core were precast, two flights per storey which were ‘glued’ together to speed construction. The ‘weld’ was formed by a high strength mix containing Densit and steel fibres bonding the concrete elements. The asymmetric plan enabled torsional modes to be investigated as part of performance research: the asymmetry is accentuated by omitting the second flight of stairs at the north end of the building.

The reinforced concrete solid flat slab floors were designed to facilitate fast construction. Soffits are unobstructed, with no downstands. The only upstands are at the ends of the building where upstand beams frame the stair and lift openings. Steel cross-bracing at each end of the fiame provides wind resistance and stability in the absence of shear walls. The whole structure is enclosed within the famous airship hangar at Cardington, and thus shielded from most wind pressures and other elemental forces.

Dimensions of all internal and all perimeter columns are identical, using high strength concrete (C85) between the ground and third floors, and C37 above that. The thin floor slabs are cast in C37 concrete.

Q I Fmflevel 9 9 I 1 II

Figure 5.2 Typical cross section

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5.3 Design and detailing As far as possible, all designs were to Eurocode 2 (ENV1992)(’4’ using sub-frame analyses. ’

With an overall depth of 250 mm, the slabs at Cardington were deliberately designed to be thin in order to test design methods, induce deflections and be at the limits of what would normally be used. The thickness was in line with that determined from preliminary optimisation studies.

To achieve a span/ overall depth ratio of 30, EC2 (and BS 8110(32)) requires the amount of main flexural reinforcement to be increased above that needed for the ultimate limit state. This can have a considerable bearing on the economics of concrete floor slab construction. So on Floor 3, the designer was told that “deflection was unlikely to impair performance”; in other words the flexural reinforcement in spans was not to be increased to cater for deflection requirements. Half of Floor 4 was designed using yield line theory: Floor 6 was designed using finite element analysis.

The traditional approach to providing punching shear reinforcement is to use links, but these are very time-consuming to fix into position. The building provided the opportunity for a range of innovative solutions to be tried out. The option of not using shear reinforcement was not feasible and had already been proved to be uneconomic with relatively small columns (see 4.3). I

Once a finalised structural design of the in-situ building at Cardington was available, member companies of Construct and UKSA (RMPG) detailed alternative arrangements of reinforcement for Floors 3 to 6 . Anthony Hunt Associates, Powell Tolner and Whitby & Bird provided alternative designs for the flexural steel on floors 3 ,4 and 6 respectively. Individual manufacturing and supply companies provided designs for their proprietary punching shear systems. Drawings, weighted bending schedules and specifications produced as part of this research project formed part of the contract documentation required for the construction. The designs, drawings, schedules, correspondence, specifications and briefs generated were brought together in four volumes that were distributed to interested parties (copies are deposited with the BCA library, BRE, and CRIC at Imperial College).

It should be noted that, in many areas, the work was co-ordinated with other research projects on the ECBP. The structure at Cardington offered a unique opportunity to demonstrate techniques and to obtain valuable comparative and quantitative data on many different projects. This opportunity was taken on several projects but the consequence was to increase the need for co-ordination and in some instances to cause delay to this project. Progress on this project was also hampered by the rate of progression the on the in-situ building at ECBP, Cardinton. Arrangements for financing the construction, the final design, research by others and construction problems on site are examples of some of the factors outside the control of this project’s partners that delayed andor had other effects on the project.

It should also be noted that specialist items were specified, designed and provided for the purposes of the research - sometimes despite the protestations of the supplier who believed their product to be unsuitable for the specific application at Cardington (see Flexural reir2forcentent in 5.5)

The amounts of correspondence involved in procuring these alternative systems were noted and studied.

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5.4 Pre-construction In the long lead up to construction at Cardington, the Steering Group for this project helped resolve many issues. These included 0

0

0

0

Agreeing which configurations of reinforcement should be considered. How they ought to be detailed and by whom Which types of generic and proprietary shear reinforcement should be investigated How to compromise between the demands of this project and those of others Specifications for designers and detailers.

The final arrangement of reinforcement used in the structure is given in Table 5.1. The configurations of top and bottom reinforcement were chosen to reflect the Steering Groupk thoughts on best practice tempered by the findings of Further cost model studies(28). The relatively late loss of Floor 7 to this project resulted in two configurationsof reinforcement being used on Floor 4. The various methods of providing flexural and punching shear reinforcement are illustrated in Figures 5.3 to 5.8.

Table 5.1 Configurations of reinforcement in the in-situ building at Cardington.

Location Main reinforcement Shear reinforcement

Grids A & B Grids C & D

Floor 1 Traditional loose bar Traditional hook-and-bob links (shape code 85)*

Floor 2 Traditional loose bar

Floor 3 Rationalised loose bar

Floor 4 Blanket cover loose bar - ROM shear ACI shear

I I

I t

yield line design and elastic ladders stirrups design (as other floors)

Floor 5 One-way mats BRC shear hoops BRC Deha stud rails

Floor 6 Blanket cover two-way mats - Traditional shape AncoPlus shear Finite element design code 85 + stud rails

Floor 7' Traditional loose bar in combination with permanent formwork

Square hollow section shear heads.

Notes * Shape code 85 to BS 4466 (BS 4466 was superseded by BS 8666 in April 2000) + Except structural steel shear heads (made from rectangular hollow sections) at positions A2 & B2 # Not within the scope of this project

Members of the Steering Group also helped in obtaining free supplies of reinforcement for the project through the RCC and UK Steel Association (RMPG).

The intention was that the detailing and fixing were to be carried out as if there were seven floors of the same reinforcement configuration. One of the main problems for the process researchers lay in the number of different configurations, which meant that in two cases only half of one floor was actually done - and then half of that had different shear provision.

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The reinforcement drawings and schedules were also prepared in different ways. The first two floors were conventionally designed and detailed by the consultant. The Floor 3 was in effect contractor designed and detailed. Floor 4 was contractor detailed and the Floors 5 and 6 were supplier-detailed. See Appendix I.

Table 5.2 Weight of flexural reinforcement in slabs

Floor Weight Notes

1 16.9 tonnes

2 17.1 tonnes

3 15.3 ' tonnes

4 14.5 tonnes

4 23.2 tonnes

5 19.9 tonnes

6 25.5 tonnes

7 d a

Different ductility bars used

Different ductility bars used

Assuming whole floor subjected to yield line design

Assuming whole floor subjected to blanket cover loose bar, elastic design

Not available for research on rationalisation ~

# Designed as if "deflection would not unduly affect performance'' otherwise an extra 1.6 tonnes would have been required to meet normal deflection criteria

.

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5.5 Commentary on reinforcement systems used General Buro Happold checked all drawings and calculations for suitability for incorporation and integrity of the whole structure

Floors 1 and 2 Floors 1 and 2 were designed, drawn and detailed by Buro Happold acting as the client’s consulting engineer. This may be thought of as being part of a traditional method of procurement.

Floor 3 Design and detailing for both flexural and shear was undertaken by Anthony Hunt Associates acting as designeddetailer to the contractor, Byme Bros. The contractor’s preferred detailers, Peter Brett Associates, also prepared alternative details to Buro Happold’s design. These were not used for construction but were retained for possible future use.

Floor 4 Here Peter Brett Associates acting as the contractor’s preferred detailer prepared reinforcement details for the main flexural reinforcement. They brought together blanket coverage elastic design, prepared by Buro Happold, for the southern half of the slab with a yield line design, prepared by Powel Tolner Associates, for the northern half of the slab.

The design of the ACI stirrups on the eastern half of the slab was carried out by CRIC and drawn up by Peter Brett Associates. ROM shear ladders were used on the western half of the slab with details being prepared by ROM. The ladders were designed as direct replacements for traditional shape code 85 links. In each case the worst case shear requirement for internal, edge and corner columns according to either the elastic design or the yield line design was used for all internal edge and corner columns.

Originally this floor was to accommodate structural steel shear heads made from rolled steel channels. Unfortunately, supply difficulties delayed and then restricted their use to two columns in Floor 6. Shear heads made from square hollow sections were used on Floor 7 - but these were outside the scope of this project.

Floor 5

Details of mats and supplementary main reinforcement were prepared by ROM on behalf of UKSA on a (contractor’s) supplier detailed basis to Buro Happold’s design and a brief given for mats by the RCC. Details of shear hoops, used in the western half of the slab and DEHA stud rails used in the eastern half of the slab were prepared by BRC on behalf of UKSA, who ensured compatibility between mats and shear systems

Floor 6 Details of mats and supplementary main reinforcement were prepared by ROM on behalf of UKSA on a (contractor’s) supplier detailed basis to Whitby & Bird’s design and the RCC’s brief given for blanket cover two-way mats. AncoPlus shear studs were used in the eastern half of the slab. Designs and details of these studs were prepared in Switzerland by Ancotech. Structural steel shear heads were used at positions A2 and B2 to designs by CRIC and as detailed by Frank Hodgson Associates. In other parts of the western half of the slab traditional shape code 85 reinforcement was used.

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Flexural reinforcement Detailing of loose bar followed traditional practice. The definition and level of rationalisation and prefabrication was left to the best judgement of the detailer (having been given a clear brief). For instance the brief for the detailer of Floor 3 was to give the specialist contractor least cost of labour and material (as might be required in a detail-and-construct contract). The brief for the detailer of Floor 4 was to give least labour cost to the specialist contractor (as might be required in a labour-only contract). Full details of design and specifications are recorded in four volumes as outlined in 5.5 Design and detailing records. A4 copies of the reinforcement drawings for Floors 1 to 6 are included in Appendix VI.

Mats The supplier detailed the mats. Initially, it had been proposed to include narrow (1 m wide) mats on half of Floor 5 but this proved impractical.

It must be recorded that the supplier felt that the slabs at Cardington were not an ideal demonstration for the use of mats as the nature of flat slabs (distribution of reinforcement within them), the long spans and amounts of reinforcement would lead to heavy and inefficient mats. Usual weight premiums of 5% to 7% are expected for one-way slabs. Here, it worked out to be as high as 16% for one-way mats. This is partly explained by the need to rationalise mats so that the number of different mats was kept down to a reasonable level while satisfying the elastic design with its intrinsic different arrangements for reinforcement. Some concern was expressed about the weights of mats and the interface of top mats with columns (in that top mats might need lifting over column starters). The weights of mats were restricted to a four-man lift. It was envisaged that some top mats would need to be lifted over column starters.

Shear reinforcement ROM Shear ladders The area and spacing of reinforcement in the vertical legs of the ladders was determined from the area and spacing of shape code 85s they were designed to replace

Figure 5.3 ROM shear ladders in place, Floor 4W

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ACI stirrups This arrangement of stirrup and longitudinal bars was designed in accordance with EC2'I4' to methods described in AC1 3 1 8(33'. In effect the stirryps are small beam cages within the depth of the slab (see Figure 5.4). They are formed into Ls, Ts and Xs when viewed on plan.

Figure 5 4 ACI shear stirrups used in Floor 4E

Layering was thought to be a potential problem. However, the top-most longitudinal reinforcement (together with the top horizontal part of the link) is in the same layer as the top-most main flexural reinforcement. Similarly the bottom-most longitudinal reinforcement (together with the bottom horizontal part of the link) is in the same layer as the bottom-most flexural reinforcement.

The shallower 'beam cage' was prefabricated. The taller 'beam cage' was made up by threading the longitudinal bars through loose links. Assemblies were prefabricated on the ground and lifted onto the correct floor where they were lifted into position by hand ('I ... A five minute job"). Cover was measured to whichever reinforcement was closest to the surface.

From the construction process point of view the ACI stirrups required no special measures as they can be detailed on normal reinforced concrete drawings and scheduled on normal RC bending schedules.

Shear hoops Shear hoops were designed to BS 81 10'32'.

In the run-up to construction, some concern was expressed over the compatibility of the shear hoop and stud rail systems with prefabricated mats (the bars in the mats cannot be adjusted in relation to each other to allow the proprietary shear system to fit). The manufacturers suggested that compatibility was not a problem as proprietary systems were bespoke to suit the job. In the event, there proved not to be a problem although there were reportedly difficulties caused by the shear hoops tangling whilst being moved

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rn I . . I L I I

Figure 5 5 BRC Square Grip shear hoops used in Floor 5W

Stud rails The stud rails were designed to against BS 8 1 10'~~) .

and, in the case of the Deha stud rails, checked

Figure 5.6 DEHA stud rails fixed to formwork in Floor 5E

Figure 5.7 AncoPlus stud rails being placed in Floor 6E

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Shear heads Originally it was intended to cast 10 structural steel shear heads in Floor 4. Due to problems with drawings, supply, fabrication in time for construction, and costs, this did not happen Only two shear heads were cast, and that was in Floor 6: one X at position B2 and one T at A2. The designs of conventional shear heads were completed by CRIC but the designers struggled to design an edge shear head with holes.

Figure 5.8 Geitlinger structural steel shear head similar to the one used at grid location B4 on Floor 6

The shear heads were made from 250 x 150 x 8 RHS’s (48 kg/m) cut vertically into channels and prefabricated into units up to 2 m long and wide. The shear heads were craned into position after the installation of the bottom mat of reinforcement. Bars in the B2 layer were cut on site as required to avoid cover problems at the top. Perimeter U-bars were fixed after installation of the perimeter shear heads and the bottom legs of the U-bars were cut off on site.

The shear head at column B2 is an enclosed type and weighs approximately 450 kg. This shear head allowed for two 600 x 600 mm holes to be made close to the column. The shear head at column A2 is also an enclosed type and weighs approximately 275 kg. and again allowed for two 600 x 600 mm holes to be made close to the column. For the purposes of research, the top reinforcement was fixed as if there were no holes and then trimmed.

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5.6 Design and detailing records The pre-contract, design and detailing and construction phases of the project generated a great deal of paperwork that was collected into four volumes and circulated to interested parties. One copy is deposited with the library at BCA.Other copies were deposited with CRIC and with B E . The contents are given in Table 5.3.

Table 5.3 Summary of design and detailing records

Vol 1 Designs general General & RC drawings Floors 1 to 6 Floors I , 2, 4 (south) and 5 Design by Buro Happold. Justification for revision to Buro Happold calculations for 8T20s at supports to become 8T16 Floor 3 Design by Anthony Hunt Associates Floor 4 Floor 4N design by Powell Tolner & Associates (Yield line) ACI shear stirrup design by CRIC Conversion of traditional links to ROM shear ladders Floor 5 Shear hoop design by BRC 1 David Crick Associates Deha stud rail design Floor 6 Finite elemects design by Whitby and Bird Justification for switching T1 and T2 AncoPlus studrail design by Ancotech (Switzerland) Structural steel shear head design by CRIC and Frank Hodgson and Associates

Vol3 Correspondence regarding reinforcement 11/97 to 7/98

General Floors 1 & 2 Floor 3 Floor 4 Floor 5 Floor 6 Floor 7

Vol2 Drawings and schedules Issue sheets Floor 1 by Buro Happold Floor 2 by Buro Happold Floor 3 by Anthony Hunt Associates Floor 4 Slab by Peter Brett Associates. Upstand beam (Floors 4,5 & 6) by Webster Negga. ROM shear ladders by ROM

Floor 5 Mats and reinforcement by ROM (on behalf of UKSA). Shear hoops by BRC / David Crick Assoc. DEHA stud rails by DEHA for BRC

Floor 6 Mats and reinforcement by ROM (on behalf of UKSA). Shear heads by Frank Hodgson & Associates. AncoPLUS studrail by Ancotech (Switzerkand). Traditional links by RCC

Floor 3 Alternatives. .Revisions to Floor 3 if it was to be subject to normal deflection criteria as other floors. Rationalised layout by Peter Brett Associates (not used for construction)

Floor 4 Alternatives. Full details and schedules for yield line design by Powell Tolner & Associates.

Vol4 Contractors detailers briefs suppliers detailer brief & records

General brief Floor 3 brief

Site records:

Floor 4 - Shear briefs Floor 5 - Shear briefs Floor 6 - Shear briefs

Bending schedule register. Drawing register and issue sheets.

Approvals to pour etc

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5.7 Process issues Off-site processes: correspondence In order to consider off-site activities, the numbers and types of correspondence were investigated. The RCC's records pertaining to its role as the contractor's design co-ordinator were used as data. These records were not complete or comprehensive nor did they necessarily account for the many communications between the parties involved in the reinforcement at Cardington. Nonetheless the RCC played a central role in co-ordinating the reinforcement for the floor slabs and it was hoped that this exercise might shed some light on the off-site processes. Again, due to the nature of the construction at Cardington, it provided an imperfect but valuable basis for comparison.

The issue sheets for calculations, drawings, schedules, specifications and records of notes of design co-ordination meetings, letters etc. were used to determine numbers of communications, originators and recipients. In order to compare like with like a communication that involved more than one floor was counted once for each floor involved. (The thinking was that if the whole of the building had been of that reinforcement arrangement then that communication would have been necessary.) They also provided the basis for drawing process maps.

Figure 5.9 shows that the traditionally reinforced slabs (Floors 1 and 2) elicited least correspondence, while contractor detailing (on Floors 3 and 4) produced more correspondence and the contractor's supplier design and detailing (of Floors 5 and 6) resulted in the most. Most of the excess correspondence on Floors 5 and 6 was due to correspondence with suppliers.

Blanket two way mats (finite element anal) Floor 6

Designed one way mats Floor 5

Blanket loose bar; (N- yield line: S- elastic) Floor 4

*8Rational" loose bar (Contractor design) Floor 3

Traditional loose bar (Buro Happ.) Floor 2

Traditional loose bar (Buro Happ.) Floor 1

0 10 20 30 40 50

' Consulting Engineer 0 Contractors co-ordinator 0 Contractors designer/ detailer

Supplier/Supplier designer 5Others ~Subcon t ' r to Cont's designer/ detailer

Figure 5.9 . Correspondence: incidences and origins for flexural reinforcement

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Struct steel heads (A2, 82) Floor 6wb

SC 85 (The rest) Floor 6w

AncoTech Shear studs Floor 6e

Square Grip Shear hoops Floor 5w

Deha stud r a i l p , 1 Floor 5

Rom Shear ladders Floor 4w

ACI stirrups (prefab on site) Floor 46

SC 85 Floors 1 ,2 & 3

1

0 4 0 12

a Contractors co-ordinator g SupplierlSupplier designer others

Figure 5.10 Correspondence: incidences and origins for shear reinforcement

A similar story emerges with traditional shear links: they elicited virtually no separate correspondence while proprietary systems resulted in at least some.

This exercise indicates that non-traditional methods involve more correspondence, therefore more relationships (and more time to develop relationships with the parties involved). On the other hand, it may just indicate that many of the processes (providing design information, design, drawing, checking, scheduling, approving) that traditionally are carried out in-house by the engineer are, in contractor detailing and contractor design-and-detailing, carried out by those contracted to do so. This requires a greater amount of communication and design co- ordination between parties. This type of work is not necessarily core to the specialist contractor's business and may provide a barrier to better integration.

Process maps From correspondence and the above section 5.6, the following process diagrams have been drawn. While these process maps should be regarded as being indicative only, they give the impression that contractor detailing and contractor design lead to a greater number of relationships, which perhaps complicates the process. This reinforces the conclusions drawn just from looking at amounts of correspondence.

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Figure 5.11 Process map for traditional construction

49


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Figure 5.12 Process map where contractor detailing is used

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Figure 5.13 Process map where contractor design is used

5 1


5.8 Construction Construction of the seven-storey in-situ concrete flat slab frame structure took place over a 14-week period commencing in January 1998. The building was considered to be sited in Bedford High Street with access from one side only. A tower crane was used but no external scaffolding. Four different types of formwork systems were employed0'):

Floors 1 & 2 - aluminium free-standing props and laminated timber beams. The main props were stabilised using a tripod during erection.

Floors 3 & 4 - table-based system comprising self-supporting aluminium frames. The system can be used either on an erect-and-dismantle-basis or the tables can be flown as a complete assembly with the formwork from one floor to the next.

Floors 5 & 6 - a tubular steel frame system delivered to the site in crates. This system was used in combination with the aluminium primary and secondary beams of the Ischebeck TITAN system..

Floor 7- the roof of the building was constructed using combinations of precast panels and in-situ concrete.

0

For the floor slabs one concrete (C37) strength was used but the workabilities and placing methods were varied. In the columns two strengths of concrete were used. Some of the concrete was placed by skip and some by pumping. Based on the results of the research undertaken, it was found possible to strike the slabs at very early ages without the need for re-propping. Some of the slabs were struck at ages as early as 25 hours from the time of start of hydrati01-1'~~).

Construction of the building was to some extent artificial due to the use of the different combinations and permutations of reinforcement. The construction time was not increased markedly by the variations in construction methods, as compared with a conventionally constructed building of this type. However it was estimated that the implementation of the best of the methods to emerge from the project would have enabled the frame construction time to have been cut by more than 50%"').

There were other problems on site that affected this project: these are reported in Chapter 7 Cardington: Analysis of construction process data and Appendix III supplement 2. With so many different sources of information and different arrangements and types of reinforcement it is testimony to the contractor and researchers that reinforcement operations on site went (relatively) smoothly.

The various stages of construction are illustrated in Figures 5.14 to 5.21

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Figure 5.15 First floor

Figure 5.14 Ground to first floor

Figure 5.16 Concreting Floor 2 Figure 5.17 Floor 3

Figure 5.18 Floor 4 Figure 5.19 Concreting Floor 5

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Figure 5.20 Floor 6

Figure 5.21 Completed in-situ building April 1998

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5.9 Other research on reinforcement The design and detailing of the reinforcement at Cardington had to be co-ordinated with other research on the reinforcement and frame. These research projects are described below. Inevitably they had effects on the data obtained for this project.

Improving rebar information and supply In its project, Improving rebar information and supply, the Loughborough University researchers used information from organisations involved with the Cardington project as a base case study. Having mapped out supply chains, they went on to look at information flows in order to recommend methods and standards for electronic data interchange (EDI).

Their final report(34) advocates the use of a comma delimited data file in ASCII format to allow free exchange of reinforcement information. Their final process diagram is presented in Figure 5.22.

Instrumentation To assist with the performance research which is being carried out on the completed structure, a large number of short additional strain-gauged bars were included within four bays of Floors 2 and 3. Measurements from a selected number of these bars were taken during the striking of Floor 3.

A small number of special instrumented bars known as Durham bars were installed on Floor 6 as replacements to normal bars. Each of these bars was sawn in half longitudinally to enable the installation of a large number of strain gauges along the centre-line of the bar, enabling very detailed information to be gained on strain and stress distribution. An initial series of tests was to be carried out, applying serviceability loads to look at the behaviour of slab/column connections in the area in which the instrumentation has been installed.

Deflection Initial measurements and data show that neither the omission of additional ‘deflection’ reinforcement on Floor 3 nor striking floors at the very early age of 25 hours had any significant effect on the long- term deflection or performance of those floors. A brief report is given in Chapter 6.

Couplers Reinforcement is normally lapped to provide continuity. However this can cause problems due to congestion of reinforcement: a possible solution is to use threaded reinforcement couplers. Under a commercial research project some column connections were carried out using couplers. The research showed its sponsors just how viable couplers are on a ‘typical’ site.

Ductility of reinforcement Ductility of reinforcement is a subject that has caused some debate in Europe and some concern in the UK. Floors 2 and 3 incorporate normal ductility reinforcement (i.e. equivalent to the 460A grade in BS4449) as opposed to normally-used higher ductility reinforcement (i.e. equivalent to the 460B grade in BS4449). There is the opportunity to examine the effects of ductility on slab design through full scale destructive testing at some future date.

The differences between the two types of steel used on Cardington were as follows:

55


Table 5.4 Bar ductilites

Ductility Annotation used Comment

Normal X Cold-workedy with three ribs around the circumference which tend to be less pronounced than traditional rebar

Hot rolled, with either two or four ribs around the circumference

High T

Figure 5.22 Rebar process flow diagram from IRIS"4'

56

6 Cardington: Structural performance Changes to reinforcement configurations may affect performance. This chapter looks at the effects of the reinforcement configurations on ultimate performance and deflection.

6.1 Ultimate strength in flexure Nottingham Trent University was commissioned to investigate alternative reinforcement configurations to assess factors of safety against failure in flexure of each configuration. A summary of their report is presented below. The full report is presented in Appendix VII.

Automated yield-line analysis(35) was applied to the first six floors of the in-situ building of the concrete frame at Cardington (including the two differing halves of Floor 4). The objective was to evaluate any variation. in flexural capacity resulting from the differing design and detailing approaches used for the floors. The results obtained are summarised in Table 6.1 below.

Table 6. 1 Flexural reinforcement, weight and load factor against failure at ultimate load ~

Floor Flexural reinforcement Weight Load factor (tonnes) against failure at ultimate

loads using fine mesh

1 Traditional loose bar 16.4 1.13

2 Traditional loose bar - high ductility 16.6 99 3 Rationalised loose bar 14.7 1.19

4 North: Yield-line design 14.2 1.16

South: Blanket loose bar 22.9 1.34

5 One-way mats 19.3 1.15 ~

6 Two-way mats 25.0 1.51

$ Weight per floor of flexural reinforcement in slabs (i.e. excluding shear reinforcement and reinforcement in upstand beams). On Floor 3, compliance with normal deflection criteria was specifically excluded from the design; an extra 1 .G tonnes would have been required to meet normal EC2 deflection rules. In the case of Floors-4N and 4s weights have been scaled from schedules from half a floor. Not analysed as very similar to Floor 1. 68

The principal conclusions arising from the investigation are:

0 The flexural strength of the floors is satisfactory in respect in that all designs provide a load factor divided by ultimate design strength of at least one.

0 The designs for Floors 1 to 5 (with the exception of Floor 4s) provide very similar load factors against failure at ultimate loads. They are all above one, but are so close to unity as to make further reinforcement economies unwarranted.

Floor 4s and Floor 6 have higher load factors and some reduction in the reinforcement steel could be made to make the load factor closer to one. This is particularly the case for Floor 6 , where the reinforcement of the uniform two-way mesh can be readily reduced to effect a proportionate reduction in load factor.

57


6.2 Deflection

The following is a reproduction of A review of slab deflections in the in-situ concrete frame building@) (at Cardington) prepared by Dr R L Vollum of Imperial College. The figures have been subsequently updated to include later data.

Introduction The design depth of floor systems is often governed by the need to control long-term deflections. Therefore, there is a need for reliable methods to calculate long-term slab deflections. It is difficult to predict slab deflections in the field since they are influenced by the following factors, which are not known at the design stage: 0 Concrete tensile strength 0 Creep and shrinkage coefficients 0

0 Long-term service loads 0

0 Exact slab thickness

Construction loads / concrete strength at striking

Exact position of the steel reinforcement

Currently there is a lack. of accurate field data on long-term deflections. The in-situ building at Cardington is particularly suitable for long-term measurements of slab deflection since the concrete properties, reinforcement details and construction loading/process are well documented.

Design of Cardington slabs The test building is seven storeys high and of flat slab construction. The grid is 7.5 m square. The structure was designed in accordance with EC2'I4'. The designer's brief included slab thickness = 250 mm, design characteristic = imposed load 2.5 kN/m* + 2 kN/m2 for ceiling, -

services, access floor and partitions.

Site North

< v ., .0 ,, 0 . 0. > 7500 7500 7500 7500

Figure 6 .1 Floor plan

5 8

6 Cardington: Structural performance

Table 6.2 Summary of floor design methods

Floor Design method Design er ~

1 Elastic sub-frame analysis 1 Buro Happold * 2 Elastic sub-frame analysis 1 Buro Happold * 3 Elastic sub-frame analysis 2 Anthony Hunt Associates

4 Elastic sub-frame analysis 1/ Yield line

Buro Happold */ Powell Tolner *

5 Elastic sub-frame analysis 1 Buro Happold * 6 Finite element analysis Whitby & Bird

* Buro Happold and Powell Tolner were instructed to ensure that their designs complied with the span to depth rules given in EC2. This was achieved by increasing the area of reinforcement over that required for strength.

Table 6.3 Summary of reinforcement details(3')

Floor Weight of flexural Rebar

1 16.9 Traditional loose bar

2 17.1 Traditional loose bar

3 15.3 Rationalised loose bar

rebar (tonnes) arrangement

4 14.5* Blanket cover loose bar

4 23.2** Blanket cover loose bar

5 19.9 One-way mats

6 25.5 Blanket cover one-way mats

* Weight if all yield line design ** Weight if all equivalent frame design

Prediction of deflections Code methods require: 0

0

Actual loading to be represented by a single long-term load Single values of concrete material properties to represent:

i) concrete tensile strength ii) elastic modulus of the concrete iii) creep, ageing and shrinkage coefficients

A realistic assessment of deflection is dependent on the choice of appropriate material properties and loading. In practice, the loading on a slab is dependent on construction sequence and building use. A realistic assessment of deflection needs to take account of loading history. This presents a problem since the majority of data on long-term deflections is from tests with constant loading as shown in Figure 6.2 but actual loading of suspended slabs is more akin to Figure 6.3.

59


I d

I

Figure 6.2 Typical load history in laboratory test

i::: Y z 5 0

0 50 100 150 200 250 300 350 400 days from hydration

Figure 6.3 Idealised load-time history for the third floor in the Cardington in-situ concrete building

Incremental finite element approach A finite element program has been developed at Imperial College for calculating slab deflections. The program incorporates a global plate stiffhess approach based on the CEBEIP Model Code varying load.

An incremental approach was used to calculate deflections due to a time-

Test programme at Cardington Objectives The main objectives were to: 0

0

Measure long-term slab deflections under specified loading Assesshmprove a non-linear finite element program developed at Imperial College for calculating long-term slab deflections under time varying loads Compare actual deflections with deflections calculated using existing methods with measured material properties

0

Deflection measurement Slab deflections were measured by precise levelling at the following times: 0 After striking

0

At end of construction of building Approximately eight months after construction started Further measurements were not taken until February 99 due to a lack of funding Intermittent periods thereafter

60


Test programme Floors 1 to 6 were loaded with sandbags to give a uniform load of 3.27 kN/m2 between grids 2 and 4. This is an imposed load at the upper end of that which might occur in a typical office. The service load includes allowances of 1.0 kN/mz for ceiling, raised floor and services and 1 .O W/m2 for partitions.

The same loading arrangement has been adopted at each floor to enable deflection measurements to be compared between floors.

E

Site South

hanger doors)

(&

A

1 2

I

4 3 I

5

Figure 6.4 Loaded area of slabs

Schedule of deflection measurements 0

0

Two readings immediately before and after application of service load, then Two readings at two-weekly intervals, then Five readings at monthly intervals, then Four readings at three monthly intervals until month 18 then Two readings at six-monthly intervals until at least month 30

Results Measured and predicted deflections are shown below for the external bays of Floors 3, 4 and 6. Deflection limits are span/250 = 30 mm and span/500 = 15 mm.

Predicted and measured values of deflection are shown in the following figures.

61


30 I

25

A

E 20 E Y

c 2 15 0 0)

U= 0) n 10

5

0 Test point 21

x Test point 25

0 Test point 34

A Test point 38

+Predicted

+No construction load

+No construction load strucl at 7 days

0 200 400 600 800 1000

Time

Figure 6.5 Comparison of predicted (external) and measured deflections at Floor 3 (external panels)

25 30 I

A I

5!! 0 0 200 400 600 800 l O O C

Time days

0 Test point 21

x Test point 25

A Test point 38

+Predicted with constructiion

+Predicted no construction

+Predicted struck 7 days no

load

load

construction load


62


0 Test point 21 35

X Test point 25 30

n Test point 34 E 25

A Test point 38 E 20 0 P +Predicted with $ 15 Q = '

; 10

E Y

construction load -0- Predicted no construction

+Predicted no construction

0 100 200 300 400 500 600 700 800

Time sixth


Discussion Following EC2, the degree of cracking in the slab can be defined by a parameter K as follows:

where w = load fct = tensile splitting strength, p = an EC2 factor that accounts for loss of bond with time.

The minimum value of K corresponds to the greatest cracking and hence greatest deflection. For cracked slabs, it can be shown that the long-term slab deflection reduces linearly with increasing K, for constant load w, if all other parameters are constant. Values of K are given for slabs 1 to 6 at striking, peak construction load and long-term load of 9.27 kN/m2 in Table 6.4. Deflections were plotted against K to determine 1) whether deflections are proportional to K and 2) the value of p that should be used with the peak construction load. A linear relationship was found if p was taken as 0.5 for the construction load even though it is a short-term load (see Figure 6.9). Table 6.4 indicates that the long-term slab deflection was governed by cracking induced at either striking or casting the slab above.

Table 6.4: Relative slab deflections and K values

Deflections Kshke Kpeak K o n g Kmin K h MaX increase in p=OS P=OS p=O .5 p=O .5 w.5 deflection sequence fcll28 = 37MPa ( 4 0 0 days) Floor 1 0.26 0.38 0.33 0.26 0.22 18.0 Floor 4 0.26 0.29 0.34 0.26 0.22 19.7 Floor 2 0.25 0.23 0.30 0.23 0.22 22.5 Floor 5 0.26 0.22 0.29 0.22 0.22 22.8 Floor 3 0.28 0.24 0.3 1 0.24 0.22 24.9 Floor 6 0.21 0.23 0.3 1 0.21 0.22 26.6

63


To investigate the influence of the peak construction load on deflection, slab deflections were calculated without the peak load from casting the slab above. The resulting deflections are compared in Figures 6.5 to 6.7 and in Table 6.5 for external panels. Table 6.5 shows that the predicted deflection without peak construction load is up to 23% less than the maximum deflection with the peak construction load. The difference was larger for slab 3 than the other slabs since the difference between &bike and Kpeak is greater. To explore this further, deflections were calculated without construction load assuming slabs were struck at 7 days. Table 6.5 shows that the resulting deflections were up to 31% less than the maximum measured deflection.

Table 6.5: Influence of peak construction load on deflection of external panels

No peak construction load Slab 1 Slab2 Slab3 Slab4 Slab5 Slab6 Cardington striking times -6.8 6.8 23.2 6.9 6.0 12.7 Strike at 7 days 14.9 15.0 30.8 21.3 11.9 23.4

Table 6.4 also gives Klong-tem with w equal to 9.27 kN/m2 and f,, equal to the specified value of 37 MPa. The resulting K value of 0.22 is close to or the minimum value for all slabs indicating that measured slab deflections are close to those that would have occurred with striking at 7 .days and no peak construction load if the characteristic concrete strength had been 37 MPa as specified. In other words, the increase in deflection due to peak construction load was largely offset by the fact that the concrete strength was greater than specified (C50 to C60).

% less than maximum measured deflection

Time, days

0 200 400 600 800 1000 30

25

E 20 E

't. 15 C O 0 a, E

0" 10

5

t

I I I I

I I I I

I

0

Awrage deflection external panels

+ floor 1

+ floor 2

-j(t floor 3

+ Floor 4 I

-0- floor 5

+ Floor 6

Figure 6.8 Comparison of mean deflections in external bays at floors 1 to 6

.In :the absence of peak construction load, it can be shown that ,the greater part of the increase in deflection due to early age striking is due to increased cracking. It follows that if Kpeak is less than or equal to Ksbike, early age striking will not significantly increase deflection. However, it should be noted that Kshke or Kpeak Will usually be more critical than Klong-tem.

This implies that deflections may be underestimated if early age strikindpeak construction loading is neglected. Construction loading can be accounted for in deflection predictions in

64


accordance with EC2 if the interpolation coefficient 6 is taken as the largest value corresponding to 1) striking, 2) peak construction load and 3) design service load, all with Pz0.5. This approach is based on analysis of data fi-om Cardington and should provide a reasonable upper bound to slab deflections. To avoid overestimating slab deflection, the peak construction load should not be overestimated and realistic concrete strengths should be used. Theoretically, the deflection should be increased to take into account the residual deflection that remains on unloading using the method of Rotilio or otherwise. In practice, this is probably unnecessary since the proposed calculation method appears conservative (see Figures 6.5 to 6.7). More test data is required to refine the proposed model since it is based on data fi-om Cardington, which is limited in scope.

It is interesting to note that the deflection of the floor slabs was relatively insensitive to variations in reinforcement between slabs. This is shown in Table 6.6. Figure 6.9 shows that the beneficial effect. of providing additional reinforcement was swamped by variations in concrete tensile strength. The correlation between K and deflection improves slightly if the deflections are normalised to account for the difference in reinforcement between the slabs.

Table 6.6: Influence of reinforcement weight on deflection in external panels Deflections increase in Weight steel Max deflection,

sequence (tonnes) ( 4 0 0 days, mm) Floor 1 16.9 18.0 Floor 4 18.9 (ave) 19.8 Floor 2 17.1 22.5 Floor 5 19.9 22.8 Floor 3 15.3 24.9 Floor 6 25.5 26.6

De 30 fle 25 cti 20 y = -128.6~ + 52.877 on 15 R2 = 0.7044 (m 10 m) 5

0 0.20 0.21 0.22 0.23 0.24 0.25 0.26

Kmin

Figure 6.9 K, average deflection in external panels

65


Conclusions

1. Cardington presented a unique opportunity to measure deflections in a real building under hown and controlled conditions. This is enabling the accuracy of methods for predicting deflections to be assessed and will enable .improved methods to be developed.

2. Slab deflections will usually be increased by construction loads owing to peak construction loads being greater than service loads.

References

1. Hossain T.R. Numerical Modelling of deflections of reinforced concrete flat slabs under service loads, PhD thesis, University of London, July 1999

2. Vollum R.L and Hossain T.R. Assessment of slab deflections in the in-situ concrete frame building, Cardington Conference, November 1998

3. HossainT. R., Vollum R.L, Modelling the deflection of reinforced concrete slabs under time varying loads, Concrete Communication Conference 2000, University of Birmingham, June 2000, pp 437-448

4. ROTILIO J-D. Moment-curvature relationship, XI11 FIP Congress and Exhibition, May 1998, The Netherlands.

66

. . . .

7 Cardington: Analysis of construction process data

The following sections 7.1 to 7.5 and Appendix III reproduce in full Lorien plc’s final report, Analysis of construction process data(38) commissioned for this research project

7.1 Introduction Reinforced concrete is a multi-component construction material, which is brought together in numerous overlapping processes on site. The interaction between these processes, differences between sites and other uncertainties, together with the almost infinite variations in design objectives in reinforced concrete all make determination of the best value generic reinforcement arrangements very difficult. The Cardington building offered the potential for most of the identified variables to be held constant and thus establish the effect of reinforcement arrangement differences only.

This report was commissioned by the RCC as part of the DETRPiT project Rationalisation of $at slab reinforcement as per Lorien’s proposal ref. MS/CJR/Doo4628, Oct 1997. It describes measurements made on the reinforcement installation process at Cardington and results obtained from the exercise together with their analysis and Lorien’s conclusions and recommendations.

’ The key deliverables from Lorien’s proposal were: Analysis of the time taken to install the reinforcement for each arrangement into the building. Analysis of the proportion of this time that extends the critical path, i.e. overall construction period. Analysis of other construction processes, associated with the reinforcement, which would affect the overall construction cost. Overall synthesis of the foregoing to produce a model which enables designers to identify best value and to design floors (and if possible other elements by extrapolation as appropriate).

0

In the event a certain amount of ‘leakage’ of uncontrolled variability occurred in the construction process, reducing the certainty of the outcome, and making it possible to give only indicative trends rather than precise answers for some of the deliverables. However, we are able to report data that strongly indicates differences between systems with the potential to yield significant value optimisation and recommend ways forward to realise these. The information is presented in the following manner:

Site measurement Results and analysis

0 Variability and uncertainty Conclusions and recommendations

67

Rationalisation of flat slab reinforcement . I.

7.2 Site measurement This section describes how the data from the construction phase of the project was collected.

The whole construction process at Cardington was recorded on video and by two automatic still cameras. In addition, site diaries were kept by BRE, the contractor and others. To ensure that the information worked on was as reliable as possible a strategy was adopted whereby the base data recorded by the fixers themselves were correlated with the video and still cameras. In detail, the methodology was as follows:

Lorien established a database into which the recorded information would be compiled. The design of this database was developed with a view to the planned analysis, and used inputs from the contractor, designers, etc. in advance of the project commencement to ensure that the identified data was capable of being collected in practice.

From this Lorien established a measurement plan, defining the information required and setting out procedures and principles aimed at eliminating uncontrolled systematic variation in the data. The resulting method statement is included as supplement 1 of Appendix 111.

An experienced industrial engineer from Lorien gave the fixing team training in the measurement procedures.

The measurement procedures were validated by comparing the data recorded on Floor 1 by Lorien’s industrial engineer with that recorded on the almost identical Floor 2 by the fixers themselves.

Data were checked on an ongoing basis as it was captured by loading it into the database, and monitoring for completeness and obvious problems.

A detailed review of all factors which could have affected the results and their interpretation was carried out towards the end of the project.

In order to clarify any anomalies discovered in the analysis phase the data were moderated and corrected, where appropriate, by reference to the photographic and video record.

68

7 Cardington: analysis of construction process data

7.3 Results and analysis This sub-section sets out the results obtained and draws conclusions fiom them. In subsequent parts uncertainty associated with these results is discussed. The data recorded are summarised in Tables 7.1 to 7.7 as follows:

Table Data

7.1 Descriptions of reinforcement systems used

7.2 Reinforcement details and quantities

7.3 Labour and timing summary

7.4 Resources per floor: flexural reinforcement

7.5

7.6

7.7

Costs per floor: flexural reinforcement

Resources per floor: shear reinforcement

Costs per floor: shear reinforcement

For convenience of comparison a convention has been adopted of applying the data pro-rata onto a per floor basis, i.e. the amount of steel, labour etc. which would have been required for a particular reinforcement system for the whole of a floor. The slight errors (- 2%) associated with asymmetry of the building have been ignored in this process.

Camera

I South for Shear I North for Shear

I I I

I

I

I I I I I

I Area Q

I I -

I I I I I

r--- --_-

- - - - - - - - ,

I

Area T I

I

I

I I I 4 I I I I

I

I

I

I

_---_-_-

Area R

4 I i Area U

- - - - - - - -

I I c I I

I

I I I

A B C D

Figure 7.1 Reference work areas with respect to grid lines

69

Rationalisation of flat slab reinforcement . ....

Reinforcement design and detail Firstly, it is useful firstly to revisit the principles underlying the selection of the various reinforcement systems evaluated in this project as outlined in tables 7.1 and 7.2.

Floors 1 and 2 Floors 1 and 2 are essentially identical. They were intended to reflect contemporary traditional construction and to provide an extended area of constant construction detail for physical tests on the structure. However, they also provide some repeat measurements to help with statistical analysis.

Floor 3 Floor 3 was intended to represent a good compromise between minimum steel mass and minimum complexity. The concrete frame contractor was invited to submit his own design to this end. He was required to use shear reinforcement based on shape code 85 links as used on Floors 1 and2.

Floor 4 Floor 4 was intended to give the least complex, most practical details, largely irrespective of weight, i.e. blanket cover of loose bars at a density dictated by the highest calculated design moment. In practice the designer selected a level somewhat below the maximum calculated, giving reinforcement levels for the blanket supplemented with extra steel in areas of higher stress, using his judgement to optimise the result, The stress levels in the floor from grids 3 to 5 were designed using elastic analysis as for Floors 1 and 2. For grids 1 to 3, yield line analysis was used.

Floors 5 and 6 Floors 5 and 6 were constructed using reinforcement mats. Floor 5 was detailed.by ROM in one-way mats' according to their estimate of best compromise between minimum steel and minimum complexity. Floor 6 reverted to blanket cover with conventional (two-way) mats but finite element analysis was used to determine the maximum stress condition, which was then used as on Floor 4.

Shear reinforcement for Floors 4 to 6 was provided by a variety of proprietary and semi- proprietary systems, each used for half a floor.

Where two reinforcement systems were used in the same floor, the flexural system was varied along one axis and the shear along the other - resulting in up to four flexurehhear combinations per floor. This was done to try and reduce the impact on the measurement of interactions between the systems.

Inevitably this collection of reinforcement solutions was developed by a number of designers, each bringing their own ideas to the problem.

The following observations refer to the data presented in Tables 7.1 and 7.2.

The design for Floor 3 uses significantly less steel than that for Floors 1 and 2 (in principle, the minimum weight reinforcement design). This has been largely achieved through a decision to relax the deflection criteria (see note 2 to Table 7.1).

The shear reinforcement detail for Floor 3 was very much more complex than that for Floors 1 and 2, though both were based on the same design principles.

# One way reinforcing mats are an array of parallel bars supported by a minimum number of light crossties. Mats are placed in a layer at right angles. The system is intended to greatly simplify the fixing process compared to loose bars on conventional, two-way, mats.

70


reinforcement 1 Traditional - grids 1 -3(’) loose bar

Happold) Pur0 I 2 -grids3-5

The weight of reinforcement used to provide blanket cover was substantially the same whether derived by elastic analysis (Floor 3) or finite element analysis (Floor 6). Yield line analysis resulted in dramatically lower weights of steel for blanket cover.

Traditiona lloose bar

Happold) (Buro

The differences between the reinforcement details for the different floors arise from both the underlying philosophies described above and variations introduced by different designers. The latter variations were very significant.

Shear reinforcement

SC85 Happold)

Table 7.1 Descriptions of reinforcement

SC 85 (Buro ,Happold)

Floor 11 1 2 Flexural

ystems used 3 1 4 15

Rational@) loose bar (Contractor design)

Blanket

I loosebar I I BRC

SC 85 (Contractor design)

6

Blanket two way mats (finite element analysis)

SC 85 apart fiom sttuct steel heads (A2, B2)

Ancotech Shear studs

Notes to Tables 7.1 to 7.3 Elastic design used except where alternative noted Designed as if “deflection would not unduly affect performance”. (1.58 tonnes extra required for equivalence with other floors). All floors include 0.60 tonnes (129 pieces) of reinforcement for two beams at ends of slab Different ductility steels used on either side of slab Slight (2%) error on split due to symmetry differences Assume 0.5 tonnes per head - total for floor excludes shear heads 20 ACI stirrups prefabricated from 758 pieces Hours adjusted & rationalised slightly to bring to consistent base (see text) Based on 5.5 days worked per week “Skilled = trained steel fixer: “unskilled” = carpenter/labourer/etc assisting (see text) No equivalcnt “start” date for Floor 7

Fixing time and elapsed time Table 7.3 is a summary of the data recorded for the times taken to fix the reinforcement described above. The data presented have been extracted from the original raw information following two stages of moderation.

Anomalies in the data were sought by examining variation between the time taken to fix different floor areas. (Notes on construction and measurement difficulties are presented in Appendix 111, supplement 2.) Some 10 specific days were identified for which the data were considered questionable. These days were re-appraised by comparison with the video’ and photographic record and the database corrected. The corrections were mainly in the nature of re-assigning time spent to different fixing areas, that is correcting simple recording errors.

Smoothing of data between floors was then carried out to, for example, account for the fact that all steel for Floors 5 and 6 was delivered together. The objective of this was to apply a minimum of adjustment to the core information to enable fair comparison between systems. All the original data have been retained and provided in database form with the original report. All recorded time associated with working with the reinforcing steel has been broken down into the processes of:

Unloading

71


0

0 Fixing shear steel 0

0 Checking 0

Movement and stacking on site. Fixing bottom mat (of reinforcement)

Fixing top mat (of reinforcement)

Breaks, standing time and other waste.

The last of these has been characterised as ‘Non-value adding’ and the remainder ‘Value adding’ (VA). The proportion of non-value adding man-hours is reasonably consistent at around 12% for all floors. All subsequent analysis considered VA time only.

It should be noted that the definition of ‘value adding’ used here is not the same as that used in conventional process analysis. Conventionally the only value adding activity is the specific action of fixing the steel in place (i.e. only part of the processes ‘Fixing bottom mat’, etc.) We have adopted a rather wider definition, which may perhaps be characterised as ‘useful work’, as a more appropriate at this stage in the analysis of the fixing process. Reversion to the purer interpretation of value adding may be appropriate when the issues revealed in the current analysis have been addressed.

Examination of the data for flexural steel fixing revealed that variation between top and bottom mat fixing times for the same reinforcement systems were substantially smaller than the time variations between the reinforcement systems. Top and bottom mat-fixing data were therefore combined to simplify subsequent analysis.

Table 7.2 Reinforcement details and quantities

Floor 1 2 3 4 5 6 Totals

16.9 1 l7’I4 (4) 15‘30 (2.4) 1454 ( 5 ) 19.98 25.52 113.59 Flexural reinforcement 1: tonnestfloor (3)

Flexural reinforcement 2: tonnes/floor (3)

23.15 ( 5 )

Flexural reinforcement 1 : pieceslfloor (bars) 2927 3032 2455 2572 960 947 13621

Flexural reinforcement 2: pieceslfloor

(mats) 220 191

3206

Shear reinforcement 1: 0.76 0.76 1.40 0.50 1.00 10.00‘6’ 5.29 tonnestfloor 10.76

Shear reinforcement 2: tonneslfloor

0.91 0.90 0.80

Shear reinforcement 1: 1420 1427 6215 20‘” 62 201 9889 pieceslfloor 1425

Shear reinforcement 2: pieceslfloor 170 116 146

See Notes below Table 7.1

Table 7.3 shows the value adding (VA) man-hours per floor broken down by the activities of fixing flexural steel, fixing shear steel and other site activities. The latter vary to some degree around the average of 13% but not, in our opinion, in any clear pattern, probably reflecting random variation in site conditions.

72


The bottom half of Table 7.3 illustrates the distribution of elapsed man days and time between the floors. A 5 % day week was worked on site and all the data reflects this base (i.e. elapsed times ignore weekends they happen to span).

Table 7.3 Labour and timing summary

Floor Totals 1 2 3 4 5 6

~

Total man hrs 187 203 212 187 157 115 1060

Value adding (VA) man hrs (*) 164 177 186 170 139 102 938

VA man hrs - flexural steel 100 133 108 133 107 69 652

61% 75% 78% 78% 77% 76% 70%

VA man hrs - shear steel 38 30 62 11 12 19 171

23% 17% 33% 6% 8% 18% 18%

VA man hrs -other moving, 27 15 16 27 20 14 119

16% 8% 9% 16% 14% 14% 13% checking etc

Total fixing days (9) 5 5 5 5 4 4 28

Total fixing man days 19 20 22 20 17 11 108

Skilled man days (Io) 15 15 15 14 12 9 79

Unskilled man days ( I o ) 4 6 7 6 5 3 30

27% 38% 48% 43% 38% 29%

Start to start elapsed days (9) 9.5 10 9 12 10

See Notes below Table 7.1

Total fixing days are elapsed times while steel was being fixed into the work and, crudely, can be considered ‘critical path time’. They are consistently five days for loose bar and four days for mats. In fact, many construction processes were operating in parallel and a precise calculation of true critical path time would be extremely complex and not, in our opinion, appropriate in this case.

The number of man days fixing per floor runs broadly with the number of man hours, averaging 8% man hours per man day for Floors 1 to 5 , but showing an intensification of work rate for Floor 6 with the average rising to some 9% man hours per man day.

The work was done by a combination of skilled steel fixers and other trades drawn in as workload demanded. This is normal site practice. The proportion of skilled to unskilled labour varied considerably through the work.

Finally the elapsed working days from floor to floor are estimated. Only the point of starting reinforcement fixing is recorded in our data set and so start-to-start elapsed days are noted. This is relatively consistent at 9 to 10 days per floor except for Floor 4, which was 12 days. This coincides with known site difficulties, discussed in Appendix In, supplement 2

Figure 7.2 and Tables 7.4 to 7.7 show the breakdown of fixing VA man-hours for all the reinforcement arrangements described. They also present the tonnes of steel and number of pieces, or components, employed and ratios of these factors; in all cases these have been compared against the ‘standard’, represented by traditional arrangements in Floors 1 and 2.

73


Time - man hours

Floor6 -4 I t I I I

Floor 5 I Floor 4

Floor 3

Floor 2 t I I I

I

Floor 1 D%l I I

llllllllllllU I I I I I

0 Delivery

0 Site movement

0 Bottom mat

Shear

U Top mat

Checking

0 50 100 150 200 250

Figure 7.2 Value adding man-hours in fixing reinforcement

Table 7.4 Resources per floor: flexural reinforcement

Man minsl Man hrs Tonnes rebar/ Pieces rebar/ Man hrs/ /floor floor floor tonne piece Floors flexural reinforcement

1&2 Traditionalloosebar(Buro 116 100% 17.0 100% 2980 100% 6.8 100% 2.3 100% Happold)

(contractor design) 3 Rationalised loose bar 108 93% 15.3 90% 2455 82% 7.1 103% 2.6 113%

4 Blanket loose bar 127 109% 23.2 136% 3206 108% 5.5 80% 2.4 101%

4 Blanketloosebar(yie1d line 138 119% 14.5 85% 2572 86% 9.5 139% 3.2 137%

5 Desienedone-wavmats“’ 107 92% 19.9 40% 1180 40% 5.4 79% 5.4 232%

analysis)

69 59% 25.5 150% 1138 38% 2.7 39% 3.6 154% 6 Blanket two-way mats (finite element analysis) (I)

Note 1. Pieces per floor = loose bars + mats

Table 7.5 Costs per floor: flexural reinforcement

Floors flexural reinforcement Steel €/tonne to match trad. cost TOTAL VA man hrs Rebar @

@, €21.65/ h r €350/ tonne - 1&2 Traditional loose bar (Buro f2,519 €5,959 €8.478 100% f35O 100%

Happold)

(Contractor design) 3 Rationalised loose bar €2,337 €5,355 €7,691 91% E401 115%

4 Blanket loose bar €2.749 f8.104 €10,853 128% E247 71%

4 Blanket loose bar (yield line €2,987 €5,089 f8,076 95% E378 108% analysis) ( I )

5 Desiened one-wav mats ‘ I ) €2.3 I6 €6.958 f9.274 109% €310 89%

6 Blanket two-way mats (finite € 1,485 €8,932 €10,417 123% €274 78% element analysis) (I)

Note 1. Pieces per floor = loose bars + mats

74

’ 7 Cardington: analysis of construction process data

Table 7.6 Resources per floor: shear reinforcement, comparison with Floors 1 and 2 (pro-rata for whole floors)

Man hrs/ Tonnes rebar/ Pieces rebar Man hrs/ Man floor floor floor tonne minslpiece Floor Shear reinforcement

1&2 SC 85 (Buro Happold) 34 100% 0.8 100% 1424 100% 44.2 100% 1.4 100%

3 SC85(contractordesign) 62 184% 1.4 184% 6215 437% 44.2 100% 0.6 42%

4 ACI stirrups ( I ) 14 42% 0.5 66% 758 53% 27.8 63% 1.1 78%

4 ROM shear ladders 8 24% 0.9 120% 170 12% 8.8 20% 2.8 199%

5 Sauare Griu shear hoous 18 54% 1.0 131% 62 4% 18.0 41% 17.4 1229%

5 Deha stud rails 5 15% 0.9 118% 116 8% 5.6 13% 2.6 185%

59% 10.0 N/A 20 1% 2.0 N/A 60.0 4232% 6 Structural steel shear heads 2o VJ, B2)

6 SC 85 (floor 6 design) 27 80% 0.8 100% 1425 100% 35.5 80% 1.1 80%

6 Ancotech shear studs 12 36% 0.8 105% 146 10% 15.0 34% 4.9 48%

Note I . Time recorded for fixing relates & to off-job, non-critical path prefabrication. Notes say assembled stirrups

installed “in 5 minutes” and no time logged. Per piece timings reflect the prefabrication process.

Table 7.7 Costs per floor: shear reinforcement comparison with Floors 1 and 2 (estimates for whole floors)

Steel f/tonne to match “trad” cost TOTAL Floor Shear reinforcement VA man hrs Rebar @

@ f21.651 hr f350/ tonne

1 &2 SC 85 (Buro Happold) €728 €266 €994 100% €350 100%

3 SC 85 (contractor design) €1,342 €49 1 €1,833 184% -€248 -71%

4 ACI stirruus ‘I) €303 €177 €480 48% €1,370 391%

4 ROM shear ladders €173 €320 €493 50% €899 257%

5 Square Grip shear hoops €390 €350 €740 74% €605 173% ~

5 Deha stud rails €1 10 €315 E425 43% €983 281%

6 Structural steel shear heads €433 €3,500 €3,933 396% €56 16% (A2, B2)

6 SC 85 (Floor 6 design) €584 €266 €850 86% €539 154%

6 Ancotech shear studs €260 €280 €540 54% €918 262%

Note 1 Time recorded for fixing relates only to off-job, non-critical path prefabrication. Notes say assembled stirrups

installed “in 5 minutes” and no time logged. Per piece timings reflect the prefabrication process.

Comparison between systems The following observations may be made:

Fixing times for the flexural reinforcement systems per floor are relatively similar, with the exception of blanket two-way, mats which were some 40% quicker to install than the other systems.

In the case of shear reinforcement, very marked variation in fixing times per floor are evident, with proprietary systems being up to 20 times quicker to install than some traditional forms of shear reinforcement.

75


Only two structural steel shear heads could be tested in this exercise. The 40% improvement over traditional fixing rate can be regarded as only the crudest of estimates, given learning curves for the fixers and site issues.

The fixing time per tonne and per piece for flexural reinforcement suggests that:

0 The rationalised design is not inherently significantly easier to fix than the traditional, and time savings have essentially all arisen from reduced mass of steel.

The blanket loose bar solutions present an anomaly in that the expected quicker fixing rate has been found with the elastic design but not the yield line design. It is possible the problems on Floor 4 have created this result and this is discussed below.

0

f? 6 Two-way mats .- E FE

a, 5 One way mats E

.- a

a, 0

c

)I

c

00 4s Blanket s 0 elastic

g 4N Blanket yield 2

rn

line w

cn c *;i 3 Rational loose .- a, bar (Cont) m -.

1&2 Tradnl. 2

a loose bar (BH) 0 >

+t t

0% 20% 40% 60% 80% 100% 120% 140% 160%

Figure 7.3 Relative fixing rates

The fixing rate per tonne and per piece for blanket two-way mats on Floor 6 (see Table 7.4) reflect the faster fixing rates anticipated for such a system. It is important to note that these data include both the fabric arid all the associated loose bars required for the system.

The data on the one-way mat solution on Floor 5 (Table 7.4) does not show the same advantage over traditional reinforcement (Floors 1 and 2). Notes from site show that some of the individual pieces of fabric in this design were awkward to man-handle and had to be craned into place. This, and other site issues, noted in Appendix ID, supplement 2, probably accounts for this observation.

As may be seen from Figures 7.4 and 7.5, fixing time per tonne and per piece are, particularly for the proprietary systems, less relevant as many of the elements are highly prefabricated. Nonetheless the following points are noteworthy:

As may be seen in Figure 7.4 the shear solution employed on Floor 3 used over four times as many components as the traditional solution on Floors 1 and 2. The shear reinforcement in Floor 3 took almost twice as long per floor to fix, although the fixing rate per tonne was identical.

The data do not clearly highlight the benefits of the ACI stirrup systems. All the time recorded in Table 7.3 is associated with prefabricating the stirrups before installation and in practice was outside the critical path for the floor. The installation of the prefabricated elements took so little time that it was not recorded.

76


It is worth noting that the shape code 85 (SCS5) shear arrangement used to supplement structural steel shear heads on Floor 6 , is virtually identical to that used on Floors 1 and 2, and yet was fixed 20% faster than Floors 1 and 2.

.. .,..... . .., . .. .. ,.. . .. .......... ... ......................... ...... ............................. . . . ... .. ... .. . . ..... .. ............... .. ......... . ... .. ..... .. ................... . ..... ... . ....

Numbers of Bars

Floor 6 b=II OAllowancb for deflgction I t

Floor5

Floor4 1

Floor 3 - + I I Floor2 1 Floor 1

0 2,000 4,000 6,000 8,000 10,000

Figure 7.4 Number of bars per floor

Shear systems: m. h rslcol umn

6E Shear studs

6Wb Struct steel heads

6W SC 85 (Flr 6)

5E Deha stud rails

5W Shearhoops

4E ACI stirrups

4W ROM Shear ladders

3 SC 85 (Cont)

-I I

-I

I 182 SC85(BH) - , I

0.00 1 .oo 2.00 3.00 4.00

Figure 7.5 Shear systems - man-hours per column

Overall cost per floor The final columns of Tables 7.5 and 7.7 combine the costs of time and steel in an attempt to give an overall cost comparison for the systems. The approach adopted has been to take typical, large contract, steel fixing rates at E150 per tonne and equivalent price for reinforcing steel (cut, bent and delivered) at €350. These figures, in our opinion, reflect market rates over the last few years and the analysis is demonstrated to be relatively insensitive to these chosen figures.

From the overal! data on the building the tonnage of reinforcement used and total hours of actual fixing time have been determined. The cost per hour for the fixing process itself of

77


€21.65 has been calculated from €150 per tonne as the fixing rate. On this basis a cost for labour and reinforcement per system per floor has been derived and compared with the "standard". As an alternative way of looking at the same data, a cost per tonne for the reinforcement has been derived which would give an identical cost per floor'to the standard solution. This gives an indication of the premium derived from design input or from proprietary pre-fabrications (particularly in respect of shear systems).

The results of the per floor cost analysis are presented graphically in Figures 7.6 to 7.8. Figure 7.6 is the data as presented. Figures 7.7 and 7.8 show the effect of altering the assumptions on fixing price per tonne and reinforcement cost per tonne up and down 10% from the starting point.

I

Cost per floor

0 Labour

"Traditional" loose bar "Rational" loose bar

"Blanket cover loose bar - elastic Blanket cover loose bar - yield line

Designed one-way mats Blanket two-way mats

SC 85 (Buro Happ.) uI

~

SC 85 (contractor design)

ACI stirrups

ROM shear ladders Square.Grip Shear hoops

DEHA stud rails

Shear heads SC 85 (Floor 6.) e

AncoTech Shear studs

E ; g 8 0

$ 2 -

- 0 0 0

8

Steel

- 0 0

w 2

- 0 0

2 5

Figure 7.6 Costs per floor

78


Cost per floor Rg3- lowsteel. h@labov

Traditional loose bar

"Rational" loose bar

Blnkt loose -elastic

Blnkt loose - yeld line

Designed 1 wayiriats

Blanket 1 waymats

SC 85 (Buro Happ.)

SC 85 (Contractor)

ACI stirrups

Rom Shear ladders

Squ Gr Shear hoops

Deha Stud rails

Shear heads

SC 85 (Floor@

hcotch Shear studs

0 Labour Steel I

Figure 7.7 Cost per floor - low cost of steel high cost of labour

Cost per floor Fig 2 low labour: high steel

TradibOMl bose bar

'Rallonar bose bar

Bhkt bose . elasbc

~ ln~bote-ye~bne Designd 1 waymats

Btanket 1 waymah

, sc 85 ( ~ u m Happ ) sc 8s (comcmr)

A C l s b m ~ s

Rom Shear Ladders

Squm Shear hoops

Deha Shd rails

Shear heads

SC85(FborG) Ancotch Shear shds

10 Labour Steel I I I

Figure 7.8 Cost per floor - high cost of steel, low cost of labour . .

79


The following observations may be made:

Of the flexural reinforcement systems the extra steel required to realise the elastic blanket reinforcement designs has resulted in un-competitive solutions. Yield line blanket designs, despite the very high (and probably anomalous) fixing man-hours per tonne, are competitive and the rationalised design is the most competitive - albeit this derives entirely from steel savings, themselves derived from a decision to relax deflection criteria. (But see Chapter 8.4, under Labour and material costs, where this deflection reinforcement is considered.)

The shear reinforcement systems show considerably more variation in man-hour requirements. Proprietary systems show savings of about 50% over the traditional systems and potential margins justify a 100% or more premium over traditional reinforcement. Interestingly, the ACI stirrups, which are essentially standard reinforcement but to a sophisticated design, are in the same cost saving bracket and amongst the most competitive solutions. This is before tahng account of the factthat the great majority of the time associated with stirrups was not on the critical path.

The more complex shear solution selected for the rationalised design on Floor 3 resulted in cost penalties which effectively eliminated the gains made in fixing the flexural steel.

The data on shear heads in the tables and charts are not illuminating but are presented for completeness.

Figures 7.7 and 7.8 illustrate that, over a reasonably wide range of steeVlabour price ratios, the ranlung in respect of economy for the different solutions remains unchanged. In fact it requires a factor of 30% distortion of costs on the base assumption to have any effect on their rank order. They also illustrate clearly the relative proportions of the costs of shear and flexural reinforcement in this data set. It is conceivable that a more sophisticated analysis, weighted for critical path time extension, would amplify the relative importance of shear reinforcement but this is beyond the practical limitations of this exercise'.

Summary In summary we may conclude that, on the basis of the data recorded, there are arrangements of reinforcement that offer cost advantages over the traditional systems used. These advantages are robust over a range of 1abour:reinforcement cost relationships. This is particularly true for shear reinforcement where proprietary systems and the ACI shear stirrup design were typically half the cost of the traditional system.

No account has been taken of the potential additional benefit of shortening the construction critical path time but a relatively sophisticated analysis of this sort is inappropriate using the existing data set which have been strongly affected by a number of factors. These factors also reduce the confidence that can be placed on the previous paragraph, and are discussed in more detail in the following sub-section.

This theme is taken up in Chapter 8.4 under Integration of results #

80


7.4 Variability and uncertainty This section discusses the nature of uncertainty, examines the circumstances of the Cardington project and its associated uncertainties and concludes by offering opinion on the confidence that may be applied to the conclusions drawn in the previous section.

Nature of uncertainty Any measurement of a parameter may be considered to be a sample value for that parameter. Taking one or a number of measures of the same parameter can be described as an experiment to determine its value. In principle the measurement obtained may be considered to be affected by both systematic and random variations. A systematic variation is one that affects all the values obtained in an experiment. A random variation introduces a random error on all measures made within an experiment. Thus if the experiment were the measurement of the length of an object, a systematic variation would be, for example, an error in the calibration of the ruler. The random variation would be operator error in reading the value (though not error associated with a wrong measuring procedure, which would produce a systematic variation one way or another).

In general, random errors are amenable to analysis and estimate, the larger the sample size the more precisely the parameter may be estimated. Systematic errors cannot be analysed or estimated from a data set. They can be ‘corrected’ for by measurement or estimate but generally experimental design should aim to minimise or eliminate them. Alternatively the scope of the experiment can be extended to encompass a large sample of the factors creating systematic variation, essentially consigning them to random variables. Thus if a population of rulers was known to have randomly distributed calibration errors, using one in a measurement experiment would introduce systematic variation of unknown value. If a large number of rulers were used the error introduced would be randomised and amenable to analysis.

Sources of uncertainty The key purpose of the Cardington project was to measure parameters associated with concrete construction in an experiment in which as many as possible of the sources of uncertainty associated with the construction process were controlled.

The problem with this has always been that the Cardington experiment is very costly and there has been a need to gain as much information as possible simultaneously on a number of aspects of concrete construction. There is additional complication in the need to enable the commercial organisations who have contributed to the funding of the project to test or demonstrate aspects of their products and, from an industry-wide viewpoint, to demonstrate the capabilities of reinforced concrete construction.

It was extremely difficult for the project’s organisers to balance all these needs in ‘order to obtain the best value for all parties concerned.

In respect of the measurement of reinforcing fixing times the following aspects were not considered ideal.

The spans and floor thicknesses selected are on the boundaries of what is currently usual to demonstrate concrete construction capabilities. This resulted in relatively high design stresses, resulting in high steel usage for blanket cover designs and problems designing convenient fabric reinforcement (a number of different sheet types were required, some rather large for convenient handling).

There were commercial and practical pressures to incorporate a large number of reinforcement arrangements, particularly in respect of shear. This resulted in the use of

81


more than one reinforcement system per floor. Apart from the obvious measurement problems, the effect of this, was that the fixers were constantly learning new arrangements so their fixing times will have been strongly affected by factors such as clarity of drawings, previous familiarity etc. etc. Other work has demonstrated at least a doubling of fixing times or rates while arrangements are learned. Ideally, the systems would have been compared at their optimum, rather than starting, rates of fixing.

Other important experiments overlapped with the fixing experiment. For example, strain gauges were fitted to the reinforcement on Floors 2 and 3; different ductility reinforcement was used on each’ side of these floors; a stop end was due to be fixed down the middle only of Floor 3.

Systematic variation The above factors will all have introduced systematic variation to some degree.

In an attempt to prevent further systematic variables being introduced into the experiment, time was spent before construction started on discussions with site personnel and RCC, culminating in a published measurement process method statement (Appendix III, supplement 1).

In practice, construction ran like any typical site with breakdowns of key equipment, late deliveries and non-availability of components leading to ad hoc re-ordering of processes (shear heads were planned to be used on Floor 4, ultimately only two were used and these were incorporated into Floor 6 )

In addition to the errors introduced by these difficulties, priority was given to completing the project inside the planned time frame, and as a result none of the precautions detailed in the method statement were taken to avoid introducing systematic errors. Specific examples are listed below.

Different mixes of skilled and unskilled operators were used on different systems.

Completion priorities resulted in variable work rate (e.g. shear steel was fixed 20% faster on Floor 6 than on Floors 1 and 2).

None of the unfamiliar systems were trial assembled off works. This was a particular issue with BRC shear hoops used in combination with one-way mats, neither of which were familiar to the fixing team. Significant difficulties were encountered which may or may not have been resolved with practice.

A summary of the problems encountered per floor is included in supplement 2 of Appendix III.

The foregoing is not intended as a criticism of the individuals involved in the construction process who were, in our opinion, singularly conscientious in trying to deliver the outputs sought. It is rather a criticism of the construction industry as a whole in that all the problems and solutions encountered were typical of general site practice.

It could be argued that the results obtained under these circumstances are more realistic. Unfortunately, the uncontrolled systematic errors will have affected the results from relatively small samples of each reinforcement system to different, essentially unknowable, degrees. We have considered attempting to estimate ‘corrections’ for the various effects but the magnitude of these adjustments rapidly exceeds the differences actually measured, and would result ultimately in a data set more reflecting opinion than measurement.

82


Random variation In addition to the systematic variation discussed above there will be elements of random variation in the data set. The very small sample sizes make a precise measure of this very difficult. To get an estimate of the random variation we have established a measurement regime for the flexural steel in which each floor was divided into six areas and the time taken to fix each area was measured separately. An estimate of the random variation for the fixing and data recording process has been made by determining the standard deviation of the time . recorded for all areas of top and bottom mat on Floors 1 and 2 (the largest sample of a single system available). The data have been inspected for trends which correlated with physical differences such as the asymmetry of the floor layout, differences between top and bottom mats, etc. but none was evident. Even when the data had been ratioed to align the small difference between the means per floor (i.e. assuming differences between means arise entirely from systematic variables) and one high and one low outlying point have been discarded, a variance (Standard Deviatiodmean) of 38% resulted. On this basis, a sample size of 40 would be required to detect a 10% difference (typical of those found) between systems at a 90% confidence level

This estimate of random variability includes errors of misattribution of time from area to area. Given the small sizes involved and the relatively imprecise boundaries this may well be a significant contributory factor. Nevertheless, this analysis gives a feel for the sample size required to enable differences of the order measured to be reported with a degree of statistically based confidence.

Implication of variables on results It is clear from the foregoing that almost all the analysis given must be prefaced with the word ‘probably’. It is certainly quite inappropriate to draw any more detailed conclusions from the Cardington data set alone.

Broadly speaking the trends seen in the data align with what may have been expected from an educated guess at the outcome, with one key exception - this is the results for the yield line, blanket cover on Floor 4. The fixing rate seems high by a factor of at least two. Examination of the data reveals no obvious source of this anomaly, other than the fact that Floor 4 was particularly beset with construction process problems.

If the yield line blanket reinforcement could have been produced in fabric and been fixed at the same rate as Floor 6 it would have produced a saving of some 30% over the traditional reinforcement. This is easily the best opportunity identifiable by applying reasonable ‘what i f analysis to the data. The observation is, of course, speculation but we consider that it merits further investigation.

It has been stated previously that all the sources of uncontrolled variation are typical of normal site operations. This work illustrates the impact of such variations on the timing and installed costs of reinforcement systems. Add this to the difficulty of actually measuring the differences and it is clear that feedback of site experience into the design process will tend to be, at best, anecdotal and hard to quantify. This may go some long way to explaining the wide variation in opinion on optimum design approaches, illustrated in no small measure by the interpretations manifested in the designs for this project.

83


7.5 Conclusions and recommendations From its inception the inevitable compromises inherent in this project were always going to lead to less than ideal data. Unfortunately, problems during the construction phase have reduced further the reliability of the available information.

The overall layout chosen for the structure was not appropriate for some of the systems investigated and it is unlikely that any of the designs was truly ‘optimised’ to minimise the combined material and labour cost. Virtually all systems were measured with the fixing team at the beginning of their learning curve, which is less informative than when measured later at ‘steady state’.

Nevertheless, the data obtained is better than any previously available to the industry and is adequate to guide further activity.

This work demonstrated that different reinforcement arrangements can have significant impact on overall costs - in the systems investigated up to 30% on flexural reinforcement and 50% on shear reinforcement This excludes any benefit from reduced critical path time whic in principle, should be of the same magnitude.

There is a considerable opportunity for cost reduction. This would require a combination of informed design, system-specific expertise and, particularly, process control and supply chain integration to eliminate delay and wasted time. All these could be achieved through adopting good practice, a technique widely followed in manufacturing industry.

The information included in this report provides pointers towards design approaches aimed at reducing costs and a methodology for measuring their costs. More data are required in order to determine the optimum design for different structural arrangements. Given the demonstrated variability of design input and site practice, a practical way to obtain these data would be to organise an industry-wide data gatheringhenchmarking exercise through which the systematic variables that have affected this experiment could be randomised. A broad outline of how this might be achieved is included in supplement 3 to Appendix 111.

84

8 Discussion

8.1 Introduction The key question in this project is "Where is the balance between rationalisation and additional cost , i.e. between extra weight of reinforcement and savings in time?" The theoretical relationship between cost and rationalisation is shown in Figure 8.1. The real answer is shrouded in many uncertainties but the information gleaned from the background studies, literature searches and the research and data from the project itself can at least provide the basis for an answer.

High

Low

plus Cost d .*- time, finance, .......

etc. etc .......... ............................. Cost d

............. ................ reinforcernert


Figure 8.1 The relationship between rationalisation of reinforcement and minimum cost

The intention in this chapter is to bring together the findings of the research and look at. costs and consider who are the winners and losers from using the different configurations of reinforcement and what should be considered best practice.

85


8.2 Background studies Optimisation I

It was shown that optimising slab thickness saves money. Reducing a 300 mm slab to 255 mm on the Cost model buildings saved 8% of frame costs. In practice this potential optimisation is often set against reducing risk in design - thicker slabs can absorb late changes and they deflect less than thinner slabs. Thus the opportunity is often lost.

Time costs For developers aild owner/occupiers, early occupation brings early and extra revenue or rental income. However, this may be secondary to the core business of predicting and managing risks and opportunities. For contractors and specialist subcontractors early completion should mean less time related overheads. However, overheads are normally costed as a single percentage addition, typically 10%. This is insensitive to the particular resource usage and thus does not reflect the often quite small changes in design that achieve production efficiencies.

Perhaps the easiest way of determining the balance between extra weight of reinforcement and savings in time is in terms of cost. But, as the University of Reading found, costs mean different things to different members of the construction team. In an early piece of research the University of Reading investigated this issue and produced a report called l7ze cost of time (3).

For the contractor, time savings result in cost savings in preliminaries and labour. For the client the time savings may well result in finance savings, which are an order of magnitude greater than those for the contractor. Time savings reduce the length of time between capital' outlay and the return on investment. This may be a vast oversimplification for individual projects (e.g. it ignores opportunity costs), but for the market as a whole, these differences should be appreciated.

The priorities of the contractor and the client inevitably differ. The following table aims to quantify the basis of the cost of time for each party to a contract.

Table 8.1 The cost of time to contractors and clients

Contractors Clients

Trade Main Speculative Owner1 occupier developer (devefoper*prelet)

Timescale of involvement Part of T2 T2

Construction costs:

=QXIO x l o x c c = ~ T , X Y ~ X C ~ n/a' T2 100x100 T2- 100 Preliminaries

n/a'

Labour 06T2.nos.rate n/a' n/a' n/a'

Plant x6T2.nos.rate n/a' n/a' n/a'

Rla terials Prime cost n/a' n/a' n/a'

Lost rentallincome No =6T2xrate

Finance costs Yes(2) Yes(2'

Opportunity costs , Perhaps Perhaps

86

8 Discussion

Key . 1.

2.

Not applicable. Assuming traditional form of contract and cause of delay down to contractor - otherwise may be charged at over-headed rate over cost. Finance costs are often approximated to = Construction c o d 2 x duration (months) x interest rate/] 2 but have been defined more accurately by the University of Reading as being:

where 1 TI

Tz = Construction time T3 CL CO = Cost of demolition, c c

= Interest rate is per unit of time = Acquisition time - the time from acquisition of the land (waiting for planning permission and design

plans) until construction starts.

= Disposal time - the time from the end of construction until the building is let or sold. = Cost of land, (including acquisition costs, compensation, fees)

= Costs of construction. (including contract value, ancillary costs etc)

Barriers to rationalisation and prefabrication The following section discusses the points made in a series of 12 interviews with members of the concrete industry by the University of Reading in the course of their work for The Concrete Society's Rationalisation of reinforcement project(2* 24* 25). The interviewees were from four engineers, five subcontractors, two fabricators and a steel mill.

Least cost It is very apparent that least cost drives decisions. Contractors buy at lowest price and clients relate minimum cost to minimum material content. Cost is regarded as being more important than time.

The problem for the concrete frame and reinforcement industries is how to quantify the benefits of rationalisation. It may seem that overall costs can be reduced but the costs of physically measurable materials are often greater. With fabric, labour savings of 60% to 70% can be achieved, output is doubled but weight increases. The most economic structures make allowances for time benefits, reduced preliminaries, additional commissioning time etc. But how are these benefits costed and allocated?

Apparent benefits must be tempered with practical realities. Innovation appears to be driven by developers and the needs of industry rather than by concrete contractors. Under traditional contracts, there is little incentive for engineers to increase overall efficiency by innovating or allowing innovations to be used. Innovations can be seen as risk. Unless one party is prepared to be a champion and shoulder risk then innovations are not used. There is little encouragement from contractors for prefabrication of reinforcement, presumably because traditional methods are a known quantity and can be managed more easily. Often, lower prices are tendered where the contractor is not to be too rushed on site.

The pricing of reinforcement The pricing of reinforcement appears to vary between contractors. The prices quoted by contractors in the series of interviews for the Concrete Society (see Appendix II) are summarised in Table 8.2. Sometimes price is based on weight alone and sometimes it is both weight and size. Little account of ease of fixing seems to be taken. Most fixers appear to be paid day rates based on bar diameters (although some contractors pay per tonne without regard to diameter). Labour rates tend to vary inversely to bar diameter. For the sake of optimisation and rationalisation some consistency over pricing should be established.

Reinforcement is a world commodity, seen as low-tech with little added value, often sold as a loss leader for other, more highly valued products. The market for loose bar is very competitive. Neither standard fabric nor special fabrics are seen as being competitive on price.

87


Table 8.2 Reinforcement prices 1998

Item Average

Bar (6 to 40 mm dia) Supply 52401tonne' Labour costs tonne^, 3,

Tie wire, spacers and chairs etc E451tonne5 Total E525ltonne

Mats BS fabric E325ltonne Bespoke fabric 53 87ltonne Labour 50.6 5/m2*

Notes 1 Dependant on market conditions 2 3 4 5 G

€275/tonne to f70/tonne. Some contractors lump all together, some go by diameter Flat slabs 10% less. Troughs 5% more. Waffles 15% more Fixing rates 20 - 32 mm bar 2.0 tonnes/day; 8 - 12 mm bar 0.5 tonnes/day Tie wire, spacers and chairs €8/tonne, wastage 2%, O/H & P 5%. For A142, A252 and A 393 mesh: equivalent to f293/tonne, €165/tonne and €106/tonne

Weight of reinforcement When estimating a price, contractors may or may not want a breakdown of sizes and links at tender stage. They certainly want to know total weight.

Usually the weights are only finalised at the end of detailed design some .time later. Traditionally final production information i.e. bending schedules, were available at tender stage: this is no longer the case. Nowadays, because the final design is not fixed, engineers will generally 'guestimate' bar weights and diameters based on experience. Quantities are based on these estimates and most usually the quantities are subject to a re-measure. Inevitably re-measurement can become antagonistic and either some equitable method of paying for reinforcement has to be found or someone has to take an unwanted risk.

Construct's Guide to contractor detailing(39) may point the way but this uncertainty does not help the initial scheme and budget phases of the project. The consequent risk may be sufficient for the decision to be taken for a project to be constructed in other materials.

This problem has been the bane of much of the concrete frame market. Without the final design, accurate estimates are time-consuming and difficult. The risk might be passed on to the contractor but the contractor might baulk at having to work up his own quantities for each tender. If he is taking risk, should he not have some control over the design? How can this situation be overcome? Indicative RC drawings or the advent of reinforciment contours from contemporary finite element design packages help. Very simple designs e.g. yield line design help reduce risk. Very quick design and detailing processes e.g. Hy-ten's Bamtec system (where a finite element design package outputs information suitable for the manufacture of reinforcement carpets) indicate how integration of software can help.

Figure 8.2 shows the relationship between complexity and weight. The bottom part of the figure is an alternative x axis that seeks to illustrate that a typical design will firstly look at a highly rationalised model and gradually refine it by doing more analysis and design. There comes a point where further work will either make the reinforcement highly detailed or increasingly rationalised. Increasingly rationalised usually means increasingly heavy.

88

8 Discussion

High

Low Prefabricate b

Manufactured mekhes ~~ ~ -


Figure 8.2 Rationalisation (and design duration) against weight and complexity

Weight and prefabrication Prefabrication increases weight. For one-way slabs this increase is usually 5% to 7%. The weight of reinforcement in the flat slabs at Cardington that were subject to elastic design, increased from 17.0 tonnes to 23.2 tonnes (i.e. 36%) for one-way mats and from 17.0 tonnes to 19.9 tonnes (i.e. 17%) for two-way mats.

The experience of one engineer interviewed in reference 24 was that changing from flat slab reinforcement to mats resulted in a weight increase from 58 kg/m2 to 65 kg/m2 i.e. 12%. For elastic design the increase appears to be case specific. Indeed the providers of the mats at Cardington were against their use as they knew the additional weight would be excessive: they were persuaded to provide the fabrics for the sake of the research. This lack of certainty about increases in weight compares unfavourably with the certainty of weights of loose bar schemes. There is a lack of confidence in price of fabrics and therefore a reluctance for contractors to price accurately.

'

Mats Rationalisation using mats would appear to have restricted application for the following reasons:

Prefabricated mats are seen as needing repetition and a regular grid and therefore they appear to have limited applicability to flat slabs, trough slabs and one-way slabs. For rationalised methods there need to be standard types of fabric. The many awkward sites with lack of repetition appear unsuited to fabric. Current standard meshes are unsuited to the generally high levels of reinforcement in many suspended slabs. Large (25 mm diameter) bars are heavy and generally regarded as being unsuited for use in fabric, certainly on most building structures. There is a need for bespoke fabric but its availability is subject to minimum quantities, lead-in times and market shortages. Loose bar is considered easy to adapt. On site, fabric is seen as not being adaptable. There can be problems integrating loose bar with fabrics.

0

89

. Rationalisation of flat slab reinforcement

Germany is often quoted as being a large user of mats, but from a UK perspective Germany tends to have regular grids, repetition, and uses thicker slabs with smaller bars. Different cultures and customs, rules and regulations etc. mean transferring methods can be difficult.

Table 8.3 Preferred sizes of tailored mats in UK

Item Minimum Maximum

Width 1200 mm 3400 mm (some machines 3000 mm

~

Length 2000 mm 12000 mm

Bar sizes (plain or deformed wire and hot rolled bar

4 mm 16 mm (if double)

Spacing *

50 mm - < 12mm 75 mm > 12mm

preferred min. spacing 10 mm preferred min. increments So mm

Weight of sheet 300 kg (1 50 kg if nested)

At the detailing stage consideration needs to be given to the type of lap for a prefabricated sheet (i.e. layered, edge, reversed, nested or flying end)

Form of contract As illustrated by Figure 8.3, specialist trade contractors, sub-contractors and engineers have different experience of the different forms of contract and methods of building procurement. This may affect their attitude towards rationalisation and prefabrication. Traditional (or JCT) contracts are price driven but it appears easier to introduce innovation into Construction Management (CM) contracts where arguments about overall gains can be won and the construction manager becomes a champion, (Similarly, the arguments can be won in Management Contracting (MC) and Design and Build (D&B) contracts.) However, JCT contracts are considered cheaper than CM contracts. Construction Management contracts may be quicker but are regarded as being 5% to 7% more expensive.

I I

I 1 HJCT I D & B O C M & M C I I I I I I I 1 I Trade

contractors ave.

Subcontractors ave.

1 Engineers ave. I

0% 20% 40% 60% 80% 100% i Figure 8.3 Contracts experience of Concrete Society interviewees (See Appendix 11)

90

8 Discussion

Adni n is t rtt i on arran gemn ts 17% right

28%

22% Analysis and engineering

33%

Figure 8.4 Engineers' design costs

However, detailing for prefabrication on slabs does take time and there is little or no advantage for the detailer or designer to undertake it. Few engineers and detailers have experience of mats. Under traditional contracts, engineers are unlikely to detail for mats as the contractor has t o be in charge of construction methods used and is unlikely to be on board when detailing is started. Engineers are even less likely to be willing to re-detail: re-detailing has to be part of the prefabrication costs or prefabrication has to be part of the initial detailing process. If this is the case then the contractor has to have some influence over design - unlikely under traditional forms of contract.

Reluctance to change Commonly rationalisation is a separate process that is undertaken after design and before detailing, or even after the start of detailing. It is thus regarded as additional (and unpaidunrewarded) work. It is possible to adopt highly rationalised methods from the start of the project but this generally requires the judgement of a highly experienced engineer - one who can justify the additional reinforcement against simplicity and time savings. There may be a reluctance from the design team to adopt highly rationalised methods because of the assumed extra cost of reinforcement. From an engineering point of view, this assumption is based on the use of elastic theory of analysis whereas other methods such as finite element and yield line give rationalised layouts, and in the case of yield line lead to less reinforcement being used.

There is a natural conservatism and reluctance to change methods: Designers become familiar with certain methods of design Labour is familiar with traditional loose bar There may be opposition from draughtsmen and drawing office staff protecting their trade There are contractors who believe that prefabrication of reinforcement for slabs on large projects is neither practical nor economic.

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8.3 Cardington The presumption behind the various arrangements of reinforcement at Cardington was that rationalisation and prefabrication would lead to greater speed and simplicity on site. And so it proved. This is perhaps at the expense of more reinforcement being used than in the traditionally detailed floors but the purpose of the exercise was to investigate the balance between material costs and time.

Figure 8.5 illustrates the man-hours spent on the different operations in fixing reinforcement the different floors. A comparison of the efficiency of fixing different types of bending reinforcement is made in Figure 8.6 with traditional loose bar taken as the benchmark (100%). Bands showing 90% confidence limits are included to give a feel for the accuracy and consistency of the data.

Delivery Site movement

0 Bottom mat H Shear 0 TOP mat

Time - man hours

Floor6

Floor 5

Floor 4

Floor 3

' I 1 t I" I 4 1 Floor 1

I

0 50 100 150 200 250

Figure 8.5 Cardington - value adding man hours spent on reinforcing each floor

3 6 Two-way mats E - FE - 8

5 One way mats

E 4s Blanket

i 4N Blanket yield 3 line m .- 3 Rational loose

eh L 9 Q: loose bar (BH)

c -91

elastic h

E

bar (Cont)

182 Tradnl.

F

I

-tt-+-t

0% 20% 40% 60% 80% 100% 120% 140% 160%

Figure 8.6 Relative 'fixing efficiency'

Despite all the preparation and care it has to be said that the data were based on a limited sample and were subject to interference from extraneous events (in other words, the data were specific to Cardington and imperfect). Many construction processes were studied at Cardington so inevitably there were compromises in the work on rationalisation of reinforcement. Nevertheless the results show significant differences in the weight of steel and

92

8 Discussion

the man-hours in fixing steel for the various configurations of reinforcement in the six floors studied.

The fixing rate measurements shown in Figure 8.6 indicates that by switching from traditional methods to two-way prefabricated mats and proprietary punching shear reinforcement systems, a 50% saving in man-hours can be achieved. Proprietary punching shear reinforcement systems are between three and ten times quicker to fix per column than traditional links.

However, the savings in man-hours need to be offset against additional materials costs and differences in process (e.g. obtaining proprietary shear systems which generally need to be designed by the specialist supplier against traditionally designed and procured shape code 85s). Also, savings in man-hours may not translate into useful project time savings. Discussions with two leading specialist contractors suggest that savings in cycle time due to improvements in reinforcement placing would be a maximum of two days per floor.

Using traditional loose bar arrangements and links, the floor plate at Cardington required 150 man-hours per floor. Using two-way mats and proprietary shear systems required only 77 man-hours (averaging the quickest three systems). The results are generally in accordance with trials undertaken in Germany in the early 1 9 8 0 ~ ~ which showed how the reduction in fixing time for fabric or other prefabricated units improves as the weight of reinforcement increases('5' 18).

The saving in man hours on site have to be balanced against cost of different design methods, e.g. finite element analysis or yield line design, andor more reinforcement (up to 50% more) and whether any savings were significant in terms of overall critical path. Major improvements in fixing reinforcement and critical path time on site can not be obtained simply through rationalising at the detailing stage: rationalisation must embrace the whole process in order to get worthwhile benefits on site.

Performance Changes in reinforcement also (theoretically) affect ultimate performance and serviceability. .All arrangements used on the project complied with the relevant design code (EC2 ENV 1992(14' - which refers back to BS 8110'32' in no small measure). but there were some differences.

U1 tima te performance According to the study by Nottingham Trent University(') the slabs all slabs had a factor of safety against failure at ultimate loads of at least 1.15. As might be expected the blanket cover and two-way mat arrangements had even greater factors of safety.

These conclusions are based on the use of yield line theory which deals with two dimensional structures where plastic yielding takes place along a fracture line (called a yield line). When all the yield lines necessary to initiate a mechanism have formed, that portion of the slab fails in the ultimate condition. Whilst not in common usage, yield line theory is well accepted and its use to assess and compare factors of safety appears to be perfectly valid.

The amounts of reinforcement in the blanket cover arrangements on Floors 4, 5 and 6 were determined by the designer considering a level of reinforcement that would, with the addition of local supplementary bars, be sufficient for all areas of the slab according to elastic orfinite element analysis. Both types of analyses suggest concentrations of stresses, particularly over columns, which appears to be an accurate reflection of the serviceability load case.

93


Elastic analysis always has to be used for the analysis at serviceability limit state. It is only at the ultimate limit state that other methods of analysis or moment distribution can be used'37). The fundamental reason why one cannot get full advantage from using non-linear or plastic analysis is that, in reality, the serviceability criteria are critical. This is evidenced by the need in the yield line design used in Floor 4 for additional reinforcement to satisfy span to depth:ratios (deflection) and concentration of support steel to within the column strip (cracking).

Yield line design gave the simplest flexural reinforcement arrangements. The finite element arrangement was simple but, to accommodate sensible maximum moments, the general level of reinforcement was high. The amounts of reinforcement used were actually dictated by considerations of serviceability, and it is perhaps with regard to this aspect that rationalisation of design needs to be investigated.

Serviceability Deflections Dr Vollum notes that ". . . . the beneficial effect of providing additional reinforcement was swamped by variations in concrete tensile strength due to variations in concrete compressive strength" and "there appears to be little benefit in increasing reinforcement area to control slab deflections". Although based on incomplete research and also being at odds with the philosophy of BS 81 lO"*', it would appear that the different arrangements of reinforcement had little effect on deflections. It would also appear that the rules on deflection in BS 8110 may need to be looked at in light of current knowledge'.

Cracking From the time of construction of the building through to 2000, hormal' cracking was observed on the top surface of almost every slab at its intersection with an internal column. This is consistent with expectations, taking into account the level of loading applied to the slab. It is within acceptable limits, and is independent of the method of reinforcing the slab.

More extensive cracking may be observed on Floor 4, grid 3, where the design philosophy changed from elastic to yield line. Here, single cracks extend into the yield line area towards grid 2 (to the half away from the hanger doors). The extent of this cracking is explained by the fact that the designed top reinforcement in the column strip on the yield line side of grid 3 was, for various reasons, omitted. The extent of these cracks is therefore not surprising and is not connected with yield line design. Whilst these cracks are unsightly and are over 1 mm in width in places, there is no evidence to suggest that they have impaired the performance of the slab. It is worth adding that the whole of Floor 4, including the half designed to yield line theory, appears to be performing satisfactorily - and deflecting less than some of the other slabs.

It is worth noting that the amount of top reinforcement used in middle strip supports subject to yield line design was low. At his discretion, the designer called for:

A142T mesh over all internal supports with supplementary T16@150bwT (both ways top) in column strips over supports i.e. 142 mm2 /m in middle strips: (142+1340) mm2 /m in column strips over supports, i.e. 9% : 91%

ti BCA's DETR Pi1 project, The influence of serviceability on the economic design of concrete stiwctiires, is indeed examining this subject. A final report is due by 2002

94

8 Discussion

BS 8110‘32’ C1 3.7.3.1 stipulates that the division of negative (support) moments between middle strips and column strips should be 25% : 75%. This more conventional distribution of reinforcement would have suggested a more even distribution of reinforcement, i.e. 0 T12@250bwT (452 mm2/m) over all internal supports with supplementary

T16@25ObwT (804 mm2/m) in column strips over supports i.e. 452: (452 + 804) or 26% : 74% 0

The relatively low amount of reinforcement in middle strip supports does not appear to have affected the amount of cracking compared with that in other slabs.

Summary From a performance point of view, reassurance may be taken from Dr Johnson’s work determining that all slabs are satisfactory in terms of factor of safety against failure in bending. Dr Vollum’s measurements of deflection show that at up to 700 days after construction, deflection is within acceptable limits. Further deflection is probably predictable and deflection appears unrelated to amount of reinforcement used (but more probably related to concrete tensile strength at first loading). Cracking can be seen at several locations in the building, particularly around columns. These cracks are not abnormally large. Larger cracks on Floor 4 adjacent to grid 3 are the results of omitted reinforcement.

It should be noted that Dr Vollum’s and other research work at Cardington continues (as at August 2000) and there is more planned research that has yet to obtain funding. The results of future research into ultimate performance (e.g. punching shear, fire, explosion) and serviceability (e.g. deflection, cracking, shrinkage, durability, etc), may demand that the findings of this report must be reviewed at some future date. However, for the present it can be stated that all arrangements of reinforcement performed satisfactorily.

Comparison of systems Equivalent costs per tonne One way at looking at the various reinforcement configurations is to look at costs per tonne that would give an identical cost per floor to the standard or ‘traditional’ solution. As noted by Lorien plc, the figures in Tables 8.4 and 8.5 give guidance on possible premiums for the various systems on a tonne-for-tonne basis just taking material and site labour costs into account. In effect they may be considered as being a measure of how the construction cost of each system compares to the standard solution. Thus, on a measure of material and fixing costs, blanket loose flexural reinforcement based on yield line design with ACI shear stirrups would appear to be most economic or best practice.

The relative economy of the systems is indicated by the final column of Tables 8.4 and 8.5. The economic ranking of the systems is robust over a relatively wide range of labour and material costs. Even using the figures indicated by the Concrete Society interviewees (see Table 8.2), where reinforcement is priced as low as €240/tonne, ‘rationalised’ loose bar and blanket cover yield line still come out on top for flexural reinforcement, as do ACI stirrups, ladders and studrail for shear reinforcement.

However time costs are more than just fixers’ costs, and the costs of preliminaries and financing should be included.

95


Table 8.4 Bending reinforcement cost equivalence

Floor Configuration Steel &/tonne so system matches the . cost of "traditional" loose bar reinforcement

1&2 Traditional loose bar €350 100%

3 "Rationalised" loose bar (contractor design) €361 * 103%*

4 Blanket loose bar €247 71%

4 Blanket loose bar (yield line analysis) €378 108%

5 Designed one way mats €310 89%

78% 6 Blanket two way mats (finite element €2 74 analysis)

* Compared with Table 7.5 figures for Floor 3 have been adjusted down (from €401 and 11 5%) to account for the extra 1 .G tonnes of reinforcement that would have been required had Floor 3 been subject to the same deflection criteria as the other floor slabs (cost of rebar @€350/tonnes becomes €5,959). No adjustment has been made to the value-adding man-hour costs of placing this steel (despite the observation "time savings [on Floor 31 have essentially all arisen from reduced mass of steel").

Table 8.5 Shear systems cost equivalence ~

Floor Shear system Steel &/tonne so system matches the cost of 'traditional' shear reinforcement

1&2 Shape code 85 €350 100%

3 Shape code 85 (contractor design) - E248 -71%

4 ACI stirrups €1,370 391%

4 Shear ladders €899 257%

5 Shear hoops i605 173%

5 Stud rails €983 281%

Time Major improvements in fixing reinforcement and critical path time on site cannot be obtained simply through rationalising at the detailing stage: rationalisation must embrace the whole process in order to get worthwhile benefits on site.

On site, rationalisation appears to help only marginally, but, nonetheless, perceptibly and usefully. Changing standard arrangements on an ad-hoc basis might confuse as much as it helps. On the other hand a radical change in methods of providing shear reinforcement can reap real rewards.

The use of proprietary systems and meshes is far more prevalent on large projects where one suspects that the value of time is valued highly. Perhaps these methods should be adopted far more widely.

Critical path time and production rates There was some consistency in the amount of time the reinforcement took to fix. This amounted to five days per floor for loose bar and four days for mats. During this period, many

96

8 Discussion

construction processes were operating in parallel and a'precise calculation of true 'critical path time' would be extremely complex and not, in Lorien's opinion, appropriate in this case. In multi-storey construction reductions in the critical path time is, of course, vital. Production rates are notoriously difficult and time-consuming to obtain, interpret and evaluate and coefficients of variation are exceptionally high. To detect a 10% difference in production rates between systems with 90% confidence limits (typical in manufacturing) a sample size of 40 would have been required for each system. Clearly obtaining this level of certainty is not feasible. It is not surprising that planners apply a significant degree of judgement to production rates in order to obtain critical path times, knowing that resources can be revised to suit production. This situation does not help instil confidence in the concrete frame market's customers.

The blanket loose bar solutions present an anomaly in that the expected quicker fixing rates were not found - probably due to the fact that only half a floor of each configuration was used.

Off-site processes: Correspondence The process maps in Chapter 5 show the differences between the different methods of procurement tried out on the frame at Cardington - traditional, contractor detailing, and contractor and supplier design. The amounts of correspondence (for correspondence read amounts of time, money, work) as illustrated in Figures 5.9 and 5.10, were significantly greater for the 'contractor's design co-ordinator' for non-traditional methods of procurement than for the traditional methods. This may have been due to unfamiliarity of the parties to the various arrangements or a manifestation of the flows of information, checks and reviews that are traditionally undertaken by the engineer.

The amount of correspondence (Table 8.6) might also have been related to timescale and criticality of information. The traditional design was completed months before construction and there was little pressure to issue information; the design could be reviewed and detailed in a leisurely way. This was not the case for the contractor detailed or contractor designed and detailed slabs, which were designed and detailed during the early stages of construction at Cardington. Issue of information became critical as construction became imminent and the designers and detailers did not have the luxury of having as much time as perhaps they would have liked to check, review, and co-ordinate. Revisions were therefore more likely to be, and indeed were, required. The philosophy of Just-in-Time delivery of information is just as important as Just-in-Time delivery of materials, but perhaps information cannot be managed as easily.

This exercise might have been imperfect but has indicated bamers to the adoption of non- traditional methods of procuring reinforcement for slabs. Whilst these methods may provide efficiencies on site there is a price to be paid by the designer andor contractor and supplier in terms of extra effort and correspondence in areas where they do not necessarily have experience. Not only is there apparently extra cost, but there is also additional risk. What are the rewards for innovation? In traditional contracts perhaps not a lot; in non-traditional contracts there is maybe some reward.

Every construction project demands an immense amount of innovation to make all the conflicting requirements come together in a satisfactory way. Innovation seemingly creates correspondence, work and the need for time and money to be spent managing it. In large projects there is surely an amount of innovation fatigue: more innovation for scant reward is not what construction teams need. Innovation has to be worthwhile and has to be proved as such.

97


The amounts of correspondence may be seen as a measure of the time, money and hassle needed to manage a system. Undoubtedly the novelty of certain systems caused correspondence to be initiated and the survey suggests that this may present a barrier to innovation.

Table 8.6 Contractor's design co-ordinator's correspondence ~~~ ~

Correspondence regarding Location Numbers of correspondents

Traditional loose bar Floor 1 2 originators ave. 3.25 recipients ~~

Traditional loose bar Floor 2 2 originators ave. 3.25 recipients

Rationalised loose bar (contractor design) Floor 3 2 originators ave. 5.5 recipients ~~ ~~ ~

Blanket loose bar; (N- yield line: S- elastic)

Designed one way mats

Floor 4

Floor 5

2 originators ave. 5 recipients

5 originators ave 4.8 recipients

Blanket two way mats (finite element anal)

Shape code 85 (SC 85)

Floor 6

Floors 1 , 2 & 3

4 originators ave 5.25 recipients

No special correspondence

ACI stirrups (prefab on site) 2 originators ave. 2.5 recipients

Shear ladders Floor 4W 2 originators ave. 4.5 recipients

Floor 4E

Stud rails Floor 5E 1 originator 3 recipients ~

Shear hoops Floor 5W 2 originators ave. 4 recipients

Shear studs Floor 6E 2 originators ave. 2 recipients ~~

SC 85 [the rest of Floor 6) Floor 6W 1 originator 1 recipient

Structural steel heads (A2, B2) Floor 6Wb 2 originators ave. 1.5 recipients

General comments The reinforcement was detailed and was to be fixed as if it were for seven floors of the same reinforcement configuration. One of the main problems for the researchers lay in the fact that the number of configurations being looked at meant that in many cases only half of one floor was actually reinforced in the same way - and then half of that had different shear provision. In order to attain 10% confidence limits in productivity results, 40 repetitions would have been needed. In reality, 40 similar floors never happens. (Proverbs"') suggests 60 repetitions for 5% confidence limits). It is not surprising that productivity information in the industry in general varies and is anecdotal.

In hindsight it was unfortunate that, due to the pressure on space and the compromises required, it was not possible to devote whole floors to blanket cover elastic design and blanket cover yield line design. The results for finite element analysis were perhaps hampered by being implemented with two-way mats (or vice versa).

98

8 Discussion

8.4 Integration of results General The intention in this section is to compare the various forms of reinforcement used at Cardington on an overall cost basis. Lorien’s research looked at material and labour costs but stopped short of looking at the benefits from the reduction of critical path time. However, the effect on critical path time is fundamental to the overall economics that should be applied to each system.

The primary objective of this project is to disseminate meaningful guidance on the rationalisation of reinforcement aimed at designers, clients, their advisors and contractors. The simplest way to put any message across is to express the research findings in terms the target audience will understand - money. The usual method is to look at costs per square metre; and these are what are investigated here. Estimates of critical path time savings are used to determine time-cost savings. These savings are added to the savings in, or more likely, additional costs of, labour, plant and materials used in the various systems in order to compare the systems on an overall cost basis.

It is recognised that the Cardington data set was affected by a number of factors. These factors, in Lorien’s opinion, rendered more sophisticated analysis inappropriate. However, for the purposes of thls study, the important issue is to understand the order of costs involved and likely winners and not be too concerned with the finer detail. The Cardington data is the best comparative data available. It was also possible to check this data against commercial data fi-om the concrete frame industry.

Labour and material rates are notoriously variable. All market rates have a certain volatility, the effects of which might invalidate this exercise of integrating results. Nonetheless the exercise provides a valuable means of comparing overall costs.

In essence, the following studies are based on considering the time and materials used at Cardington in relation to current commercial rates and likely finance charges. The research by Lorien and the University of Reading provided the tools with which these aspects were comprehended and compared. The Cardington time data was checked against other data.

To provide the overview, and to integrate the findings, a spreadsheet was constructed based on using: 0 RCC’s Cost model as a basis for comparison. 0 Productivity information from Cardington and el~ewhere‘~’)

Weight information from Cardington 0 Cost information from Clapson(’’), Loried3’), University of Reading (26) and others(2” 40)

0 The cost of time to the relevant parties to the contract as in The University of Reading’s report cost oftinze(’) Commercial cost and time data from correspondence

Method Labour and material costs Labour and material costs were derived from Cardington and other data(”* ”* 26* 38* 40) . The timings from Cardington were checked by using commercial labour rates per tonne and per hour.

Whether the rates used are valid for individual projects is debatable. On an individual project, there are so many variables that contracting companies have to rely on the experience of their

99


estimators and make suitable allowance. Nonetheless, in relation to the rates used here, members of the concrete frame industry have been consulted and the answers are based on their collective understanding, and were considered valid. It is hoped that they are valid for the general case.

Plant costs It is assumed that placing and fixing is by hand and that the type and size of crane, or other type of plant, required will be unaffected by the type of reinforcement configuration employed. Any savings due to plant time savings is assumed to be included in with time- based preliminaries costs.

Critical time In Lorien's research, time was measured as being either value-adding or non-value-adding. Non-value-adding time might be in the form of waiting, obtaining information, eating etc. Time could also be on the critical path or not.

In order to assess critical path time, the value-adding time data from Cardington was used. A number of assumptions were made (e.g. constant productivity and gang sizes) and checked against specialist subcontractors' assessments.

The assessment of critical path time is almost always subjective. Whilst there are some ground rules, assessing critical times appears to be more of an art than a science. Planners may apply basic rules but they apply many modification factors based on the many variables of the specific project and individual company methods. Planners tend to guard, their methods jealously. On site, managers have many ways of changing outputs on site (increase labour, overtime, etc.). Taking into account the problem of acquiring accurate productivity data('9) and the vagaries of construction sites, it would seem unwarranted, for this exercise at least, to apply more scientific methods. Lorien also warned against using the existing dataset for relatively sophisticated analysis! !

Time costs The basic costs of time have been determined in their research by the University of Reading. In order to assimilate these costs with those for material and labour, it is necessary to relate them all to actual buildings or at least financial breakdowns on actual buildings. Finance charges (or the cost of time) increase with size of building. But each project differs in size, complexity, location, time frame and market conditions. Here, in order to provide overall guidance, it is necessary to assess the finances on a range of buildings and it is advantageous to base studies on simple physical models. The opportunity has been taken to use the relatively simple models in the RCC's Cost model study(*') to provide guidance data. The study provides a suitable basis as it gives a breakdown on costs on a range of buildings, and the concrete model buildings used flat slab construction with spans the same as those used at Cardington.

Apportioning costs and savings Costs cannot simply be added together. As recognised by the University of Reading the costs (and savings) vary for different parties to a contract. So the incentives and therefore priorities may also be different.

Different forms of contract affect how costs or savings are apportioned between parties. Therefore, the effects on specialist subcontractors, main contractors and clients were studied with respect to traditional forms of contract and two of the main alternatives, Construction Management, and Design and Build. As illustrated, there are considerations as to whether the innovative methods are adopted pre- or post- contract i.e. in the original design or as an

100

8,Discussion

alternative to an existing design. Having investigated all these options, the costs and savings were apportioned and, by use of a spreadsheet aggregated and presented in a series of graphs which are presented in Appendix VIII.

Caveats It is far from easy to arrive at costings with any great certainty or precision. Many factors influence decisions taken on individual projects - not just financial factors. Prices change with market demand. Other processes in the construction process may be critical; labour can be switched from one task to another. Yet, investigating ballpark figures will, it is believed, give an insight into the significance and value of rationalisation of reinforcement. Making educated guesses (or speculating) using measured and historical rates should, it was hoped, give useful guidelines.

There are many dangers and difficulties in arriving at these costings. Amongst these is putting too much credence on the data from Cardington. Wherever possible they have been calibrated against commercial knowledge or specialist opinion. Another presumption is that rationalisation as innovation has benefits quantifiable in terms of Urn2. It presumes that performance and quality are not compromised, indeed, may be improved.

Labour and material costs Labour content check Figure 8.7 attempts check the Cardington time data by comparing the material and labour costs found at Cardington for the various configurations of flexural reinforcement with average historical commercial rates for the various configurations of flexural reinforcement. There appears to be reasonably good correlation.

.- J 0 0 - L m

al M 0

L c 0

n

-

% p:

1

5 E 6

U

U)

- c

3

2" c 1 5 m

Type of reinforcement

--.CMakrial+Labour (msd time)Am - -Mterial+Labour( timeltonne)Am i --cMaterial+Labour(msd time) High ... __ ... I Material+Labour ( timellonne) High --c Material+Labour (msd time) Low - - I. Material+Labour ( timellonne) Low

Figure 8.7 Comparison between costs of reinforcement (labour plus material): derived from Cardingtod3@ (msd time) and commercial rates(*') (timekonne)

101

Rationalisation of flat slab reinforcement . _

The main anomalies are that the figures derived from Cardington data appear to show blanket cover loose bar (yield line design) and mats in a relatively unfavourable light compared to more traditional methods of reinforcing flat slabs.

Of more significance is the range between high and low costs (approximately 30% of the average) which is indicative not only of fluctuations in market prices but also lack of hard productivity data and perhaps different pricing policies.

Costs/m2 to the quantities and timings fi-om the Applying the range of supply and fming costs

Cardington project gwes a range of 'material and labour costs' for the different configurations of reinforcement at Cardington. These are presented as Figures 8.8 and 8.9.

(15, 25. 38. 40)

Bla-k&loosebar(yieJd l i n e d )

0 5 10 15 a3 25

costurrp

Figure 8.8 Costs of material plus labour for flexural reinforcement

Labour and Material Costs Shear reinforcement

Ancotech Shear stuc s

SC 85 (Floor 6 design) 1 S. S. shear heads

Deha Stud rai s

Square Grip She; r hoops

ROM Shearladde-s

l r ACI stirrups

' SC 85 (Contract r b design)

SC 85 (Buro Hap .) I

I 1 0.00 1.00 2.00 3.00 4.00 5.00 6.00

Cost Elm2

Figure 8.9 Costs (material plus labour) per square metre for punching shear reinforcement

102

8 Discussion

The range is indicative only and subject to commercial pricing that may differ from time to time. Products also change (e.g. ROM shear ladders have been developed M e r since their use at Cardington). Part of the spreadsheet from which Figures 8.8 and 8.9 are derived is presented in Appendix IV. The spreadsheet used costs from a series of interviews conducted by the University of Reading(2s).. Table 8.7 is included to put these figures in context.

Table 8.7 Base reinforcement supply and fixing costs - materials and labour only

Supply only Fix Total &/ton &/ton &/ton

~~ ~~

Cost comparisons -1991"

Cut & bent bars 225 130 355

Standard fabric 245 65 3 10

Purpose made fabric 275 65 340

Other prefabricated units 485 65 550 ~

Cost comparisons 1997

Cut & bent bars 242 240' 5252

Standard fabric 3253 1054 4702

Purpose made fabric 387 1 054 535*

Lorien 1998

Cut & bent bars 350 150' 500

Notes a Estimate of average market prices, Autumn 1991 - Clap~on('~) b Estimate of average market prices, Autumn 1998 - University of Reading (") c Figures represent an estimate Autumn 1998 - Loried3') I Labour costs €275 to €70/tonne depending on diameter. However some contractors lump all diameters together,

some go by diameter. Flat slabs 10% less, troughs 5% more, waffles 15% more 2 Tie wire, spacers and chairs €8/tonne, Wastage 2%, Overheads and Profits 5% 3 €320 to f335ltonne 4 Based on A393 at €0.65/m2 5 €15O/tonne for fixing is equated to €21.65/hr material and labour costs

Punching shear reinforcement Figure 8.9 is based on 43m2 of floor per column (in line with the Cost model study buildings). At Cardington each column actually supports an average of 34m2 of floor. Even with this 25% increase in relative costs , the costs of shear systems are relativeley small compared with the cost of flexural reinforcement.

It must be emphasised that this figure graph is based on very little data, particularly with respect to productivity. Indicative prices from manufacturers were only that - indicative for the purpose of research. As an indication, the labour and material costs of the various systems are in the order of:

€25/column for ACI stirrups €50 - €SO/column for traditional links €75/column for proprietary stud rails, ladders, etc €2OO/column for stuctural steel shear heads.

103


Comparing costs Lorien chose to make comparisons on the basis of how highly a type of reinforcement could be priced to be directly comparable with traditional methods. This is a valid technique, particularly in the manufacturing field. Lorien also showed that the relative economics of the various systems were not too sensitive to the relation between labour and material costs.. However they did not account for the possible finance cost savings on the whole building through reduction of critical path time which was held to be tremendously important.

Critical time In order to assess differences in critical path time a number of assumptions were made and then applied to the labour times measured at Cardington. The assumptions were based on best estimates then compared with specialist trade contractors' thoughts on critical path time savings. Whilst the original assumptions gave a basic agreement, the actual numbers used were adjusted to give a best fit match. Thus a permanent fixing gang of three working an 8 hour day was assumed. Further, it was assumed that 66% of the flexural bar fixing operation was on the critical path and 100% of shear reinforcement fixing was on the critical path.

Although inconclusive, the graph in Figure 8.10 shows some consistency between measured and perceived time saving. The main anomalies occur in regard to blanket loose bar for both elastic and yield line design and for structural steel shear heads. The difference for blanket loose bar was probably due to the fact that only '/z a floor of each was used. It is acknowledged that using two systems on Floor 4 severely affected time and productivity measurement. The procurement and placement of structural steel shear heads at Cardington was very difficult and this is perhaps reflected in the difference between measured time and perception of their potential for saving time.

2.00

p 1.00

d 0.00

5 rn v) v) 2,

-1 .oo

Time savings per floor - measured and perceived

-1 I ~. 4

Figure 8.10 Time savings per floor cycle - comparing as 'measured' at Cardington and as perceived by specialist subcontractors

The assumptions about gang sizes, working day and criticality mode made above are not unreasonable for a building the size of that at Cardington. But as illustrated by events at Cardington gang sizes (gang numbers), working hours and criticality of operations can vary enormously. In practice site staff can juggle resources to achieve the desired end e.g. gang size or gang make-up can be changed.

104

8 Discussion

Building I M4C3 M4C5 M4C7 M4C9

Time costs

M62C3 M62C5 M62C7 M62C9

The financial effects of reduction of critical path time have been defined by the University of Reading(3). Essentially there are two parts: savings in finance charges and increased rental, which is in the domain of the client, and savings in preliminaries which is in the domain of contractors. Depending on the form of contract and within that contract when any savings are made, savings in preliminaries may be in the domain of the client.

Finance charges The potential financial cost savings per day can be calculated using the Reading formula (see note 2 to Table 8.1 and Section 4.4).

By integrating data from the RCC Cost model study (as amended and expanded by the original cost consultants) it is possible to estimate the value of one day. In order to aggregate these effects with those for the costs of materials and labour, it is necessary to assign some figures to the variables (e.g. cost of borrowing, yields required etc). These numbers vary with market conditions, but again, historical data can be applied in order to derive indications of costs or cost savings. Again, the models from the RCC's cost model study were used. A summary of these model buildings is given in Table 8.8 and the assumptions used and part of the workings are shown in Appendix V. The variables used are shown in Table 8.9

Nominal location

Specification

Reading

Square in plan -1500 m2/floor Curtain wall glazing

Air conditioned Concrete structure - flat slabs

Construction cost 1992/3 E3.4m E5.4m E7.4m E9.4m

Construction cost 2000 E4.7m f7.5m E10.4m E13.2m

Construction period 52 wks 57 wks 62wks 67 wks

Rochdale

Rectangular in plan -1500 m*/floor Traditional brickwork cladding

Naturally ventilated Concrete structure - flat slabs

E2.8m E4.5m E6.lm E7.8m

E3.9m E6.3m f8.6m El.Om

53 wks 58 wks 63 wks 68 wks

RCC Cost model study buildings The M4C3 building is assumed to be located in Reading, a three-storey concrete frame, square in plan with air conditioning and curtain walling. The M4C7 building is similar but with seven storeys. The figures for the M4C5 and M4C9 buildings were interpolated from the M4C3 and M4C7 buildings.

The M62C3 building is assumed to be located in Rochdale, a three-storey concrete frame with two rectangular wings, natural ventilation and traditional brick cladding. The M62C7 building is similar but with seven storeys. Again figures for the M62C5 and M62C9 buildings were interpolated from data for the M62C3 and M62C7 buildings.

All buildings incorporated concrete flat slabs on a 7.5 m x 7.5 m grid. Floor plates had an area of 1500 m2 and based on a 1991 survey the three-storey buildings were considered to be 'average sized' multi-storey offices.

Land acquisition and demolition costs were taken from conversations with the Cost model sttrdy's cost consultants who also advised on cost indices from the original study done in 199213 to the present (2000).

105


Table 8.9 Assumptions made in determining the cost of time for the model buildings - finance

Variable Value Comment

Time saving dT

Nominal rate for borrowing pa

Developers profit required

Fees as percentage of construction cost

Yield required

Costs of ownership/renting as %pa

Gross yield required

Savings (cost) of preliminaries

Savings (cost) of materials /m2

Savings (cost) of labour /m2

Savings (cost) of materials /column

Savings (cost) of labour /column

0.167

9%

25%

12%

9%

1%

10%

0

0

0

0

0

Weeks. i.e one day in a six day week assumed.

Say base plus 3%

Typical - dependent on project and markets

Design fees for new budd includmg Architect, Structural Engineer, Services Engineer, Quantity Surveyor and Letting Agent. Dependent on type ofproject

Indicative for high quality office accommodation. Dependent on project, location and markets

Say Typically dependent on project and markets

1 } Allowed for elsewhere and ignored } for the purposes of assessing differences }in financecharges 1 ' 1

Rental Typical 2000 rentals for Reading and Rochdale were determined from discussion with a developer. These values were used to moderate costs and values to maintain the relationship Property value = Net irzcome/yield within reasonable bounds.

Preliminaries As mentioned above, savings in main and specialist subcontract preliminaries may, depending on the type of contract and its stage, be shared amongst the parties to the contract. Before considering the distribution, it is worth understanding the size of those savings. The assumptions made to derive savings in preliminaries are shown in Table 8.10 below.

Table 8.10 Assumptions made in determining the cost of time for the cost model buildings for preliminaries

Assumption Value used Reference

Preliminaries: main contractor time-based preliminaries as a percentage of cost of construction

Preliminaries: specialist subcontractor time-based preliminaries as percentage of cost of subcontract

% of savings passed on by specialist subcontractor to main contractor

% of savings passed on by main contractor to client

5.50% From Cost model study("). 5.5% may be considered as being low. Depending on the type of project - main contractor preliminaries can often be between 8% and 12% or even 5% to 15%

From discussions with main and specialist subcontractors

10%

50% From discussions with main and specialist subcontractors. Contractors are not obliged in all contractual cases to pass any savings on and 50% may be regarded as a maximum. In other cases it may be a matter of negotiation. Similar figures are used when passing on labour andor material cost savings.

50%

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8 Discussion

Theoretical savings per day The theoretical savings in preliminaries, finance charges and, in the case of owner/occupiers (or speculative developers with rental agreements in place), gain in rent, may be added together. Figure 8.1 1 represents savings available to each of the parties involved in a contract per day saved, (assuming no net labour or material costs).

The data used in the graph are based on the assumptions given in the previous two tables and on the Cost model buildings. The graph should be regarded as being indicative only. The three storey buildings may be regarded as ‘average’ size (4500 m’), so the graph may be regarded as being applicable to the median to large end of the market. The savings per square metre per day saved are also be expressed in tabular form in Table 8.1 1 :

Depending on contractual arrangements, parties may or may not have to pass any savings on. The parties are defined here as being

Specialist subcontractor; 0 Main contractor (or construction manager) 0 Clients subject to finance charges e.g. speculative clients without pre-let;

Owner/ occupier client subject to finance charges and rental income (e.g. speculative client with pre-let agreement or owner/ occupier).

Figure 8.12 shows that in monetary terms the potential savings are far greater for clients than they are for contractors. Savings increase with size of building - very significantly so for clients. It becomes worthwhile to pay for one-off costs of innovation. Ignoring opportunity costs, time savings are far more valuable for owner/occupiers, andor developers with tenants than for speculative developers without tenants.

Possible savings per day saved

T-

P n

U 0

m 2 4000 U)

w

6000

.-

2000 K L -e--- *- -- -- - 0 4

M4C3 M4C5 M4C7 M4C9 M62C3 M62C5 M62C7 M62C9

Model building a

- -Specialist trade contractor - prelims -Main conlrador - prelims

-+- Owner/ Occupier et& finance 8 rent +Speculative developer. finance

Figure 8.11 Possible savings per day saved (assuming no net labour or material costs)

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If one day is saved every storey then the savings go up pro-rata with number of storeys. So saving one day per floor on a nine storey building might save an owner/occupier client about €0.81 x 9 x 12086 m2 = €88,000 on a E13.2 million project in Reading. (This figures is approximately in line with the level of liquidated and ascertained damages (LADS) that might be applied to late delivery of these projects.) For the specialist trade subcontractor the savings would be approximately €8,700.

If nine days can be saved on a nine-storey building then the theoretical €7.29/m2 (9 x €0.81) time-related savings make a large impact on the premium for using say blanket cover two- way mats and finite element design. On smaller buildings the effect is much smaller (e.g. on M4C3, 3 x €0.78 = €2.34/m2) and therefore perhaps changing from least material cost solutions for flexural reinforcement may not be warranted.

For shear systems costing say €75/column (say €1 .50/m2) if the system saves even two days overall it is a worthwhile proposition for owner/occupier clients.

Assuming one day is equivalent to 0.2 of a week instead of 0.167 of a week would increase monetary savings by 17%. Assuming the main contractor‘s preliminaries were 10% as opposed to 5.5% would almost double the value of the savings from their preliminaries costs.

From Table 8.11 it can be seen that there are greater savings for the main contractors and clients on the M4 buildings than on the M62 buildings. This appears to be because the latter projects are more expensive, pro-rata and, for owner /occupiers, rentals are greater for the M4 buildings. The fact that the two halves of the M62 slabs could be effectively phased may also lessen the effect of one day’s saving on the critical path of the slab construction process. Savings for the specialist subcontractors are more per day on the M62 buildings due to the fact that the structural content on the M62 buildings is proportionally more than on the M4 buildings and specialists are less affected by critical time.

Table 8.11 Value of time: Savings €/m2 per day saved’

M4C3 M4C5 M4C7 M4C9 M62C3 M62C5 M62C7 M62C9

Specialist subcontractor - 0.11 0.10 0.09 0.08 0.17 0.13 0.11 0.09 prelims

Main contractor - 0.20 0.17 0.15 0.14 0.21 0.16 0.14 0.12 prelims

finance Speculative developer - 0.22 0.23 0.24 0.24 0.22 0.20 0.20 0.20

Owner/ occupier - . 0.78 0.80 0.81 0.81 0.64 0.57 0.54 0.53 finance & rent

NB # Indicative only.

The savings appear to be are broadly in line with the extent of a party’s involvement on a project. It is revealing to compare savings with contract involvement. Figure 8.12 shows the percentage saving for contractors, notably specialist subcontractors, falls with increased size of project whereas it is constant for clients.

108

8 Discussion

0.14%

0 0 + 0.04%. 0 3 s

0.02%

0.00% M4C3 M4C5 M4C7 M4C9 M62C3 M62C5 M62C7 M62C9

Model bullding - -Specialist sub mntrador prelims -Main mntraaor Drdims

+owner/ Ocnrpier etc. finance L rent + Speculative developer ~ finance

Figure 8.12 Possible savings per day saved expressed as a percentage of contract value (assuming no labour or material costs).

Apportioning costs and savings In addition to considering the total and proportional savings, it should be recognised that different forms of contract and indeed the stage of contract both affect how costs or savings are apportioned and allocated between parties. There are considerations as to whether the non- traditional methods are adopted pre- or post- contract i.e. in the original design or as an alternative to an existing design. Table 8.12 attempts to identify who gains and who loses from saving time and or materials. It also looks at when the innovation takes place.

Innovating pre-contract Whatever form of contract, any savings in time or costs in adopting innovative methods pre- contract should be reflected in the tender figures, and the beneficiary in monetary terms is the client. Contractors benefit in that those who adopt more efficient methods are likely to win more work. If they are confident that a particular innovation will not be used by competitors, then they may be inclined to increase their margin.

As indicated in Table 8.12, when innovating pre-contract, the benefits of good innovation will ultimately pass through to clients (not necessarily the same individual client) as they are adopted in following tenders and proposals. They become norms against which other innovations are judged.

Innovating post-contract Many innovations are introduced post-contract. The pace of change and the duration of large contracts mean that new innovations are constantly being introduced. Their applicability to a particular project may only become apparent once contractors have been appointed and serious work has started on the project. The work may have been let on outline information.

The beneficiaries of innovation post-contract vary depending on the type of contract and individual circumstances. It is difficult to generalise but the essence of who should benefit is set out in the three lower parts of Table 8.12. Again, many assumptions have been made but

109


the important issue is to recognise that there are differences in contractual arrangements that lead to different motivations.

For instance, once a traditional contract is let, no design changes should, strictly, be made. In practice contractors often make suggestions for alternatives. The incentive for the contractor is cost savings. The incentive for the client is a share of the savings from preliminaries, labour and materials and quicker delivery. It is a commercial decision just how much of the savings are passed on. Should the client initiate quicker delivery then he will usually have to bear all acceleration charges (consisting of additional labour and material costs etc).

With Construction Management contracts, the benefits of innovation would be expected to be shared between client and contractor; the construction manager should also share in any savings (for the purposes of this section the construction manager is equated to a main contractor). With Design and Build contracts, the benefits would be expected to reside with the contractor.

Assuming the saving passed on by specialist subcontractors to main contractors is 50% and the savings passed by main contractor to clients is 50%, the savings accruing to each party may be quantified as in Table 8.13. Again, many assumptions have been made over contractual arrangements and who pays for what. Nonetheless, the figures in this table were used to apportion and aggregate savings.

110

8 Discussion

Table 8.12 Who gains from saving time on a concrete frame contract and who gains from using less (and pays for additional) material?

Contractors Clients

Specialist Main Specialist Owner/ subcontractor developer occupier (labour only) (developer with

pre-let)

Innovating pre contract (traditional, design and build, construction management)

Preliminaries* 3 3 + + Labour 3 3 + + Materials Xb JC + +

~ _ _ _ _ _ _

Innovating post traditional contract

Preliminaries* .I= 4 3 + + Labour .I= X d 3 +, e +, e

Materials X" $3 +. e +, e

Innovating post construction management contract

Preliminaries* .If- d f 3 .I .I Labour .If> X d 3 .I .I Materials X X .Ig J g

Innovating post Design and Build contract

preliminaries* 4% .If X

Labour .If= .If X

Materials .If- .If X

Note Rental income, finance charges and opportunity costs are allowed for elsewhere.

Key * 3

4

a b

d

e

f

h g

X

C

As charges for plant are time-based, 'Plant' is assumed to be included in with preliminaries. Savings passed on. If the innovation over 'traditional methods' is part of a tender then any savings (or costs) would be expected to be passed on, at least in part from specialist subcontractor to main contractor to client. If the innovation is introduced post-tender then different situations arise. Savings accrued by relevant party No saving accrued by relevant party Accrued from contractors Assuming 'labour only' subcontract otherwise savings passed on Savings accrue from both 'labour only' and 'supply and fix' subcontracts Some savings in main contractor's labour and plant is inevitable, but items such as tower crane and banksmen may be considered as included in preliminaries. Savings from subcontractors should be passed on. If the innovation is at clients request then he may have to bear all additional costs (or acceleration charges). Savings should be shared. Assuming 'supply and fix' subcontract. Savings (or costs) maykhould be shared between specialist subcontractor, CM or D&B contractor and in the case of CM, the client. Assuming 'labour only' subcontract otherwise savings shared The client usually pays for materials under CM contracts.

111


Table 8.13 Savings accruing to parties to a contract.

Contractors Clients

Specialist sub- Main contractor

Innovating pre contract (traditional, Design and Build, Construction Management)

Spec. subcontr: Preliminaries 0% 0% 100%

Main contr: Preliminaries 0% 0% . 100% ~~

Labour 0% 0% 100%

Materials 0% 0% 100%



Main conrt: Preliminaries 0% 50% 50%

Labour 50% 25% 25%


Innovating post Construction Management contract


Main contr: Preliminaries 0% 50% 50% ~~ ~ ~

Labour 5 0% 25% 25%




Main contr: Preliminaries 0% 100% 0%

Labour 50% 5 0% 0%


Theoretical overall savings per m2 for flexural reinforcement General The following setions compare the savings from different ways of reinforcing flat slabs. They are based on graphs that are reproduced in Appendix VI11 which themselves were derived from a spreadsheet based on the assumptions made in the preceding sections.

The effects of adopting different methods of reinforcement vary depending on which party to a contract is considered. They also vary according to which stage of a contract ‘traditional’ forms of reinforcement are replaced. These points are discussed under the next few headings.

112

8 Discussion

The graphs Each graph has four sets of lines representing the savings (or losses) expected for four main parties to a contract -

0 Specialist trade subcontractor 0 Main contractor (or equivalent) 0

0

Speculative clients (clients experiencing finance charges) Clients subject to finance charges and to obtaining rent

Each set of lines is based on four points. The left hand point represents an average sized multi-storey building of 4500m’ gross floor area on three storeys, M4C3 (M4 relates to the M4 corridor, nominally Reading, C to concrete and 3 to three storeys). The right hand point of each four represents a large building of approximately 13,000 m’ gross floor area on nine storeys (M4C9). The intermediate points relate to buildings of approximately 7000m2 on five storeys (M4C5) and 10,000 mz on seven storeys (M4C7).

Innovating pre-contract (all forms of contract) Please refer to Figures A-VIII. 1 to A-VIII.4

If innovations (or rationalisation) takes place before a contract is let then apart fiom helping to win the contract which has its own value, no savings accrue to contractors.

Based on the data from Cardington, the best approach for clients appears to be to use rationalised loose bar. The speed advantage of two-way mats with finite element design appears to be advantageous for owner/ occupier clients when the project is two or three times the average sized multi-storey building - say above 10,000 m2 and/or seven storeys.

Savings calculated using specialist subcontractors’ perceptions of time are broadly in agreement with those based on the Cardington data. The striking exception is the contractors’ perceptions of time for blanket cover yield line design. This indicates the possibility for very large savings to be made across all building sizes. The perceived time savings from using two-way mats suggest that for owner/occupier clients, the two-way mats should be used in buildings larger than, say, 6,000 m’.

In all cases results for one-way mats confirmed the view of the supplier that their use was perhaps inappropriate for this type of construction.

Graphs for the M62 corridor, have similar sets of lines and points. It will be noted that the savings over traditional methods are generally less than for the M4 buildings. As noted in relation to Table 8.1 1 , this is probably due to the fact expensive pro-rata and, for owner /occupiers, rentals are therefore time is more critical.

Innovating post traditional contract Please refer to Figures A-VIII.5 to A-VIII.8

that the M4 buildings are more greater for the M4 buildings and

When the innovation occurs post traditional contract, contractors derive some benefit. From the Cardington data it is most advantageous for the specialist subcontractor to use two-way mats as he is not paying for the material under the labour only subcontract assumed! Otherwise the rationalised approach appears to give best value for all parties. For owner/occupiers or clients with a pre-let on larger than average buildings, there would be savings in using two way mats on large buildings. Again the perceptions of specialist subcontractors are in line with what was measured on Cardington and, again, there would appear to be worthwhile savings from using yield line design.

113


Innovating post Construction Management contract Please refer to Figures A-VIII.9 and A-VIII.10

As main contractors share in the time savings they, as well as specialist subcontractors, benefit by using two-way mats on all types of building. For owner/occupier clients, two-way mats have advantages over rationalised loose bar only in large scale buildings (say greater than 7500 m2 and/or five storeys). As the balance of who pays for extra material shifts fi-om contractor to client so the attractiveness of using more material shifts from client to contractor.

Innovating post Design and Build contract Please refer to Figures A-VIII. 1 1 and A-VIII. 12

Here it would be in the client's interests to go for two-way mats as in this case he does not pay for the additional material! , The opportunity for reinforcement arrangements based on yield line design is again clear. Otherwise rationalised arrangements appear to be most economic.

Theoretical overall savings per m2 for shear reinforcement General The graphs showing savings by using innovative shear reinforcement were constructed and are presented in the same manner as those for flexural reinforcement in Appendix VIII. All graphs are based on Cardington data and timings.

Please refer to Figures A-VIII. 13 and A-VIII.2 1

Commentary on shear systems The graphs for shear systems are consistent in showing that there are benefits in using ACI shear stirrups or proprietary shear systems in all the flat slab buildings considered in all the contractual situations considered. Clients always benefit. Contractors benefit if the systems are introduced post-contract and no one loses. The benefits are greater for larger buildings and for the M4 buildings.

The ACI stirrups came out particularly favorably. They are liked because: 0

0

0

They are cheap and easy to fix They may be prefabricated off the critical path They can be designed, detailed and procured in the same way as the usual flexural reinforcement There is no specialist design or separate design/ procurement process required.

However, one of the assumptions made was that the alternative systems used at Cardington would give equal or better structural performance and quality. It would appear that the ACI stirrups are performing perfectly adequately: there is no suggestion that they are not. These shear stirrups are used in the USA and are covered by the Building Code Requirements, in ACI 3 1 S(33). In Cardington they were designed on the basis of this code. Unfortunately there is no direct equivalent design method accepted in UK and even in America there appears to be some doubt about their use particularly when holes are present near columns. As the USA market moves towards post-tensioned floors, it appears(4') to be moving away from stirrups towards stud rails. These points were covered in correspondence parts of which are reproduced below.

114

8 Discussion

“The ACIprocedure is what is used for design of these elements, but it is dejinitely vague when it comes to designing around openings near the column. A number of creative details such as using two stirrup cages to flank the opening, are used but no consensus exists. ”

“Your ‘ACI stirrups’ seem to be what was typically used here (and is still used in some parts of the country). They have somewhat gone out of fashion because they are less effective and create more congestion in the thinner post-tensioned slab sections now being used, particularly at the slab/column joint. (The Americans are using 7 to 8 inch thick (180 to 205 mm) flat slabs for many of their hotel and condominium projects with spans in the 26 to 30 foot range (7.9 m to 9.1 m)). Shear reinforcement systems are less congestive around the columns.”

“I agree with all of your reasons for wanting to use them, especially the abilig for the rebar supplier to supply, but the stud rails definitely allow easierplacement of the slab rebar andp/t around the column. ”

It must be stated that American subcontractors supply shop drawings for review prior to construction so have a better chance of using the system they prefer. *

In conclusion, there are some questions against the design of ACI stirrups, especially with regard to holes. Their general use cannot therefore be advocated before these questions are resolved. At the time of writing (September 2000), The Concrete Society’s Shear Reinforcement Working Party is considering the design of ACI stirrups to BS 81 10 and EC2.

‘Traditional’ links were used in Floors I and 2 using Buro Happold’s design, in Floor 3 to the contractor’s design and in Floor 6 to Buro Happold’s design again. The three lines on the graphs for shape code 85 links illustrate the vagaries of nominally the same design and the importance of the time element in the overall costs. The base line (zero savings) is an average of the traditional links used in Floors 1 and 2. In Floor 3, the contractors design resulted in many more links to be fixed. In Floor 6 the link spacings reverted to those used on Floors 1 and 2 yet the fixing was 20% quicker than on Floors 1 and 2. The savings on Floor 6 might be regarded as representing the difference in measuring virtually the same operation at two different levels. The savings, of between €1.00/m2 and €2.50/m2 for clients with pre-lets, might be indicative of the variations or errors in measurement or of quality of labour, or may be an indication of the effect of the regularity of the mesh to which the shape code 85 links were fixed.

On Floor 3 the links were placed at closer centres than was necessary, consequently there were many more links to be fixed (6215/floor c.f. 1425/floor on Floors 1, 2 and 6). The additional cost of theses links, of between approximately €3 .00/m2 and €8.00/m2 for clients with pre-lets, is indicative of the importance of time costs.

There remains the ‘no link’ option where flexural reinforcement is increased to the extent that traditional punching shear reinforcement is not required. This option is only really viable with relatively large columns and/or thick slabs. The commercial pressure to minimize size of columns and thickness of slabs would appear to make the no-link option even less viable on medium-rise buildings. On a cost of material and labour basis, the no-link option might be economic where the cost of the additional reinforcement is less that the cost of proprietary systems i.e. less than approximately €2/m2 overall. This equates to say, €8/m2 locally over columns - equivalent to approximately 1500 mm2/m both ways locally over columns. However the advantage is lost if the thickness of the slab needs to increase or complicated reinforcement details arise.

115


If traditional shape code 85 links are used then their numbers should be minimized by designing for and specifying the maximum possible spacings.

Of course the designer must be considered. He/she has a choice of several systems and does not want to be, nor should be, drawn to the expense of redesigning. He/she may be assured that each proprietary system appears to give the same order of benefit to clients and contractor and the choice might be based on designer or contractor preference in terms of design or procurement process.

The savings are a function of number of storeys and to lesser extent type of contract. Time savings are of the utmost importance. In money terms contractors do not benefit nearly as much as clients do from saving time.

Margins of error Margins of error are very difficult to estimate. On Floor 3, for instance, in order to make the floors directly comparable on a structural basis, it was assumed for the purposes of costing that the rationalised arrangement of reinforcement contained 17.0 tonnes of flexural reinforcement rather than the 15.3 tonnes actually placed. On the materials side this assumption may have added approximately E0.50/m2 to the costs of measured material and labour. It is generally held that rationalisation leads to increased weight so this adjustment might be regarded as an underestimate. A 5% increase in weight over the traditionally designed and detailed Floors 1 and 2 would indicate an additional E0.30/m2 increase due to material costs: a 10% increase in weight would indicate an additional i0.60/m2.

It was also assumed that the value adding time to place this rationalised reinforcement would have remained at 108 hours. Lorien reported that the rationalised design is not inherently significantly easier to fix than the traditional and that time savings essentially all arose from reduced mass of steel. From the bending schedules it would appear that 2488 bars were fixed in Floor 3. If deflection had actually been a requirement it is estimated that another 154 bars would have been required (see Appendix I). While these additional bars represent some 6% of bar numbers they comprised no additional bar marks. Actual fixing may have increased by 6% but unloading, storing, finding, lifting and sorting would have been largely unaffected. A 6% increase in labour would have represented approximately an additional €0.25/m2 using the Cardington data or €0.36/m2 using data derived from commercial rates. In total, the margm of error on material and labour costs on Floor 3 might be in the order of €0.50 to €1.00/m2.

There were also anomalies in the data from Cardington. For instance the expected quicker rate for fixing blanket loose bar to the yield line design was not found. It is possible the problems on Floor 4 (explained in Appendix I11 Supplement 2) created this result. But there were construction problems on all floors. The shape code 85 (SCSS) shear arrangements on Floor 6 were fixed some 20% faster (or up to E2/m2 cheaper) on Floor 6 than on Floors 1 and 2 although they were identical.

There is also the problem of assessing market rates for materials and labour. Upper and lower bounds have been considered but these may be inappropriate for use on a specific project to be undertaken by a specific contractor.

It is the author’s opinion that errors in the assessment .of savings possible in flexural reinforcement may be in the order of €Urn2. As discussed above, the savings emanating from the shape codes 85s on Floor 6 of between €1.00/m2 and E2.50/m2 might be indicative of errors in measurement. At these levels the gist of the conclusions and recommendations are unaffected. However, the need for better productivity data is highlighted.

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8 Discussion

8.5 Best value

If best value is indicated by the form of construction that is most economic for each party to the contract, then the recommendations are given in Table 8.14.

Table 8.14 Flexural reinforcement: best practice from Cardington weight and time data

Party Specialist Main contractor Client Owner/ occupier subcontractor client2

Innovating pre- No real preference other than to win Rationalised loose bar Rationalised loose bar. contract (all Large buildings - two- forms of clients way mats and and finite contract) element design

work i.e. rationalised loose bar as for

Innovating post Two-way mats Rationalised loose Rationalised loose bar. Rationalised loose bar. traditional and finite bar. Very large buildings - Large buildings - two- contract element design two-way mats and and way mats and finite

finite element design element design

Innovating post Two-way mats Two-way mats Rationalised loose bar. Rationalised loose bar. Construction and finite and finite element Large buildings - two- Management element design design way mats and finite contract element design

Innovating post Rationalised Rationalised loose Two-way mats and Two-way mats and Design and loose bar. bar. finite element design finite element design Build contract

Notes Large buildings are defined here as those substantially larger than 4500m2 total on three floors I 2

Clients subject to finance charges e.g. speculative clients without pre-let Owner/ occupier client subject to finance charges and rental income e.g. speculative client with pre-let agreement or owned occupier.

It may be seen that, depending on the circumstance (e.g. who pays for materials) different arrangements of reinforcement provide best value to different parties to the contract. From the work at Cardington it would appear that, generally, rationalised loose bar is the most economic form. In large buildings, those substantially larger than 4500m2 total on three floors, two-way mats come to the fore.

However, using the perceptions of time saved according to the specialist subcontractors the best value method in most cases would appear to be the use of yield line design, as shown in Table 8.15

With regard to shear reinforcement, ACI stirrups came out very favourably but, as explained previously, there are some questions about their design, especially with regard to their use near to holes. Their general use cannot therefore be advocated until these questions are resolved. In the meantime, proprietary systems such as stud rails and shear ladders are to be recommended. They provide benefits across a wide range of buildings, contracts and parties to a contract. Best practice for use of shear reinforcement based on the Cardington data is summed up in Table 8.16.

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Rationalisation of flat slab reinforcement s .

Table 8.15 Flexural reinforcement: best practice according to the perceptions of specialist trade contractors

Partv

Specialist sub Main contractor Client ' Owner/ occupier contractor client2

~

Innovating pre- No real preference other than to win Yield line Yield line contract (all forms of contract)

Innovating post Two-way mats Yield line Yield line Yield line traditional and finite Very large buildings - contract element design two-way mats and finite

work i.e. use those preferred by client

element design

Innovating post Two-way mats Two-way mats and Yield line Construction and finite finite element Management element design design contract

Yield line

Innovating post Yield line Yield line Two-way mats and Two-way mats and Design and Build finite element finite element design contract design

Notes Large buildings are defined here as those substantially larger than 4500m2 total on three floors 1 2

Clients subject to finance charges e.g. speculative clients without pre-let Owner/ occupier client subject to finance charges and rental income e.g. speculative client with pre-let agreement or owner/ occupier.

Table 8.16 Shear reinforcement: best value from Cardington weight and time data

Specialist Main contractor Client ' Owner/ subcontractor occupier client'

No real preference other than to win work i.e. use those preferred by clients

Innovating pre-contract (all forms of contract)

ACI shear stirrups' then shear studs and shear ladders


Innovating post Construction Management contract

ACI shear stirrups3

then shear studs and shear ladders -


Notes 1 Clients subject to finance charges e.g. speculative clients without pre-let 2 Owner/ occupier client subject to finance charges and rental income e.g. speculative client with pre-let

agreement or owner/ occupier 3. The general use of ACI stirrups cannot at present be advocated.

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8 Discussion

8.6 Concluding remarks

The above integration of results is a subjective assessment. There are many caveats: Are these man-hour savings realisable as overall time savings on a contract? If the reinforcement becomes a quicker operation does another process become critical? Are we more interested in certainty of delivery (e.g. hit the pour date) than in overall timescales? Are there effects that are determined by size of building i.e. is practicality dependent on size (e.g. numbers of types of mats, timescales, rentals etc.)? How does the data from the one-off Cardington project relate to the real world? All productivity data are generally of uncertain quality Markets conditions vary

However, the results from Cardington appear to tie in with commercial realities. There are, therefore, opportunities to save time - not just man-hours on site but also contract duration which, on an individual project basis, saves preliminaries and finance costs. For instance, Lorien’s work found that an additional 7 tonnes of reinforcement in the form of mesh cut out 70 man-hours of traditional fixing on Cardington. Other methods of design can reduce the amounts of reinforcement required while maintaining the timebenefits. On a national basis the potential savings have to be significant.

The exercise of integrating the results has at least put a measure on the savings achievable and allowed current best practice based to be identified on a basis of overall economy.

The role of designers In using innovation, the designer must not be forgotten. Successful innovation should mean win-win situations all down the supply chain. Whilst the benefits of innovation have been described for contractors and clients, the additional effort and risk taken by engineers in using or allowing innovative methods is often not recognised or rewarded and this may be the reason for their inclination towards conservatism.

Innovation and rationalisation depend on designers and engineers to put it into effect. In this study, they have not been considered in any great detail as there was very little data. At the outset, it was hoped to keep records of design and detailing time for the options used at Cardington. However, this proved impossible as the design, detailing and procurement was so fragmented. Thus it would appear to be a very difficult exercise to benchmark or compare the work involved for one type of design against another.

When designing a structure, there are very many variables that affect the choice of design method. Those in charge of design make a choice based on their own and their clients’ preferences. This research has identified a number of areas for improvement, which, if they are to be implemented, will mean change in the current traditional methods’. Change costs money and it may also be perceived as involving some risk.

The process of getting proprietary items to site, (design, specification, nomination, discussion with constructors, checks, approvals, procurement) all cause additional effort and often the

It would appear that most flat slabs are currently (2000) designed using elastic methods of design, an #

increasing number are being designed using finite element analysis and very few are designed using yield line methods.

I19


benefit goes to the contractors or client only. Should designers be expected to make extra effort or take additional risk to make construction easier on site? Are they rewarded if they do?

Traditionally, the engineer does, and should, embrace and promote innovations. Faced with fixed competitive fees, extreme time constraints and Professional Indemnity Insurance there seems little incentive to change or to take risk - other than to be known for innovative and competitive design. In the long term, of course, innovative firms will be more efficient because of it and will have more work. However, the pace of change is so fast, and the applicability of innovation often becomes apparent only after a design is complete or at least well undenvay, that a designer's reluctance to change mid contract will not help in reducing the overall costs. Assuming delays are not caused to the construction process, a major barrier to reducing overall costs might be the lack of reward to the designer.

Do contractors have the skill or inclination to take responsibility for design and design co- ordination? In some cases, notably in the larger firms, the answer is undoubtedly yes, but in the majority of the industry the answer is less certain. Many larger specialist subcontractors put the design out to contract and reap the benefits of discussing their preferences with their designer.

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9 Conclusions

General The research investigated the costs of labour, materials and time, particularly with respect to the in-situ building at Cardington. It has highlighted the nature of the costs involved, the difficulties of obtaining reliable cost and time data and the different perspectives of the different stakeholders. This information has been drawn together in order to make comparisons and provide best practice guidance on the reinforcement of flat slabs.

Broadly speaking the conclusions align with what may have been expected by an experienced designer. However, rationalisation of flat slab reinforcement is a complex subject. It is much more than the mechanics of what a reinforced concrete detailer does on a drawing board or computer. Importantly, this research gives the basis upon which sensible decisions can be made on rationalising the whole process of reinforcing flat slabs.

Cardington Cardington provided a unique opportunity to compare methods of reinforcing flat slabs. The data obtained were for very many reasons imperfect. The overall programme of research was ambitious and may well have been over ambitious to the detriment of the research data for this project.

The data was clouded by the events on site, events that occur on any construction site. Manu- facturing levels of statistical correctness would have required between 40 and 60 samples or repetitions of the same operations. This is clearly not feasible on real construction sites. At Cardington, the reinforcement could have been assembled and disassembled a number of times. This procedure was considered but not adopted as besides additional expense, it this would have introduced unusual working conditions and probably flawed the data.

Due to the pressure on space and the compromises required, it was not possible to devote whole floors to blanket cover elastic design and blanket cover yield line design nor investigate a floor of blanket cover loose bar using finite element design. The advantages for finite element analysis were perhaps reduced on Cardington by being implemented with two-way mats (or vice versa): two-way mats are known to add weight.

Many of the findings in this report are based on data from Cardington. The data gave strong indications that were substantiated by comparisons with commercial information. They were better than any previous research data and were held to be a sound basis for subsequent comparisons.

Value chains and process To make concrete frames more competitive, the processes of optimisation and rationalisation of reinforcement need to be undertaken more widely. There needs to be some consistency and openness in methods of pricing reinforced concrete frames and some guidance on what constitutes economic practice.

One large area of risk is the weight of reinforcement in a project. Neither clients nor contractors like risk. One way of increasing certainty by making the amounts of reinforcement

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/


known at the earliest opportunity is to integrate computer programs for design with those for detailing, and indeed those for fabrication of bars’.

Within a supply chain, value can ‘leak out’ of the system unless each customer/supplier interface is well managed. No one organisation has control of the complete value chain. A vertically integrated solution must be found. This is most likely to result from either improved management of design and construction operations, by the use of information technology, by the greater use of non-traditional contracts or by education of all members of the supply chain.

Pricing policies and productivity rates are currently not transparent. The customer receives conflicting information about the most economical or best practice. Any optimisation currently undertaken may be carried out on a basis that does not hold true for the industry as a whole. There are very many options, not only in terms of the reinforcement and proprietary systems but also in terms of contract. So it is perhaps not surprising that it is only specialists who can work out what is best for the project in hand. Relationships are complex and there is a lack of hard data at a level of detail that would permit significant analysis. Even so, there appear to be opportunities to make significant cost savings. These would require a combination of informed design, system specific expertise and, particularly, process control and supply chain integration to eliminate delay and wasted time. All these could be achieved through management practices widely adopted in manufacturing industry.

Rationalisation of reinforcement has been defined as being the elimination of redundant variation. This can be achieved by applying best practice consistently.

Winners and losers Costs are different for different parties in the supply chain. Different contractual arrangements can make it difficult to identify the winners and losers from using rationalised or innovative reinforcement.

Depending on forms of contract, differences in preliminaries and construction costs may or may not be passed on to the client. If all savings are passed onto the client then there is little or even no incentive for the contractor to save costs or time - other than to win work. Conversely, if savings are retained or at least shared with the client then there is incentive for the contractor to save both time and costs. From this comes the current drive towards partnering and vertical integration.

Certainly the client, who ultimately pays for projects, should win, but the ideal is for win-win situations to occur right throughout the supply chain without risk. This situation is not always certain. For instance the difficulties and expense born by designers are not always recognised or rewarded. The incentives to innovate are unclear.

Correspondence Innovation appears to create correspondence on a project. Traditionally reinforced slabs (Floors 1 and 2) elicited least correspondence, contractor detailing (on Floors 3 and 4) more correspondence and contractor supplier design and detailing (of Floors 5 and 6 ) most correspondence. Most of the excess correspondence on Floors 5 and 6 was with suppliers. The more innovative systems resulted in more correspondence.

The amounts of correspondence may be seen as a measure of the effort (i.e. time and expense) required in managing the systems at Cardington. In practice, this management (design, design

’ Indeed this is beginning to happen with the introduction of Hy-ten’s BAMTEC system, which integrates finite element design with prefabricated roll-out ‘carpets’ of reinforcement.

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9 Conclusions

checks, procurement, delivery fixing etc) would cost someone his or her time. Traditionally this would be a designer who would probably go un-rewarded for the extra effort and risk involved in innovating. Undoubtedly the novelty of certain systems at Cardington caused correspondence to be initiated but this survey suggests that this may represent a significant barrier to innovation. For it to succeed, innovation has to be worthwhile and be seen to 'be worthwhile by everyone in the supply chain.

This exercise indicates that non-traditional methods involve more correspondence. In turn, this signifies that there are more relationships that probably need unproductive time to develop. On the other hand, it may just indicate that many of the processes (providing design information, design, drawing, checking, scheduling, approving) that are traditionally carried out in-house by the engineer are, in contractor detailing and contractor design-and-detailing, carried out by other parties contracted. This requires communication and design co- ordination. Design co-ordination is not necessarily core to the specialist subcontractor's business and might prove to become a barrier to better integration.

The philosophy of Just-in-Time delivery of information is just as important as Just-in-Time delivery of materials, but the management of design and management of information flow is not traditionally carried out by specialist subcontractors. If information arrives too early it can be changed by events. If it is, too late it causes delay.

Costs and time costs It was shown that optimising slab thickness saves money. Changing from a 300 rnrn slab to 255 mm slab on the Cost model study buildings saved 8% of frame costs. This potential optimisation is often set against increasing risk as thinner concrete slabs have less capacity to absorb late changes and the potential for higher deflections.

From productivity studies it has been demonstrated that different reinforcement arrangements can have significant impact on the costs of materials and labour. In the systems investigated at Cardington there was the potential to save up to 30% on flexural reinforcement and 50% on shear reinforcement over traditional methods of reinforcing flat slabs. This excluded any benefit fi-om reduced critical path time.

On the basis of the data recorded at Cardington, there are arrangements of reinforcement that offer significant cost advantages over the traditional systems. These advantages are maintained over a range of labour: reinforcement cost relationships.

Cost comparisons based simply upon the tonnage of reinforcing steel give a misleading indication of the benefits of alternatives. A cost structure based on weight alone will almost always inhibit prefabrication. Time costs are exceedingly important and should, wherever possible, be recognised and reconciled with the initial cost of materials and labour.

Time costs The cost of time depends upon the perspective of the user. For developers and owner/occupiers, early occupation brings early and extra revenue or rental income to counter costs of financing the project. For contractors and specialist subcontractors early completion should mean less time-related overheads.

The monetary value of potential savings for a owner/occupier client or speculative client with tenant can be in the order of 10 times greater than those for a subcontractor (see Table 8.1 1). In practice, finance costs dominate. The gross numbers and the opportunities to achieve savings are greater on larger buildings - and so are the pressures to achieve. Nonetheless, time costs should be considered on all buildings. On concrete-framed buildings, they should be

123


used to help determine the amount of rationalisation, the right configuration of reinforcement and, indeed, the appropriate type of analysis for the project in hand.

Finance costs have been defined as follows:

where F Finance costs j T,

T2 Construction time. T3 CL CD Cost of demolition, C,

Interest rate per unit of time Land acquisition - the time from acquisition of the land (waiting for planning permission and design plans) until construction starts.

Disposal time- the time from the end of construction until the building is let or sold. Cost of land, (including acquisition costs, compensation, fees)

Costs of construction. (including contract value, ancillary costs such as access roads, planning offsets and professional fees)

This formula gives the basis for designers to compare possible time cost savings against the costs of additional reinforcement.

It is interesting to note that the cost savings per square metre from saving time appears similar across a range of buildings (see Table 8.1 1). The same is true on a Urn2 basis for each of the parties involved. Large buildings would appear make the gross potential savings increasingly worthwhile.

Critical time Undoubtedly there are time savings to be gained by using rationalised configurations of reinforcement but their effects on critical time appear to be anecdotal or, at best, hard to judge. Critical path time on site cannot be obtained simply through rationalising reinforcement at the detailing stage: rationalisation must embrace the whole process of concrete frame construction in order to get worthwhile benefits on site. There is little advantage in completing reinforcement more quickly unless following work e.g. concreting, can start earlier.

The assessment of critical path time is almost always subjective. Whilst there are some ground rules, assessing critical times appears to be more an art than a science. Planners may apply basic rules but they apply many modification factors based on the many variables of the specific project and individual company methods. On site, managers have many ways of changing outputs on site (increase labour, overtime, etc.). There are acknowledged problems of acquiring accurate productivity data(”) but these data are required and need to be made available to designers if they are to become party to optimisation and rationalisation in the name of overall efficiency.

Design and performance Performance From a performance point of view, all the design methods used on the in-situ building at Cardington, elastic plane frame (equivalent frame), finite element and yield line, have to date produced satisfactory designs. Presuming all slabs continue to perform satisfactorily then the savings in reinforcement and, therefore, savings in time depend on the design method used.

It has been stated that all the sources of uncontrolled variation on data recorded at Cardington are typical of normal site operations. This work illustrates the impact of such events on the timing and installed costs of reinforcement systems. Add to this the difficulty of actual measurement and it is clear that feedback of site experience into the design process will tend

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9 Conclusions

to be very subjective. This may go some long way to explaining the wide variation in opinion on optimum design approaches, illustrated in no small measure by the interpretations manifested in the designs for this project.

Yield line design Yield line design appears to provide a great opportunity to reduce costs in flat slab construction

If the reinforcement for yield line design blanket cover could have been prefabricated and been fixed at the same rate as on Floor 6 it would have produced a first cost saving of some 30% over traditional methods. This is easily the best opportunity for improving concrete construction identifiable by applying reasonable 'what if analysis to the data obtained from Cardington.

Taking into account possible savings in critical path time, backed by the perceptions of specialist subcontractors for time savings, the case for yield line design is even more compelling.

Yield line design is a technique that has been around for many years but its commercial exploitation appears to have been curtailed by a lack of understanding, the fear that it is an upper bound solution (i.e. either a correct or a too high solution), and the lack of computer support. Unusually, it requires designers to use. their judgement.

If the opportunity is to be grasped then the industry design and construction teams must be given the opportunity to understand and use yield line design with confidence.

Shear reinforcement The ACI shear stirrups were typically half the cost of the traditional shear reinforcement system, although the data did not clearly highlight the benefits of the ACI system'. Unfortunately, their general use cannot be advocated before questions regarding design are resolved".

Commercially available shear stud and shear ladders provide an effective and efficient substitute for traditional shear links. At first sight the costs of proprietary systems may be off- putting compared with shape code 85 reinforcement. However, on a cost/m2 basis, the costs of these shear systems is relatively small compared with the cost of bending reinforcement and the potential time benefits are so large that their use is advocated for all but the smallest buildings.

It should be recognised that the procurement process for these systems differs from the traditional method of specifying bars on a bending schedule.

Structural steel shearheads are comparatively very expensive and disruptive to the process of constructing a flat slab. Nonetheless in highly serviced buildings, where large holes are required close to columns, their expense might be warranted.

All the time recorded against the ACI stirrup system was associated with prefabricating the stirrups pre- #

installation and was, in practice, outside the critical path for the floor. Also the procurement process was indistinguishable from that for the bending reinforcement, bars being delivered with the loose bending reinforcement.

There is a question about the design of ACI stirrups, especially with regard to holes. At the time of writing (Sept 2000). The Concrete Society's Shear Reinforcement Working Party wqs due to consider the design of ACI stirrups

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Simplicity The simplicity of the reinforcement arrangements again depends on the design methods. Yield line design gave the simplest flexural reinforcement arrangements. The finite element arrangement was simple, but to accommodate two-way mats and practical maximum bending moments, the general level of reinforcement was high.

I

The amounts of reinforcement used on all floors bar one (Floor 3) were actually dictated by considerations of serviceability, and it is perhaps with regard to serviceability that rationalisation of reinforcement design needs to be investigated.

Prefabrication It should be clear that prefabrication of the reinforcement could provide several benefits for all members of the construction team. These benefits include: e Simplified detailing resulting in lower design office costs. e Easier identification of steel on site 0 A reduction in the site labour cost of receiving, sorting and fixing. 0 Accurate placing of reinforcement. .

Improved site planning and organisation Faster steel fixing

e Faster completion of structure Lower overall costs Increased profits Improved return on capital

Deflection It would appear that the different design bases and arrangements and of reinforcement had little effect on deflections and the rules on deflection in BS 81 10 may need to be looked at in light of current knowledge@

Best practice There is a balance between first costs and time costs and costs are different for different parties to a contract, particularly when innovation or rationalisation is considered post contract. Based on the data from Cardington, the best valuehest practice approach to reinforcing flat slabs according to party to a contract and type of contract is shown in Table 10.1.

Current evidence suggests that there is little advantage in straying away from traditionally designed rationalised bending (or main) reinforcement on all but the largest buildings. For shear reinforcement the use of proprietary shear systems, specifically stud rails and shear ladders, appears to be almost always worthwhile.

0

examining this subject area. A final report is due by 2002 BCA’s Pi1 project “The.influence of serviceability on this economic design of concrete structures” is indeed

126

10 Recommendations Best practice guidance

In fulfilment of the primary objective of this project - to reduce the costs of flat slab construction by disseminating meaningful guidance on the rationalisation of reinforcement for flat slabs. The following table presents the preferred reinforcement configurations dependant on the party concerned according to the type of contract and the contract stage. It is based on the premise that best practice equates to most economic.

Table 10.1 Best practice from Cardington data -preferred reinforcement configurations for flat slabs dependent on party and contract

Partv

Specialist Main Client ’ Owner/ occupier subcontractor contractor client2

Main reinforcement (i.e. flexural reinforcement top and bottom) ~~ ~~

Innovating pre-contract (all forms of contract)


lnnovating post Construction Management contract


No real preference i.e. select to suit clients

Two-way . Rationalised mats and loose bar3. finite element design

Two-way Two-way mats and mats and finite element finite element design design

Rationalised Rationalised loose bar3. loose bar’.

Rationalised loose bar’

Rationalised loose bar3. Very large buildings - two-way mats and finite element design’ .

Rationalised loose bar. ’

Two-way mats and finite element design

Rationalised loose bar. Large buildings - two- way mats and finite element design

Rationalised loose ba?. Large buildings -two- way mats and finite element design

. I

Rationalised loose bar3. Large buildings - two- way mats and finite element design3

Two-way mats and finite element design

Shear reinforcement

Innovating pre-contract No real preference i.e. select to suit clients

Shear studs and shear ladders4 (all forms of contract)

Innovating post Traditional contract Construction Management contract Design and Build

Shear studs and shear ladders4

contract

Notes Large building arc defined here as those substantially larger than 4,500 m2 total on three floors 1 2

3

4

Clients subject to finance charges e.g. speculative clients without pre-let Owncrl occupier client subject to finance charges and rental income e.g. speculative client with pre-let agreement or owner/ occupier Based upon specialist trade contractors perception of time, the most economic option for these circumstances would be yield line design ACI shear stirrups actually came out on top but there are some questions against their, use especially with regard to holes. Their general use therefore cannot therefore be advocated before these questions are resolved.

I27


Yield line design Yield line design appears to provide a great opportunity. If the opportunity is to be grasped then the reinforcement industry must present designers and the wider construction industry with comprehensive design guidance and design aids providing them with the opportunity to understand and use yield line design with confidence. A raft of information, based mainly on simple design guidance should be put in place so designers can use their judgement to give their clients economic designs incorporating best practice.

The possibility of providing a set of standard designs coupled with the use of standardised meshes should be investigated.

costs A costing structure based on weight will almost always inhibit prefabrication and innovation. Time costs are exceedingly important and time costs saved through prefabrication or innovation often outweigh considerations of materials and labour alone.

The value of potential time savings must be acknowledged by design and construction teams. It should be possible for cost consultants to put a value on time, perhaps in terms of €/m2/day, so that innovations might be judged against it. Cost comparison should, as far as possible, embrace whole construction costs - including the effects of time.

Process integration In order to fully embrace the benefits of innovative techniques the concrete frame industry should aim for greater vertical integration by: 0

0

0

Improved management of design and construction operations, The use of information technology, Greater use of non-traditional contracts and Education of all members of the supply chain

For instance, the certainty of using the correct weight of reinforcement in estimates could be increased by integrating design programs with detailing programs and ultimately with bar bending machines.

In order to achieve optimisation of flat slab design and rationalisation of reinforcement, some consistency in presenting productivity rates and methods of pricing should be established and made known to designers and specifiers.

Further research Benchmarking-1 Studies to identify and analyse value chains in detail should be encouraged. The best practice information included in this report provides pointers towards low cost design approaches. More data are required in order to determine optimum design for different arrangements of reinforcement. With the demonstrated variability of design input and site practice, a practical way to obtain these data would be to organise an industry-wide data gatheringhenchmarking exercise (see Lorien's proposals, Appendix I11 supplement 3). Of particular interest would be productivity rates for reinforcement derived from loose bar finite element designs and loose bar yield line designs.

Benchmarking-2 Optimisation of design on an industry-wide basis cannot be achieved without the provision of clear rates for productivity, labour and material, and standardisation of the design, construction and procurement process. Of course much of this information is commercially

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10 Recommendations

sensitive and might be very difficult to obtain from commercial organisations. In addition it would most probably be very difficult to compare one rate with another. But this is the very essence of benchmarking - harmonising methods and methods of measurement so that best practice can be determined and promulgated. *

ACI stirrups The system referred to as ACI stirrups possesses many process benefits. Their use requires no special measures as they can be detailed on normal reinforced concrete drawings and scheduled on normal r.c. bending schedules. To fix on site they were ' I . . . . a five minute job" and off the critical path. Unfortunately the design method is empirical with apparently little theoretical basis and makes no provision for holes in the slab close to where they are used. ACI stirrups should be fully researched so they can be proved technically and so that design rules can be developed and accepted by statutory bodies. In particular, research is required to assimilate design methods with those of BS 81 10 and EC2 and to determine how they may be used in close proximity to holes.

Influence of designers The designer has great influence over how and when innovative techniques are used. An interesting area of study would be look at this influence and the degree of take up of innovation in regard to rewards and risk. A study of how the designer is affected by the various contractual arrangements would also be an interesting area for further research@.

Just-in-Time delivery of information is just as important as Just-in-Time delivery of materials. The concrete industry should do whatever it can to help designers design effectively and efficiently so information is provided in a profitable and timely manner.

A study of how innovations zffect the designer under the various contractual arrangements would be an interesting area for further research.

@ This area is being looked at under the research project Assessing Concrete Technology Innovation using Value Engineering (ACTIVE) which is currently (Sept 2000) being undertaken by the Post Graduate Research School, School of Architecture, Oxford Brookes University.

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References EGAN, J (Chairman), Rethinking Construction, The Stationery Office, London, 1998.

THE CONCRETE SOCIETY. Towards rationalising reinforcement for concrete structures, Slough, 1999. Ref. CSTR 53

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DARZENTAS, A I; DEASLEY, P J, & ROGERSON, J. Re-engineering the concrete frame process, Quality & Process Improvement Department, University of Cranfield, 1998.

PALLETT, P. Guide forflat slab fornzwork and falsework (Draft), Construct, Crowthorne, 1999.

CHANA, P. The European concrete building project, The Structural Engineer, Vol78 No 2, 18 Jan 2000.

JOHNSON, D. Bending strength evaluation of the ECBPfloor slabs, Department of Civil and Structural Engineering, The Nottingham Trent University, October 1998. (Included as Appendix VII)

VOLLUM, R L. A review of slab deflections in the in-situ concrete frame building, Dept. of Civil and Environmental Engineering, Imperial College, November 1999 (interim report on an ongoing TMR research programme).

BRITISH CEMENT ASSOCIATION. Improving concrete frame construction, European Concrete Building Project Best Practice Guide, BCA, 97.50 1, Crowthorne, 2000.

BRITISH CEMENT ASSOCIATION. Concreting for improved speed and eflciency, European Concrete Building Project Best Practice Guide, Crowthorne, 2000. Ref. 97.502

BRITISH CEMENT ASSOCIATION. Early age strength assessment of concrete on site, European Concrete Building Project Best Practice Guide. Crowthorne, BCA, 2000. Ref. 97.503.

BRITISH CEMENT ASSOCIATION. Improving rebar information and supply (IRIS), European Concrete Building Project Best Practice Guide. 97.504, Crowthorne, BCA, 2000. Ref. 97.504.

BRITISH CEMENT ASSOCIATION. Early striking and loading ,offlat slabs for efjcieiztflat slab coiistruction (Draft), European Concrete Building Project Best Practice Guide, Crowthorne, BCA, due 2000.

BRITISH STANDARDS INSTITUTION. Eurocode 2: Design of concrete structures; Part I : General rules and rules for buildings together with UK National Application Document, London, BSI. DD ENV 1992-1 -1 : 1992.

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CLAPSON, J D. Rapid reinforcement technology - catching up with established practice in Europe. Text for paper for Advances in Concrete Construction at Westminster College 1994.

KHOSROWSHAHI. F. The optimum project duration and cost curve for Hong Kong public housing projects, Engineering, Construction and Arclzitectural Management 1997,4

BENNETT, D & MacDONALD, L A M. Economic assembly of reiizforcement, British Cement Association, Slough (now Crowthorne), 1992.

HERRKOMMER, F & m T T O N , P & BRODMEIER, J. Bauen mit BetonStahhlmatten Ein aktueller Uberblick. (Construction with steel wire mesh fabrics) Beton Herstellung Verwendung, Vol 33, No 11 November 1983.

PROVERBS, D G, HOLT, G D & OLOMOLAIYE, P 0. A comparative evaluation of reinforcement fixing productivity rates amongst French, German and UK construction contractors, Engineering, Construction and Arclzitectural Management 5,4. pp 359- 358, Blackwell, 1998.

CHRISTIAN J, & HACHEY D, Effects of delay times on production rates in construction. Journal of Construction Engineering and Management, Vol 12 1 No 1 ASCE, March 1995.

THE STEEL REINFORCEMENT PROMOTION GROUP. Guidelines for economical assembly of reillforcement, Technical Policy Note No 2, June 1988.

THEOPHILUS, J. Rationalised reinforcement design, Concrete, Vol29, No 4 MarcWApril 1995.

CLARKE, J L. Rationalisation of reinforcement, Concrete, June 1999.

GRAY, C. The rationalisation of reinforcement, University of Reading. Report for Concrete Society Rationalisation of Reinforcement project. (Unpublished)

Committee papers, Concrete Society Rationalisation of Reinforcement project. (Unpublished)

UNIVERSITY OF READING. A study of reiizforcement procurement. Concrete Society Report 121. Crowthorne, The Concrete Society, 1999. Ref. CS 121

GOODCHILD, C. Cost model study, Crowthorne, British Cement Association, 1993.

WEBSTER, M. Further cost model studies: RCC’s findings, Concrete, MarcWApril 1995.

ALLEN, J. Reengineering the design and construction process. The Structural Engineer, Vol76 No 9 , 5 May 1998.

MOSS, R & MAW, J. Re-engineering the processes for in-situ concrete construction. Paper at IStructE, 13 May 1999.

GOODCHILD, C & MOSS, R. Reinforcing Cardington. Concrete, Vol 33 No 1, January 1999.

132

References

32

33

34

35

36

37

38

39

40

41

42

43

44

BRITISH STANDARDS INSTITUTION. BS 81 10 Part 1 1997, Structural use of concrete, Part I . Code ofpractice for design and construction, London, BSI.

AMERICAN CONCRETE INSTITUTE. Building Code requirements for reinforced concrete (ACI 3 1 XM-89) (revised 1992) and Commentary (ACI 3 18RM-89) (revised 1992), Detroit 1992.

UNIVERSITY OF LOUGHBOROUGH. Improving rebar information and supply (IRIS), Dept of Civil and Building Engineering, (Report in fulfilment of contract for Building Research Establishment) February 2000.

JOHNSON, D. Mechanism determined by automated yield line analysis. Tlie Structural Engineer, (1994) Vol72, No 19, pp 323-327.

CEB-FIP Model Code 1990, Thomas Telford London, 1993.

JACKSON, P A. The stress limits for reinforced concrete in BS 5400, The Structural Engineer, Volume 65A, No 7, July 1987.

LORIEN plc, Rationalisation ofjlat slab reirforcement: Report on analysis of construction process data reiilforcemeiit. (Report in fulfilment of contract) Cardiff, 1999. (Included as Chapter 7)

CONSTRUCT. A guide to contractor detailiiig of reiiforceineiit in concrete, Crowthorne, British Cement Association, 1997

Telephone conversations with industry contacts

E-mail correspondence with North American engineers.

GOODCHILD, C H. Economic concrete frame elements. Crowthorne, British Cement Association for Reinforced Concrete Council, 1997. Ref. 97.358.

BEEBY, A W. Early striking of forinwork and forces in backprops. Final report, December 1998, School of Engineering, University of Leeds.

BRITISH CEMENT ASSOCIATION. Rationalisation ofjlat slab reinforcement (Draft), European Concrete Building Project Best Practice Guide, Crowthorne, BCA, due 2000.

133

Appendices

Appendix I = Details of reinforcement

Supplement 1 Proposals for reinforcement of the in-situ concrete building at Cardington - as built.

Supplement 2 Summary of bending schedules used at Cardington.



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Appendix I1 - Summary of interviews for Concrete Society research

Initial work on The Concrete Society's Rationalisation of reinforcement project(2) included a report by the University of Reading(24) based in part on a series of 12 interviews with members of the concrete industry(25). Whilst the interviews were unpublished, they provide interesting insights into some of the attitudes towards rationalisation and prefabrication. An outline of these interviews is given below:

Interview A (Engineer) Lack of experience with mats Fabric not competitive on price Fabric not adaptable Cost more important than time Lack of confidence in price Reluctant to price accurately Clients believe min cost = min material How to quantify benefits? Design costs Fee 1.5 to 1.7% of contract of which:

30% Getting arrangements right 30% Analysis and engineering 25% Rebar and detailing 15% Contract Administration

Rationalisation - restricted application Prefabrication increases weight 58 kg/m2 to 65 kg/m2 Labour saving 60 - 70%

Interview B (Engineer) Experience: JCT 50% D&B, MC 50% No experience of prefab No encouragement for prefab from contractor Least cost drives decisions Get better price if contractor not rushed No incentive for efficiency Conservatism

Interview C (Engineer) Experience: JCT 70% D&B, CM, MC 30% Design costs

30% Getting arrangements right 30% Analysis and engineering 15% Rebar and detailing 25% Contract Administration

Prefab on slabs does not save detailing time Awkward sites with heavy (25 mm diam) bars unsuited to fabric.

Prefabrication on beams and columns saves detailing time Innovation driven by developers and needs of industry {Not contractors!} Guestimates bar weights and diams Unlikely to re-detail Standard mesh not heavy enough Lack of repetition Opposition from draughtsmen protecting trade Germany - regular grids, repetition, standardised depths and sizes - different customs, rules etc Rationalisation needs repetition

Interview D (Engineer) Experience: JCT 90% D&B, CM, MC 10% Design costs

25% Getting arrangements right 40% Analysis and engineering 25% Rebar and detailing 10% Contract administration

Traditionally use loose bar Estimates done by experience Prefab needs regular grid Flat slab OK Trough slabs OK for prefab

Interview E (subcontractor) Experience: JCT 100% - supply & fix 50%; fix only 50% Labour rate varies inversely to bar diam Fixers paid day rate based on bar diams. No experience of mats Weight and size of rebar determine price

138

Appendix I1 - Summary of interviews for Concrete Society research .. .

Interview F (subcontractor) Experience: JCT 100% - supply & fix & fix only Labour costs

Flat slabs €1 50/tonne Trough € 175ltonne Waffle €195/tonne Tie wire, spacers and chairs €12/tonne Wastage 1.25%

Interview G (specialist subcontractor) Experience: CM & MC 70% D&B 30% Reinforcement

Bar (6 to 40 mm) supply €245/tonne Labour €250/t for 8 mm

to E70/t for 40 mm €275/t for links

Spacers etc €4/t plusO/H&P@ 5% = €525/tonne

Labour E0.50/m2

(Flat slabs 10% less one price for all diams) Mesh

Loose bar easy to adapt. Loose bar very competitive Labour familiar with loose bar Can be problems integrating loose bar with fabrics

Interview H (specialist subcontractor) Experience: CM & MC 100% Reinforcement

Bar (6 to 40 mm) supply Labour &220/tonne Spacers etc E70Itonne o m w 5%

€250/T

&525/tonne (Flat slabs 10% less one price

for all diams) Mats

A142 0.75 €/m2 A252 1.30 i/m2 A 393 2.10 i/m2 Labour 0.80 i/m2 Laps 3% O/H&P 5%

For rationalisation need standard types of fabric. Need prefab scheme drawings with tender

Interview J (specialist subcontractor) Experience: CM 90% Don't believe prefab of slabs on large projects is practical or economic Generally prefab beams, walls and columns on site Need space for on-site prefab Reinforcement rates Bar (6 to 40 mm diam) supply €240/t

Labour 40 mm diam. Links €275/t

E200/t for 8mm to €lOO/t for

Special fabrics are not cheap Fixing rates: Two-man team 20 - 30 mm bar 2.0 tfday

8-12 mm bar 0.5 tfday

Interview K (fabricator) Experience: 90% loose bar 10% fabric Price driven Prefab needs regular grid Most economic structure : include time benefits, reduced prelims, more commissioning time Loose bar costs €235/t C&B delivered; sensible price €265/t; comes in at €2 15 from mills BS fabric €320/t Output doubled with fabric Project in Bridgend: loose bar costs €250/t C&B delivered; fabric €390/t

Interview L (fabricator) Experience : 45% loose bar 55% fabric was 70% loose bar & 30% mesh Loose bar - low tech and little added value Contractors buy lowest cost Fabric subject to market shortages Prefab should reduce engineer's time - does it? Loose bar costs €235/t BS fabric 6-12 mm €320/t Bespoke fabric €375/t

Interview M (mill) Rebar is commodity JCT contracts cheaper than CM CM quicker but maybe 5-7% more expensive CM onus on tenderer to work up quantities - on JCT its to SMM7

139

... _ . Rationalisation of flat slab reinforcement , -

Appendix I11 Supplements to Chapter 7, Cardington: Analysis of construction process data

Supplement 1: Data recording for Cardington project - Method statement published prior to construction

Supplement 2: Notes on construction and measurement process difficulties

Supplement 3: Industry wide data collection and analysis: Outline proposal from Lorien

140

Appendix I11 Supplements to Chapter 7, Cardington: Analysis of construction process data , ”.

Supplement 1: Data recording for Cardington project - Method statement published prior to construction

The key data required from the site is the time taken to undertake the following processes: 1. Unloading steel from delivery wagon and placing into stockpile 2. Collecting steel from stockpile and placing on formwork 3. Fixing chairs, bottom mat and edge steel 4. Fixing shear steel 5. Fixing top mat 6. Inspecting steel and giving approval for concrete pour (including rework)

For each process it is necessary to measure: 0 Total elapsed time 0 Total man hours, broken down into:

Value adding Normal non-value adding Cardington related non-value adding

Note - it will be assumed that the proportions of e,dpsed value adding and non-value a d n g time are in the same proportions as value adding and non-value adding man-hours. (Notes should be made if this is not the case).

These measurements are required for each of the reinforcement variants being investigated. There are a number of features of the experiment which need to be addressed:

Floors 4, 5 & 6 have different flexural reinforcement on their north and south sides and different shear reinforcement on their east and west sides.

The fixing schedule is constrained (as on real sites) by the concreting schedule, the more so because of the early striking experiments.

Some of the combinations and layouts are novel and unfamiliar to the fixing team.

The following precautions will be taken to minimise the difficulties identified above: 0 On EVERY floor the time taken to fix the top and bottom mats will be noted for the north

and south halves of the floor separately. This is to enable us to estimate correction factors to apply from the floors where the steel is identical in each half. Similarly the times for shear reinforcement will be noted on east and west sides. As far as it is possible all reinforcement should be fixed at a consistent work rate for different systems in order that differences in difficulty of fixing are reflected in the times recorded. Where faster fixing rates are required to meet concreting timetables, etc. this should be recorded. To minimise learning curve effects, each reinforcement layout should be considered by the fixing gang pre-fixing and the job worked through mentally. The fixers should be given training on proprietary shear systems where they are unfamiliar and any combinations of shear and flexure reinforcement that may create clashing and other problems should be tried out before fixing ‘for real’.

141


It is likely that fixing bottom mat, shear and top mat may will, in practice be carried out concurrently. It must be accepted that the break down between these three processes will therefore, to some extent, be an estimate. It is important that the total combined elapsed time and man hours consumed for all three processes is as accurate as possible (i.e. not subject to cumulative errors). Similarly, even if the three processes go on sequentially any gaps between them (other than those relating to Cardington effects) should be recorded as non-value adding activity in the following process.

It is possible that the steel representing two or more variants will be delivered together. As above, the total times taken for the whole delivery should be recorded and, if practical, an estimate of the proportions of time associated with each variant made.

Some fixing operations may require crane operations (e.g. placing mats). Time arranging items on the floor for subsequent crane installation will be counted into process 2 and craning into place will be included into, for example, process 4.

ACI stirrups are prefabricated prior to installation. The prefabrication time will be recorded as ,

a separate process (2a) but included into process 2 for analysis purposes.

Measurement of times will be determined by a time sheet compiled by the fixing foreman cross referenced with video and still photography and the BRE site diary.

For the first floor, Lorien will provide an engineer experienced in process monitoring to trial the foregoing with a view to refinement if required.

142

Appendix I11 Supplements to Chapter 7, Cardington: Analysis of construction process data _,

Supplement 2: Notes on construction and measurement process difficulties

General 0 The fixing gang varied from 1 to 6 people throughout. Three of the people used were

experienced fixers, the others carpenters, etc. None of the systems being used were tried out off the job to reduce learning curve issues. The fixes had previous experience of ROM shear ladders and little experience of any of the other systems used. Various other features affected progress are broken down per floor as follows:

0

0

Floor 1 0 Problems with formwork. 0 Reinforcement still being fixed during casting, otherwise nothing abnormal.

Floor 2 0 200 extra bars fitted with strain gauges in areas U & T(see Figure 7.1). Some 7 hours

associated ‘waste’ were identified on time sheets but U & T were perhaps a further 7 hours slower than the norm. Tower crane broke down on 11 February and reinforcement pulled up on rope. No formal time loss recorded. Possible 6 man-hours wasted in comparison with other areas? X & T steels (see Chapter 5.9, Other research, ductility) delivered together and had to be sorted out on site. This activity does not seem to have been separately logged.

0

Floor 3 0 200 strain gauge bars fitted. 8 hours of waste logged but again affected areas were

perhaps 7 hours slower than the norm. Floor was run tight onto concreting dead line. Formwork wasn’t ready thus floor was released a bit at a time for fixing and the crane was not fully available. Stop end fixed down middle of floor led to fixing problems. Timesheets don’t specifically identify losses.

0

0

Floor 4 Programme shortened by 2 days leading to pressure on fixing. Shear heads did not arrive to plan - substitute with ROM shear ladders. 2 days delay for shear ladders caused programming problems. Crane breakdown for 1 !4 days. ACI shear links prefabricated off job and required only 5 minutes to drop into place (NE3 time recorded is therefore not on critical path).

Floor 5 0

0

0

0

0

Fixers not used to welded mesh. Shear hoops new system to fixers. No practice. No experience of combining with fabric. Deha stud rails new system - no experience. 20 fabric mats delivered late and therefore upset sequence. 25% of mats, approximately, too heavy for hand lifting and needed crane support.

143


0 Edge steel proved hard to fix and had to be 'fiddled in' after top steel. Some of this time is included in top mat, some in bottom. Spacers and chairs provided were not ideal for fabric - needed to be longer. 0

Floor 6 0 Floor had to be left open for late delivery of shear heads, shear heads in area, rest SC85

links. Top steel had to be lifted, cut and refitted around shear heads. Mats could be man handled (four-man lift) without crane.

0

0

144

Appendix I11 Supplements to Chapter 7, Cardington: Analysis of construction process data

Supplement 3: Industry wide data collection and analysis: Outline proposal from Lorien

Introduction We have recommended that a benchmarking/data gathering exercise be used to enable a move from the indicative data contained in this report to firm recommendations that could lead to significant cost reductions in the industry. This appendix sets out the broad principles we would recommend. We would be happy to help you develop them further.

The subject is presented in the form of answers to the questions. 0

0 What data to collect? 0

Why data gathering and benchmarking?

How to collect and analyse it?

Data gathering and benchmarking The key problems leading to uncertainly in the conclusions presented in this report have been: 0 High levels of systematic variation introduced by ‘site practice’.

Variations in interpretation of concept by different designers.

Without some performance feedback, it seems improbable that the concrete frame industry as a whole will spontaneously start to tackle the issues of process re-engineering required to address the first of these points. Unfortunately the current level of uncontrolled process variables makes it difficult or impossible to get reliable measures of differences between the various approaches.

This problem could be addressed by collecting a much larger population of data through which the systematic variables per site would become randomised. This would, of course, require significant effort from contractors. By running an ongoing benchmarking exercise as the data are collected the participants would get feedback to help them improve their productivity which, hopefully, would compensate them for their efforts and motivate them.

This approach would have the disadvantage of tending to skew the data as efficiency improved throughout the sampling period. This minor inconvenience and slight reduction in precision would, in our opinion, be more than off set by strengthening commitment from of the data collectors and the immediate taking of ‘early wins’ by the industry.

The amount of data required will depend upon the ambitions of the participants and the magnitude of differences between systems they are interested in detecting. Broadly speaking, the sample size required to detect differences are small and/or the variability is high. It is illustrated in the body of this report that if the variability in the industry as a whole is found to be in the same order as that found at Cardington then tens of samples will be required per system to detect differences between them of 10%.

Data to be collected We believe that two key pieces of information should ultimately flow from this work:

1. The overall most economic reinforcement solution for a given structure in terms of: Reinforcement used Fixing time costs Critical path extension

145


2. Data to enable the overall costs of these activities to be driven down through design feedback, process re-engineering, etc.

We consider that these two objectives have subtly different key measures associated with them: 0 Absolute cost per m2 floor space. 0 Cost per m2 floor space, normalised to a ‘standard’ structure.

The latter measure will be required to separate the effects caused by the choice of span, floor thickness, number of storeys, etc. from the effects of reinforcement arrangement, site operations, etc.

Critical path extension time is relatively complex to measure but in many instances it is of very significant value. We would recommend measuring the more straightforward steel and fixing man hour data and then build in critical path measurement at a later date.

,

In summary we would propose starting out to collect data per site on: 0 Structure details 0 Areas of suspended floor 0 Reinforcement usage 0 Man hours reinforcement fixing

Collecting and analysing data This report has demonstrated a methodology for collecting reinforcement fixing data. Key features are: 0

0

Breaking down the activities into generic categories Standardised methodologies for recording the data

The former would be the product of a well-designed scheme; the latter would derive from written procedures and, possibly, a small amount of training.

The following would need to be broken down into a manageable number of defined categories: 0

8

Structural element (column, beam, slab), Structural design solution (flat slab, column and beam) (RCC’s Publication Economic Concrete Fraiiie Elements (42) provides a suitable basis) Reinforcement arrangement (traditional, blanket cover, mesh) Work activity (fixing, site movement, standing time)

0

Finally, some structural parameters would need to be developed which could be used to ‘standardise’ the data. These could be, for example:

Floorarea Number of floors Diagonal distance between columns Maximum stress in the floor plate

It would be necessary to determine the relationship between these parameters and reinforcement usage and fixing time. We see this as the most technically challenging aspect of this proposal. Our approach would be to create a synthetic data set, including estimates of fixing times and steel usage, and then experiment with regressing the parameters chosen in

146

Appendix' I11 Supplements to Chapter 7, Cardington: Analysis of construction process data

order to select the best indicators. As real data became available this could be used to check the relationships derived from synthetic data.

From the foregoing, it would be possible to take collected site data and reasonably quickly build a database from which to identify the most cost effective reinforcement arrangements for different structures and give feedback to participants of the importance ranking of their input data compared with other contributors' data.

In practical terms, the scheme would have to be run through a trusted third party such as Construct, RCC or The Concrete Society. .

147


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Appendix VI

Reinforcement drawings for Floors 1 to 6 (reduced to A4 size)

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Appendix VI1

Bending strength evaluation of the ECBP floor slabs.

171

Bending Strength Evaluation of the ECBP Floor Slabs

Dr David Johnson

Department of Civil and Structural Engineering The Nottingham Trent University

Burton Street Nottingham NGl 4BU

2 October, 1998

172

Bendinn Strength of ECBP Floors

EXECUTIVE SUMMARY

Automated yield-line analysis [*I has been applied to the first six floors of the in-situ building of the European Concrete Building Project (ECBP) at Cardington (including the two differing halves of floor four). The objective was to evaluate the bending strength of the floors and thereby to establish any variation in bending capacity resulting from the differing design and detailing approaches used for the floors. The results obtained are summarised in Table 1 below.

Table 1: Bending strength summary

Floor Bending Reinforcement Weight‘ LF” (tonnes) (fine)

1 st Traditional loose bar. 16.4 1.13 2nd Traditional loose bar - high ductility. 16.6 § § §

3rd Rationalised loose bar. 14.7 1.19

4th N:Yield-line design.

S:Blanket loose bar 5th One-way mats.

14.2 1.16

22.9 1.34

19.3 1.15

6th Two-way mats. 25.0 1.51

’ Weightlfloor of bending reinforcement in slabs (i.e. excluding shear reinforcement and reinforcement in upstand beams). On floor 3, compliance with normal deflection criteria was specifically excluded from the design; an extra 1.6t would have been required to meet normal EC2 deflection rules. In the case of floors 4N and 4s weights have been scaled from schedules from half a floor. ” Load factor against failure at ultimate loads. ’” Not analysed as closely similar to floor 1.

The principal conclusions arising from the investigation are that:

the floors are satisfactory in respect of bending strength in that all designs provide a load factor over ultimate design strength of at least unity. the design for Floors 1-5 (with the exception of Floor 4s) provide very similar load factors and are sufficiently closely designed to make further reinforcement economies unwarranted. Floors 4 s and 6 have higher load factors and some reduction in the reinforcement steel could be made to make the load factor closer to unity. This is particularly the case for Floor 6 , where the reinforcement of the uniform two-way mesh can be readily reduced to effect a proportionate load factor reduction.

173

Bending Strength of ECBP Floors

b l 400 x 250 RC Column

3

400 x 400 1 RC Column

4

INTRODUCTION

B U stand beam ] I

I 1 I

I I I I I I

I

I

upstand beam 1.5 1.5 1.5

I _ _ - L 1 .

Figure 1: General arrangement of typical floor of ECBP

Floor Slab Arrangement

The floor slabs of the European Concrete Building all have a similar plan (figure l), being a rectangular (three by four) arrangement of twelve 7.5 m square flat slabs supported by 400 x 400 internal columns and 400x 250 perimeter columns. There are seven floors in total and each was individually designed (table 2) to explore different aspects of flat slab reinforcement arrangements

The floors were designed for a combined (dead + live) ultimate load of 14.7 kN/m2 (uniformly distributed) and line loads of 11.95 kN/m along the longer edges and 6.75 kN/m along the shorter edges. The various designs all complied with EC2, using dead and live load factors, gk = 1.35 and qk = 1.50, respectively. The material properties employed were C30/37 concrete (fk = 30 N/mm2; f,, = 37 N/mm2) and fy = 500 N/mm2 for the reinforcement.

174


North Grids 1-3

Table 2: Bending reinforcement arrangements for the ECBP floor slabs

South Grids 3-5

I Floor I

Normal ductility

Bending Reinforcement

High ductility

4th

I lst I

Normal ductility High ductility

Blanket cover loose bar to sensibly minimise number of bar marks

Traditional loose bar to sensibly minimise steel content

Traditional loose bar to sensibly minimise steel content

I 3rd I Rationalised loose bar to minimise costs

Elastic design I Yield-line design

I 5th I One-way mats with local supplementary loose bars

I 6th I Two-way mats with local supplementary loose bars

I 7th I Hybrid of pc Omnia plates, Densit and in-situ ~

Automated Yield-Line Analysis ‘*I

To determine the mode of collapse, a quarter of each floor (on the assumption that the general arrangement is approximately symmetric) was first analysed using a general triangulated mesh of the form shown in figure 2a. From the detailed design drawings, the reinforcement in each of the rectangular subdivisions of the mesh was determined. These reinforcement details were converted into ultimate moments of resistance, as detailed in Appendix 1, for positive and negative bending and a system of moment of resistance “zones” was thereby established (figure 2b).

‘ column a. b.

Figure 2: Quarter slab model a. triangulation b. typical moment of resistance zones

175


Following the quarter slab analysis, a more refined mesh was devised which was specific to the collapse mode identified by the preliminary analysis. The finer mesh provided a more accurate representation of the geometry (for example the positions of the voids and the finite sizes of the columns) and also of reinforcement curtailment positions. Having obtained a yield-line pattern, it is possible, in theory[31, to vary the positions of the yield-line intersection points and hence to determine the critical positions for these points, which are such as to minimise the load factor. In the present case, however, no geometric optimisation of the yield-line pattern was undertaken. This was partly because the effects of geometric optimisation were expected to be small, in view of the fineness of the meshes used, and partly because optimisation would have been difficult due to the many different moment of resistance zones employed.

TYPICAL RESULTS

8.888

-8. 143

-8.286

-8.429

.... ” 0 -0.571

-8.714

-8.857

-1 .EBB .......................

a. b. Figure 3: Typical quarter slab (floor 1) a. yield-lines b. contour plot of collapse mode

In the case of the first floor, the quarter slab analysis predicted the yield-line pattern shown in figure 3a, with the associated contour collapse mode shown in figure 3b. The collapse mode is local to the north bays (AI-Cl in figure 1) and involves a negative yield-line along the penultimate row of columns and local “fan” collapses around the perimeter columns. In order to position the void more accurately, the north bays were meshed more finely. This also enabled the finite size of the columns to be incorporated and for the inclusion of the local reinforcement at the slab edges to be modelled. A second analysis was undertaken then undertaken, the results of which are shown in figure 4.

The fine analysis of figure 4 shows that the collapse mode has been modified by the new meshing so that the collapse is now of the “propped cantilever” variety, in which the hogging yield-line is along the faces of the penultimate row of columns and is accompanied by a sagging in-span yield-line, which is continuous rather than being affected by the local column fan mechanisms of the coarse analysis (figure 3). The finer analysis also increases the load factor significantly from 1.01 to 1.13. This is partly attributable to the decrease in effective span, due to the representation of the finite size of the columns, and is also no doubt partly caused by the incorporation of

176


..............................................

(b A

U 3

n n ..............................................

a.

b. Figure 4: Typical finer analysis (floor 1) a. yield-lines b. collapse mode contour plot

COMPLETE RESULTS

The results for each floor, including the two separate designs used for the upper and lower halves of floor 4 are presented in Appendix 2. For each floor, the yield-line pattern and a contour plot of the collapse mode are provided for both the quarter slab mesh and for the refined mesh analysis. In the case of floor 6, two fine analyses are shown since the yield-line pattern and contour plot of the first fine analysis (figure A2- 13) indicated that collapse of the north bays might be the critical mode, rather than the west bays mode predicted by the quarter slab analysis (figure A2-12). The second fine analysis (figure A2-14) did, in fact, result in a lower load factor, showing that the north bay collapse mode was the critical one. It is quite common for several collapse modes to coexist at very similar load factors and mode predictions are therefore much less reliable than load factor values, the latter, of course, normally being of primary interest.

A summary of the collapse load factors and collapse modes resulting from the various analyses is given in table 3 . From the table it may be observed that the finer analyses all produce increased load factors over the quarter slab results, although the increase is small in the case of floor 4, where the improved representation of the reinforcement arrangement presumably led to a weaker moment of resistance pattern, the effect of

177


The “fine” load factors all exceed unity, and are therefore “safe” in the sense that the yield-line analysis predicts that the design ultimate load will be achieved, with some reserve of capacity, dependent on the degree to which the load factor exceeds unity. For most floors the reserve is modest and there is no significant scope for reinforcement economy. Floors 4 s and, more especially, Floor 6, do, however, have load factors significantly in excess of unity and some economy could be made. This would be particularly straightforward in the case of Floor 6, where the uniform reinforcement provided by the two-way mats could be readily reduced so as to lower the capacity by up to 50%. The T12 @, 150 mm both ways T&B, could, for example, be reduced to T10 @, 150 T&B to achieve this.

Table 3: Load factor and collapse mode summary

~

Floor Bending Reinforcement Weight‘ LF“ LF Collapse mode (tonnes) (quarter) (fine)

~~

1 st Traditional loose bar. 16.4 1.01 1.13 North bays.

2nd Traditional loose bar - 16.6 1.01 § § § North bays high ductility.

3rd Rationalised loose bar. 14.7 1.05 1.19 West bays.

4th N:Yield-line design. 14.2 1.01 1.16 Internal bays.

S:Blanket loose bar 22.9 1.32 1.34 South bays 5th One-way mats. 19.3 0.99 1.15 North bays.

6th Two-way mats. 25.0 1.33 1.51 North bays.

Weightlfloor of bending reinforcement in slabs (i.e. excluding shear reinforcement and reinforcement in upstand beams. On floor 3, compliance with normal deflection criteria was specifically excluded from the design; an extra 1.6t would have been required to meet normal EC2 deflection rules. In the case of floors 4N and 4 s weights have been scaled from schedules from half a floor. ” Load factor against failure at ultimate loads. ”‘ Not analysed as similar to floor 1.

CONCLUSIONS

This investigation has been limited to an examination of the bending strength capacity of the floors of the ECBP in-situ building and the conclusions drawn are therefore subject to reservations in respect of serviceability and shear capacity requirements, neither of which are considered by the automated yield-line approach. Serviceability

178


constraints, in particular, will almost certainly affect any attempts to reduce the load factors of table 3 closer to unity.

Subject to the above reservations, the principal conclusions arising from the investigation are that:

0 the floors are satisfactory in respect of bending strength in that all designs provide a load factor over ultimate design strength of at least unity.

0 the load factors of the various floors increase with weight of bending reinforcement in a general sense, although some layouts are clearly more effective than others. The yield-line designed floor (4N), for example, provides the same load factor as the one-way mat arrangement of floor 5, even though the weight of bending reinforcement in the latter floor is some 35% more.

0 the design for Floors 1-5 (with the exception of Floor 4s) provide very similar load factors and are sufficiently closely designed to make further reinforcement economies unwarranted. The closeness of the load factors to unity for several of the floors may appear strange, given that most were designed elastically and might therefore be presumed not to allow for moment redistribution effects, which are incorporated in the yield-line analyses. However, moment coefficients provided by Codes of Practice commonly make some provision for redistribution (often based on yield-line results) and the automated analyses therefore suggest that these provisions are effective and relevant.

Floors 4 s and 6 have higher load factors and some reduction in the reinforcement steel could be made to make the load factor closer to unity. This is particularly the case for Floor 6 , where the reinforcement of the uniform two-way mesh can be proportionately reduced to effect a proportionate load factor reduction.

ACKNOWLEDGEMENTS

This investigation is part of the DETR “Rationalisation of Flat Slab Reinforcement” Project, managed by the Reinforced Concrete Council, and partial funding from the project, which operates under the “Partners in Technology” scheme, is gratefully acknowledged. The software used was developed by Dr A. C. A. Ramsay, former Research Fellow at Nottingham Trent University.

REFERENCES

1. ALLEN, J. D. (1998). Reengineering the design and construction process. The Structural Engineer, Vol. 76, No. 9, pp 175-179.

2. JOHNSON, D. (1 994). Mechanism determination by automated yield-line analysis. The Structural Engineer, Vol. 72, No. 19, pp 323-327.

179


3. JOHNSON, D. (1 995). Yield-line analysis by sequential linear programming. International Journal of Solids and Structures, Vol. 32, No. 10, pp 1395-1404.

180


APPENDIX 1

181


Taking as an example, the evaluation of the moment of resistance of T20@ 150B, then:

A, = 2090 mm2;fy = 0.87~500 = 435 N/mm*;d = 210 mm (see Figure Al. 1)

1 I d 0

I

Figure Al.1: Slab section with T20-250B reinforcement

Thus: 43 5x2090x0.9x2 10

1 o6 M" = = 172 kNm/m assuming I , = 0.9d

To check lever arm assumption:

= 0.105 whence I , = 0.87d , acceptably close to assumed value. 1 7 2 ~ 1 0 ~

1 o3 x2 1 o2 x37 k =

182


APPENDIX 2

r

183


LOAD FACTOR = 1.01243382987 CARlQUAR.flS

___ POSITIUE YIELD-LINE ___ NEGATIUE YIELD-LINE

a.

0.000

-0.143

-0.286

-0.429

-0.571

-0.714

-0.857

-1 -000

b. Figure A2-1: Floor 1 - quarter slab a. yield-lines b. contour plot of collapse mode

184


LOAD FACTOR = 1.12729755559 CAR1TOPF.HS

- POSITIVE YIELD-LINE

a.

~ NEGAT IUE Y IELD-L [NE

0.000

-0.143

-0.286

-0.429

-0.571

-0.714

-0.857

-1.000

b.

Figure A2-2: Floor 1 - fine analysis a. yield-lines b. contour plot of collapse mode

185


LOAD FACTOR 1.00849439862 CAR2QUAR.M

- POSIT IUE Y IELD-L INE ~ NEGATIUE YIELD-LINE

a.

0.000

-0.143

-0 .286

-0.429

-0.571

-0 .?I4

-0.857

-1.000

b.

Figure A2-3: Floor 2 - quarter slab a. yield-lines b. contour plot of collapse mode

186


LOAD FACTOR = 1.04543617346 CAR3QUAR.MS

~ POSITIVE YIELD-LINE

a.

~ NEGAIIUE YIELD-LINE

0.000

-0.143

-0.286

-0.429

-0.571

-0.714

-0.857

-1 .e00

b.


187


LOAD FACTOR = 1.18952060020 CABS IDE .nSH

~ POSITIUE YIELD-LINE ___ NEGAT IUE Y IELD-L [NE

a.

0.037

-0.111

-0.259

-0.407

-0.556

-0.704

-0.852

-1.000

b.


188


LOAD FACTOR = 1.01057380516 CAR6QUAR.HS

- POS I1 IUE Y IELD-L INE - NEGATIUE Y IELD-L INE

a.

0 .a00

-0.143

. -0.286

-0 .a9

-0.572

-0.714

-0.857

-1 .a00

b.

Figure A2-6: Floor 4N- quarter slab a. yield-lines b. contour plot of collapse mode

189


LOFID Ft3CIOR = 1.16204881148 CFIR6INTF.M

~ POSITIUE YIELD-LINE

a.

~ NEGFITIUE YIELD-LINE

0.000

-0.143

-0.286

b.

Figure A2-7: Floor 4N- fine analysis a. yield-lines b. contour plot of collapse mode

190


LOAD FACTOR = 1.32237805888

E CAR4BQUA.HS

~ POSIT IUE Y IELD-L INE ___ NEGATIUE YIELD-LINE

a.

0.000

-0.143

-0.286

-0 .429

-0.571

-0.714

-0.857

-1.000

b.

Figure A2-8: Floor 4s- quarter slab a. yield-lines b. contour plot of collapse mode

191


I

LOAD FACTOR = 1.34249568598 CAR4BBOT.HS

~ POS IT IUE Y IELD-L INE

a.

___ NEGATIUE YIELD-LINE

0.000

-0.143

-0.286

b.

Figure A2-9: Floor 4s - fine analysis a. yield-lines b. contour plot of collapse mode

192


LOAD FACTOR = 0.99317438822s

___. POS I1 IUE Y IELD-L INE - NEGATIUE YIELD-LINE

a.

0.000

-0.143

-0.286

-0.429

-0.571

-0.714

-0.857

-1.000

b.


I93


LOAD FACTOR = 1.15ZZ?518788 CARSTOPF.HS

- POS IT IVE Y IELD-L INE

a.

~ NEGATIVE YIELD-LINE

b.

8 .808

-8.143

-8,286

-8.429

-8.571

-0.714

-0 .857

-1.808


I '.. ' ,

194


LOAD FRCTOR = 1.32951961863 CAR6QUAR.tlS

~ POSITIVE YIELD-LINE

a.

~ NEGAT IUE Y IELD-L INE

0 .e00

-0.143

-0.286

-0.429

-e .571

-0.714

-0.857

-1 .e00

b.


195


LORD FACTOR = 1.63532943705 +--

I I

I

~ POSITIVE YIELD-LINE ___ NEGATIVE YIELD-LINE

a.

0.000

-0.143

-0.286

-0.429

-0.571

-0.714

-0.857

-1.000

b.

Figure A2-13: Floor 6 - fine analysis 1 a. yield-lines b. contour plot of collapse mode

196


LOAD FACTOR = 1.50527589999 CAR6TOPF.HS

- POSITIVE YIELD-LINE - NEGhTIUE YIELD-LINE

a.

b.

Figure A2-14: Floor 6 -fine analysis 2 a. yield-lines b. contour plot of collapse mode

197

Appendix VI11 Charts showing potential savings for flexural reinforcement and shear systems

General The following charts compare the savings from different ways of reinforcing concrete flat slabs. They were derived from a spreadsheet combined the figures in Appendices IV and V in accordance with Table 8.13. The assumptions made are discussed in Section 8.4.

Each graph has four sets of lines representing the savings (or losses) expected for four main parties to a contract -

0 Specialist trade subcontractor 0 Main contractor (or equivalent) 0

0

Speculative clients (clients experiencing finance charges) Clients subject to finance charges and to obtaining rent

Each set of lines is based on four points. The left hand point represents an average sized multi-storey building of 4500m2 gross floor area on three storeys, M4C3 (M4 relates to the M4 corridor, nominally Reading, C to concrete and 3 to three storeys). The right hand point of each four represents a large building of approximately 13,000 m2 gross floor area on nine storeys (M4C9). The intermediate points relate to buildings of approximately 7000m2 on five storeys (M4C5) and 10,000 m2 on seven storeys (M4C7).

The baseline of i0.00/m2 savings represents the traditional reinforcement used on Floors 1 and 2. Margins of error on each line are discussed later in this chapter.


...

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Appendix VI11 Savings for shear systems

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Appendix VI11 Savings for shear systems

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c-

RATIONALISATION OF FLAT SLAB REINFORCEMENT

C. H. Godchild

CVSfB Fl 1666.982.24

BRlTISH CEMENT ASSOCIATION PUBLICATION 97.376

BCA Rational is at Ion of Flat Slab Reinforcement

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