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Office Cost Study

COMPARING THE EFFECTS OF STRUCTURAL SOLUTIONS ON COST


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Overview of the study 4

Building designs and structural options 6

Costs and programme data 9

Cost comparison and conclusions 12

Office Cost Study

Contents


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The Concrete Centre aims to assist all those who design and constructin concrete to realise the full potential of the material.To this end, The

Concrete Centre undertakes independent research to identify, whennecessary, the true facts of building in concrete.A complete cost modelstudy was commissioned in order to compare the costs of constructingoffice buildings with different structural solutions, the results of which arepresented briefly in this summary document. For comprehensive analysisof the study, the complete report is available, details of which can befound below.

The design teamThe success of any project depends on the skills and knowledge of thewhole design team.To harness the experience representative of a realproject, top practitioners in each field were represented in the study inorder to form a rounded view of the design. The cost model study was

undertaken by staff from Allies and Morrison (architectural design); Arup(structural design); Davis Langdon (quantity surveying) and Mace(programming).The report was designed to be a fair comparison of costsfor two common-place office building types.The designs were selected togive no bias towards concrete solutions, for example, the proven benefitsof thermal mass were not considered.

Further publicationsTo assist all members of a design team, The Concrete Centre produces arange of technical design guidance to enable effective concrete construction.Complementary documents that may be of interest as further reading arelisted below:

Concrete Frame Elements - Intended as a pre-scheme designhandbook, this publication helps designers to choose the most viableconcrete options. Based on BS 8110, it includes cost-optimisedspan-to-depth charts for various types of in-situ, precast andpost-tensioned floor and beam construction over a range of spansand loads. Other information, such as preliminary column sizing andreinforcement estimates is included.

Utilisation of Thermal mass in Non Residential Buildings - Thisguide provides detailed guidance on the use of thermal mass as a

sustainable method of cooling which avoids or reduces the need for air conditioning.This publication should assist designers wishing to exploitthermal mass and includes chapters on concrete floor options,integration of services, acoustic considerations and surface finishoptions.The guide also includes a number of case studies.

Cost Model Study School Buildings - This publication is parallel tothis study of office buildings and reports a comprehensive andindependent cost study which was undertaken to provide a comparisonbetween six structural frame options for a typical secondary school.Budget costings were assigned to all elements of construction andadjustments were made to reflect time-related costs attributable todifferences in the construction programme.

For more information on these, and other publications from The ConcreteCentre, visitwww.concretecentre.com/publications

The full technical report isavailable from The Concrete Centre.

To purchase your copy visitwww.concretebookshop.comor call0700 4 607777

Ref:CCIP-010Price:45

Office Cost Study

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The full Commercial Buildings Cost Model Study was undertaken to provide

a comparison of the construction costs associated with building options.

PurposeCost is a major criterion in assessing design and constructionalternatives, and construction professionals require current studiesin order to inform their decisions. However, the value of a cost study isoften found to be not so much in the cost results, but in the detailed andrigorous assessment of how structural frame choice can affect the cost of other items, such as cladding, internal planning, services, fit-out, etc.

The Concrete Centre commissioned a commercial buildings cost modelstudy, which was designed to provide a detailed cost comparison. The factthat the study also acts as an independent assessment of current building

types means that it will be of enduring value to quantity surveyors,architects, engineers and other construction professionals.

This publication provides a brief overview of the findings. For moreinformation on the results found in this publication, please refer to thefull Cost Model Study - Commercial Buildings [1].

MethodologyThe commercial buildings cost model study was undertaken in 2006 andcompared the costs of constructing three- and six-storey commercialbuildings, using a variety of short-span and long-span reinforced concreteand steel-frame options, taking into account construction, full fit-out, andthe effect of programme times on cost.

The buildings were notionally located in two different locations an out-

of-town business park and central London - and were based on appropriatestructural grids commonly in current use, with specifications suited tocontemporary market conditions.

The designs were taken to normal outline design stage, the only differencesbeing directly attributable to the structural frame material. Budgetcostings were assigned to all elements of construction, from substructure,superstructure and external envelope through to preliminaries, with theexception of external works, which were considered to be too highlysite-specific to permit accurate costing. Adjustments were made to thecostings to reflect time-related costs attributable to differences inconstruction programmes.

Whilst identifying the variation in the costs of frames, the studyalso considered the effects that the choice of framing material andmethod of construction had on other elements of the building,as well as the other benefits that the choice of frame can generate(see page 13).

Overview of the study

Office Cost Study

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Slimdek is a registered trademark of Corus UK Ltd.


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ImpartialityThe study was undertaken on a completely independent basis by leadingpractitioners in their field:

Allies and Morrison Architectural design Arup Structural design

Davis Langdon LLPQuantity surveying Mace Programming

The structural design for all options was carried out by Arup and costswere prepared by Davis Langdon, based on pricing data obtained fromthe practices national cost database of recent projects and, therefore,reflecting the current marketplace. The cost models were developed usingcurrent best practice for both concrete and steel as determined by theprofessional team.

ConclusionsThe main interest in any cost model study is the process of designing

and costing alternative methods of constructing otherwise identicalbuildings, as this raises many interesting issues for those commissioning,designing and constructing buildings.

Overall, the results of the commercial buildings cost model study showedthat when developing modern commercial buildings, concrete-framedsolutions are highly competitive with steel-framed solutions and thestudy proves conclusively that construction costs for concrete are up to6% lower than for steel.

However, the study also demonstrates the need to consider all elementsof the building cost, rather than simply the cost of the structure, andhighlights the extent to which elements other than the structure areaffected by the choice of frame solution (see Table 1). In particular,when the overall procurement and construction programme is considered,modern concrete-framed buildings CAN be procured and constructed ina shorter time frame (up to nearly 12% faster) than steel-framed buildings.Concrete-framed buildings can also offer a range of other added-valuebenefits at no extra cost, such as fire resistance, durability, robustness,safety and ease of service integration, making concrete the idealconstruction material for commercial buildings. By adding to this itsenergy efficiency, concrete truly deserves a position as the constructionmaterial of choice for new commercial buildings in the UK.

Office Cost Study

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Proportion of overallconstruction cost (%)

Variation dictated by structuralsolution, (% of package cost)

Variation dictated by structuralsolution (/m )2

Proportion of overallconstruction cost (%)

Variation dictated by structuralsolution,

Variation dictated by structuralsolution (/m )2

Substructure ExternalCladding

InternalPlanning

Finishes M&E

3

7.3

3

3

2.2

6

17

4.2

9

21

6.2

22

2

13.3

4

1

22.2

4

11

-

-

10

1.6

2

34

1.6

8

33

1.4

8

Prelims

11

5.2

8

12

6

12

B u

i l d i n g A

B u

i l d i n g B

(% of package cost)

Table 1: How much does the choice of structural solution affect the cost of other packages? *

* Packages not affected by structural solution are not listed.Building A - three-storey 4,650m2 office in an out-of-town business park location. For flat slab solution: total construction cost 1,460 /m2, superstructure costs 122 /m2.Building B - six-storey 16,500m2 office in central London. For flat slab solution: total construction cost 1,676 /m2, superstructure costs 110 /m2.


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The design brief for the Commercial Buildings Cost Model Study asked for outline designs of two multi-storey buildings on open, clear sites - one inan out-of-town south-east England business park location and the other in central London.

The precise location, size and design of the buildings were based on thedesign teams judgement of current commercial practice and marketrequirements in terms of performance and cost, whilst also avoidingunduly favouring either concrete or steel. For example, concretesthermal mass can significantly reduce the use of mechanicalventilation, and therefore costs but this was not considered. TheConcrete Centre has produced literature on thermal mass, pleaserefer to www.concretecentre.com/publications for details.

Building AA three-storey office building in an out-of-town business parklocation in the south east of England that is air-conditioned, withcurtain walling and some natural ventilation.

The building was chosen to reflect a framed building of average size(4,650m2) in a commercial/business park setting. It is representative of a typical low-rise building in the centre of current development activity.

The form of Building A is an L-shape with a full-height atrium and acentral service core, with secondary stairs and service access locatedtowards the ends of the building and a limited amount of undercroftparking. Air conditioning is provided by a fan-coil system providing fullclimate control when active.

The internal environment is designed to maximise daylightingand allow some mid-season free cooling from natural ventilation,which saves energy and lowers CO2 emissions.This is achieved withfloor plates 23.5m wide, configured around a grid of three bays of 7.5m,allowing a degree of cross-ventilation from the perimeter windows.

The building envelope comprises grid stick curtain wall cladding,incorporating floor to ceiling double glazing units and aluminium

clad insulated spandrels, permitting good daylighting to most of theworking areas.

An indicative plan and section for Building A, showing the building form,areshown in Figure 1.

A 7.5 x 7.5m grid was established by the design team as optimum andwas adopted for all frame options for Building A in the study.There weresix options developed in total, three concrete- and three steel-framed.

Designs and options were selected as typical of current practice and demand.

Building designsand structural options

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Figure 1: Building A - Three-storey

SECTION A-APLAN

A A

Typical column spacing7.5m each way.

7.5m

7.5m


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Building BA six-storey office building containing retail space at groundfloor level located in central London, that is air-conditioned withcurtain walling.

The building was chosen to reflect a high-quality building of average size(14,200m2 of offices and 2,300m2 of retail space) in central London. It isacknowledged that a building of this type in London would normally havea basement. However, with the likelihood that such a basement wouldbe formed in concrete, it was considered that inclusion of this elementwould unduly favour the concrete-framed options above ground.The basement construction has therefore been excluded from the study.

The form of Building B is rectangular, arranged around a central atrium andincorporating a fan-coil unit air-conditioning system, with service coreslocated towards the ends of the atrium.The form of the building is designedwith a low envelope to volume ratio, which helps minimise heat loss duringthe winter and heat gains in the summer.The building is fully sealed, requiringfull climate control year round.The building envelope comprises unitisedcurtain walling, incorporating floor to ceiling double glazing units and stoneclad insulated spandrels.

The floor plate depths are 9.5m to the core walls on the E-W axis and15.5m to either the core walls or the atrium on the N-S axis. The buildingcan be operated with single or split tenancies, with splitting by verticaldivision and requiring a glazed wall to the atrium.

An indicative plan and section for Building B, showing the building formand column layout are shown in Figure 2.

The 9.0 x 7.5m structural grid for Building B is more representative of the current market for a city centre site. It also permitted explorationof a long-span option for both materials in the study, by creating a15.0 x 9.0m grid. Eight options were developed in total; three concrete-and three steel-framed options for the short-span solutions (7.5m)

and one concrete and one steel option for the long-span solution (15.0m).

Office Cost Study

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Figure 2: Building B - Six-storey

SECTION B-BPLAN

Note: Internal columns omitted in 15m long-span option

Note: Internal columns omitted in 15m long-span option

The concrete frame can be left exposed for both aesthetics and to utilise the thermalmass of concrete, thereby reducing energy use.

B

BTypical column spacing 7.5m and 9m.

9m

7.5m


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Short-span options - Building A and B Long-span options - Building B onlyOption 1 - Flat Slab Option 2 - Composite Option 3 - PT Flat Slab Option 7 - PT Band Beams

Reinforced in-situ concrete at slaband columns

Steel beams and metal decking,acting compositely with in-situconcrete oor slabs. Steel columns

Post-tensioned in-situ concrete atslab and reinforced in-situ concretecolumns

Post-tensioned in-situ concrete atslab and band beams with reinforcedin-situ concrete columns

Option 4 - Steel + Hollowcore Option 5 - In-situ + Hollowcore Option 6 - Slimdek Option 8 - Long-Span Composite

Steel beams acting compositely withprecast concrete hollowcore oor slabs. Steel columns

Reinforced in-situ concrete beamsand columns with precast concretehollowcore oor slabs

Slimdek system comprising asymmetricbeams and metal decking, actingcompositely with in-situ concrete oor slabs. Steel columns

Long-span cellular steel beams andmetal decking, both acting compositelywith in-situ concrete oor slab.Steel columns

Office Cost Study

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Structural optionsIndicative diagrams and descriptions for each of the concrete and steel options investigated are shown below.


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Prices were prepared by Davis Langdon, based on pricing data obtained from the practices national cost database of recently tendered projects. Ratesfor Building A are based on construction in the south-east and rates for Building B are based on construction in central London. The pricing base dateis June 2006. (Costs can be adjusted to allow for regional differences see Table 7 on page 9.)

Element Short-span optionsFlat Slab Composite In-situ + Hollowcore PT Flat Slab Steel + Hollowcore Slimdek

Element to ta l () Element total () Element total () Element total () Element to tal () Element to ta l ()Substructure 199,480 189,765 202,641 200,512 195,452 192,107

Frame/upper oors 564,827 568,078 591,645 642,599 643,704 872,208

Roof nishes 241,208 241,208 241,208 241,208 241,208 241,208

Stairs 63,000 63,000 63,000 63,000 63,000 63,000

External cladding 1,166,600 1,174,480 1,187,720 1,154,800 1,199,980 1,175,460

Internal planning 141,230 154,110 145,255 139,740 156,630 153,900

Wall nishes 51,010 50,040 49,684 48,820 52,240 50,240

Floor nishes 274,432 274,432 274,432 274,432 274,432 274,432

Ceiling nishes 125,308 125,308 125,308 125,308 125,308 125,308

Fittings 60,000 60,000 60,000 60,000 60,000 60,000

Sanitary 208,890 208,890 208,890 208,890 208,890 208,890Mechanical 1,285,834 1,311,551 1,285,834 1,285,834 1,311,551 1,285,834

Electrical 637,811 650,567 637,811 637,811 650,567 637,811

Lifts 70,000 70,000 70,000 70,000 70,000 70,000

BWIC 172,470 172,470 172,470 172,470 172,470 172,470

Contingency 394,658 398,542 398,692 399,407 406,907 418,715

Preliminaries 735,000 715,000 755,000 745,000 715,000 715,000

Overheads and prot 383,505 385,646 388,175 388,190 392,840 402,995

TOTAL 6,775,263 6,813,088 6,857,765 6,858,021 6,940,180 7,119,578

Office Cost Study

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The tables in this section highlight the itemised costs and timescales for construction of Building A and Building B.

Costs and programme data

Table 2: Building A - Summary of costs as total of element

Element Short-span optionsFlat Slab Composite In-situ + Hollowcore PT Flat Slab Steel + Hollowcore Slimdek

/m2

% /m2

% /m2

% /m2

% /m2

% /m2

%Substructure 43 2.9 41 2.8 44 3.0 43 2.9 42 2.8 41 2.7

Frame/upper oors 122 8.3 122 8.3 127 8.6 138 9.4 139 9.3 188 12.3

Roof nishes 52 3.6 52 3.5 52 3.5 52 3.5 52 3.5 52 3.4

Stairs 14 0.9 14 0.9 14 0.9 14 0.9 14 0.9 14 0.9

External cladding 252 17.2 253 17.2 256 17.3 249 16.8 258 17.3 253 16.5

Internal planning 30 2.1 33 2.3 31 2.1 30 2.0 34 2.3 33 2.2

Wall nishes 11 0.8 11 0.7 11 0.7 11 0.7 11 0.8 11 0.7

Floor nishes 59 4.1 59 4.0 59 4.0 59 4.0 59 4.0 59 3.9

Ceiling nishes 27 1.8 27 1.8 27 1.8 27 1.8 27 1.8 27 1.8

Fittings 13 0.9 13 0.9 13 0.9 13 0.9 13 0.9 13 0.8

Sanitary 45 3.1 45 3.1 45 3.0 45 3.0 45 3.0 45 2.9

Mechanical 277 19.0 283 19.3 277 18.8 277 18.7 283 18.9 277 18.1

Electrical 137 9.4 140 9.5 137 9.3 137 9.3 140 9.4 137 9.0

Lifts 15 1.0 15 1.0 15 1.0 15 1.0 15 1.0 15 1.0

BWIC 37 2.5 37 2.5 37 2.5 37 2.5 37 2.5 37 2.4Contingency 85 5.8 86 5.8 86 5.8 86 5.8 88 5.9 90 5.9

Preliminaries 158 10.8 154 10.5 162 11.0 160 10.9 154 10.3 154 10.0

Overheads and prot 83 5.7 83 5.7 84 5.7 84 5.7 84 5.7 88 5.7

TOTAL 1,460 1,468 1,477 1,477 1,495 1,534

Table 3: Building A - Summary of costs as /m 2 and % of build cost


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Element Short-span options Long-span options Short-span optionsFlat Slab PT Flat Slab Composite In-situ +

HollowcorePT Band Beams Long-Span

CompositeSteel +

HollowcoreSlimdek

Element total()

Element total()

Element total()

Element total()

Element total()

Element total()

Element total()

Element total()

Substructure 891,672 865,937 815,468 885,169 907,622 848,868 860,967 852,231

Superstructure 1,811,939 2,016,344 1,878,457 1,846,453 2,227,681 2,201,664 2,275,704 3,011,992

Roof nishes 545,080 545,080 545,080 545,080 545,080 545,080 545,080 545,080

Stairs 132,000 132,000 132,000 132,000 132,000 132,000 132,000 132,000

External cladding 5,951,060 5,849,590 5,957,935 6,053,840 6,086,885 5,957,935 6,208,265 5,974,270

Internal planning 297,080 293,790 355,728 300,225 301,360 355,638 366,552 356,352

Wall nishes 234,455 229,931 256,770 233,226 227,825 241,566 264,162 263,112

Floor nishes 1,167,221 1,167,221 1,167,221 1,167,221 1,167,221 1,167,221 1,167,221 1,167,221

Ceiling nishes 702,366 702,366 702,366 702,366 702,366 702,366 702,366 702,366

Fittings 132,500 132,500 132,500 132,500 132,500 132,500 132,500 132,500

Sanitary 824,000 824,000 824,000 824,000 824,000 824,000 824,000 824,000

Mechanical 4,544,360 4,544,360 4,635,247 4,544,360 4,544,360 4,635,247 4,635,247 4,544,360

Electrical 2,690,688 2,690,688 2,739,502 2,690,688 2,690,688 2,739,502 2,739,502 2,690,688

Lifts 600,000 600,000 600,000 600,000 600,000 600,000 600,000 600,000

BWIC 601,800 601,800 601,800 601,800 601,800 601,800 601,800 601,800

Contingency 1,584,467 1,589,670 1,600,806 1,594,420 1,626,854 1,626,404 1,654,152 1,679,848

Preliminaries 3,350,000 3,310,000 3,350,000 3,470,000 3,310,000 3,350,000 3,270,000 3,270,000

Overheads and prot 1,563,641 1,565,717 1,577,693 1,579,401 1,597,694 1,599,707 1,618,771 1,640,869

TOTAL 27,624,328 27,660,993 27,872,572 27,902,748 28,225,936 28,261,499 28,598,289 28,998,690

Element Short-span options Long-span options Short-span optionsFlat Slab PT Flat Slab Composite In-situ +

HollowcorePT Band Beams Long-Span

CompositeSteel +

HollowcoreSlimdek

/m 2 % /m 2 % /m 2 % /m 2 % /m 2 % /m 2 % /m 2 % /m 2 %Substructure 54 3.2 53 3.1 49 2.9 54 3.2 55 3.2 52 3.0 52 3.0 52 2.9

Superstructure 110 6.6 122 7.2 114 6.7 112 6.6 135 7.9 134 7.7 138 7.9 183 10.3

Roof nishes 33 2.0 33 2.0 33 1.9 33 1.9 33 1.9 33 1.9 33 1.9 33 1.9

Stairs 8 0.5 8 0.5 8 0.5 8 0.5 8 0.5 8 0.5 8 0.5 8 0.5

External cladding 361 21.5 355 21.0 362 21.3 367 21.6 369 21.5 362 21.0 377 21.6 363 20.5

Internal planning 18 1.1 18 1.1 22 1.3 18 1.1 18 1.1 22 1.3 22 1.3 22 1.2

Wall nishes 14 0.8 14 0.8 16 0.9 14 0.8 14 0.8 15 0.9 16 0.9 16 0.9

Floor nishes 71 4.2 71 4.2 71 4.2 71 4.2 71 4.1 71 4.1 71 4.1 71 4.0

Ceiling nishes 43 2.5 43 2.5 43 2.5 43 2.5 43 2.5 43 2.5 43 2.4 43 2.4

Fittings 8 0.5 8 0.5 8 0.5 8 0.5 8 0.5 8 0.5 8 0.5 8 0.5

Sanitary 50 3.0 50 3.0 50 2.9 50 2.9 50 2.9 50 2.9 50 2.9 50 2.8

Mechanical 276 16.5 276 16.3 281 16.5 276 16.2 276 16.0 281 16.3 281 16.1 276 15.6

Electrical 163 9.7 163 9.7 166 9.8 163 9.6 163 9.5 166 9.6 166 9.5 163 9.2

Lifts 36 2.2 36 2.2 36 2.1 36 2.1 36 2.1 36 2.1 36 2.1 36 2.1

BWIC 37 2.2 37 2.2 37 2.1 37 2.1 37 2.1 37 2.1 37 2.1 37 2.1

Contingency 96 5.7 96 5.7 97 5.7 97 5.7 99 5.7 97 5.7 100 5.8 101 5.8

Preliminaries 203 12.1 201 11.9 203 12.0 211 12.4 201 11.7 203 11.8 199 11.4 198 11.2

Overheads and prot 95 5.7 94 5.7 95 5.7 95 5.7 97 5.7 97 5.7 98 5.7 99 5.7

TOTAL 1,676 1,678 1,691 1,693 1,713 1,715 1,735 1,759

Table 5: Building B Summary of costs - as /m 2 and % of build cost

Office Cost Study

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Table 4: Building B - Summary of costs as total of element


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Building A

Flat Slab 10 4 10 50 64

PT Flat Slab 10 4 11 51 65

In-situ & Hollowcore 10 4 13 52 66

Composite 10 12 8 48 70

Steel & Hollowcore 10 12 7 48 70

Slimdek 10 12 7 48 70

Building B

PT Flat Slab 10 6 17 66 82

Flat Slab 10 6 18 67 83PT Band Beams 10 7 17 66 83

In-situ & Hollowcore 10 6 22 70 86

Steel & Hollowcore 10 16 21 65 91

Slimdek 10 16 21 65 91

Composite 10 16 23 67 93

Long span Composite 10 16 23 67 95

StructuralOption

Frame procurementtime (weeks)

Frame lead time(weeks)

Frame constructiontime (weeks)

Overall constructiontime (weeks)

Overall projecttime (weeks)

Table 6: Programme timescalesA comparison of the overall programme times, showing each of the periods from procurement to completion, is given in tabular form below:

Office Cost Study

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Table 7: Regional cost adjustments

* Note: The procurement element is identical for each option at ten weeks, comprising two weeks for collation of information, four weeks for bidding, three weeks for bid evaluationand one week for award of contract, assuming a traditional approach to works package sub-contracting.

% AdjustmentRegion

Building A Building B

Outer London +7 -6

Central London +13 0

East Anglia -3 -14

East Midlands -5 -16

Northern 0 -11

Northern Ireland -23 -32

North West -5 -16

Scotland 0 -11

South East 0 -11

South West -4 -15Wales - 7 -18

West Midlands -4 -15

Yorkshire & Humberside -1 -12

Source:Davis Langdon.Adjustments based on 2008 figures.


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The study showed that a concrete frame can produce the mosteconomic solution.

Cost comparison and conclusions

Building A Flat Slab Composite In-situ +Hollowcore

PT Flat Slab Steel +Hollowcore

Slimdek

Substructure

B a s e c a s e

f o r c o m p a r

i s o n

4.9% +1.6% +0.5% 2.0% 3.7%

Frame andupper oors +0.6% +4.1% +13.1% +13.9% +54.1%

Externalcladding +0.8% +2.1% 1.1% +3.3% +0.9%

Internalplanning +10.0% +3.3% 0% +13.3% +10.0%

Wall nishes 1.9% 2.6% 4.3% +2.4% 1.5%M and E, liftsand BWIC

+1.6% 0% 0% +1.6% 0%

Contingency +1.0% +1.0% +1.2% +3.1% +6.1%

Time-relatedpreliminaries

3.9% +3.9% +2.0% 3.9% 3.9%

Overheadsand prot

+0.6% +1.2% +1.2% +3.0% +5.4%

Building B Flat Slab PT Flat Slab Composite In-situ +Hollowcore

PT BandBeams

Long-SpanComposite

Steel +Hollowcore

Slimdek

Substructure

B a s e c a s e

f o r c o m p a r

i s o n

2.9% 8.5% 0.7% +1.8% 4.8% 3.4% 4.4%

Frame andupper oors +10.9% +3.6% +1.8% +22.7% +21.8% +25.5% +66.4%

Externalcladding 1.7% +0.1% +1.7% +2.3% +0.1% +4.3% +0.4%

Internalplanning 1.1% +19.7% +1.1% +1.4% +19.7% +23.4% +20.0%

Wall nishes 1.9% +9.5% 12.7% 2.8% +3.0% +12.7% +12.2%M and E, liftsand BWIC

0% +1.4% 0% 0% +1.4% +1.4% 0%

Contingency +0.3% +1.0% +0.6% +2.7% +2.6% +4.4% +6.0%

Time-relatedpreliminaries 1.5% 0% +4.6% 1.5% 0% 3.1% 3.1%

Overheadsand prot

+0.1% +0.9% +1.0% +2.2% +2.3% +3.5% +4.9%

Office Cost Study

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Table 9: Summary comparison chart Building B

Table 8: Summary comparison chart Building A

The overall conclusion of the Commercial Buildings Cost Model Study isthat, for a range of structural options commonly used in the constructionof office buildings, using a concrete frame can give the most economicsolution.

Choosing concrete can produce savings of up to 6% in overallconstruction costs.

The charts below summarise those elements where costs are directlyaffected by the choice of frame and show the percentage variation in costfor each structural frame option, when compared with the flat slab optionas the base case.

The largest cost and the main source of savings, as can be seen fromthe Tables 2 to 5, lies in the superstructure, when the frame, claddingand internal planning are all taken into account and here concretehas a definite advantage. With regard to finishes and preliminaries,

other than time-related aspects, there are minimal differencesbetween options.


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Substructure - foundationsConcrete construction is generally heavier than steel-frame construction,

and this is reflected in the higher cost of foundations to the concrete-framed options. However, although foundations for the concreteoptions can cost more, they account for a relatively small proportionof the overall cost.To some extent this cost premium can be offset byadopting post-tensioned slabs, which are typically some 15% lighter.

Frame and upper floorsTo compare flat soffit solutions with similar floor-to-floor heights, andhence similar cladding areas, the concrete flat slab option and thesteel Slimdek option can be considered. This shows that concrete ismore favourable in price by 66%. Comparisons of other solutions canbe made by reviewing Tables 2 to 5.

External claddingThe thinner the overall structural and services zone, the lower thecladding cost. Given that the cladding on the building in the studyrepresents between 17% and 22% of the construction cost, minimisingthe cladding area represents considerable value to the client.The minimumfloor-to-floor height is almost always achieved with a post-tensioned (PT)flat slab and separate services zone, offering the potential for additionalstoreys in high-rise buildings and thus improved rental or sales return.Smaller floor-to-floor heights have reduced cladding areas and hencelowered costs, and of increasing importance is the potential benefit thata reduced cladding area has on the buildings energy use.

Internal planningA premium is incurred on steel-framed options in sealing and firestopping at partition heads against the irregular soffits of the steeldecking and around irregularly shaped intersecting frame members;this is taken into account in the cost study. Unless this is considered atan early stage it can result in expensive and time-consuming remedialwork late in the construction programme.

Mechanical and electrical servicesMechanical and electrical services represent a large proportion of theoverall construction costs of the buildings, averaging up to 34%.However, a flat soffit provides a clear zone, free of any downstandbeams, enabling more services to be pre-fabricated off-site and makingthem much simpler to install. This not only improves the speed andquality of installation, but reduces the risk of errors, provides flexibilityin the design and frees up design resources to be used elsewhere.It also provides cost-in-use benefits in the form of reduced maintenancedowntime, as equipment can be changed-out much more quickly,giving greater flexibility and less disruption to an occupiers businessoperations. Flat soffits also offer greater flexibility to accommodatenew service requirements, layout or cellular arrangements, as well asbuilding refurbishments.

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ProgrammesConcrete-framed options offer a significant advantage in lead times a saving of up to 10 weeks and yet the overall construction programmetimes are almost identical. Inevitably, different planners would produceslightly different programmes based on a considerable number of variablefactors. For example, construction management and design and buildapproaches lend themselves to concrete construction, where the ability toaccommodate late information and variations is particularly beneficial.

Nett lettable areaThere are two main areas in which differences in nett lettable area are found:

Core areas the area occupied by concrete cores tends to be slightlysmaller than that needed for steel cores, due to the allowance for steelbracing zones and the fact that structural concrete walls also serveas partitions.

Stairs are typically re-sized as a result of the reduced storey heightmodule, producing slightly increased net lettable areas.

A previous cost model study on commercial buildings [2] found that, onan overall basis, the difference can be as much as 1.5% extra nett lettablefloor area, and this finding is still valid.

Finance costsThe results of considering finance costs for the periods that have beenidentified in the programmes for procurement, lead time and constructionare presented in table 10.This assumes a rate of 7.75% p.a. and is presentedas an additional cost compared with the option with the shortestduration.

This comparison takes no account of differences in cumulative financecosts arising from the different cash flow profiles experienced with thediffering forms of construction. For example, the steel-framed optionsrequire greater expenditure earlier on than the concrete-framed options,where concretes pay as you place principle works in the clients favour.

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Note: The post-tensioned band beams option has been compared with the long span composite option.

Building B Flat Slab PT FlatSlab

Composite In-situ +Hollowcore

PT BandBeams

Long-SpanComposite

Steel +Hollowcore

Slimdek

Construction cost in /m 2 1,676 1,678 1,691 1,693 1,713 1,715 1,735 1,759

Overall programme in weeks 83 82 93 86 83 95 91 91Savings in nance costs @ 7.75% p.a. (/m 2) +0 -1 +7 +2 +0 +8 +6 +6

1,676 1,677 1,698 1,695 1,713 1,723 1,741 1,765

Building A Flat Slab Composite In-situ +Hollowcore

PT Flat Slab Steel +Hollowcore

Slimdek

Construction cost in /m 2 1,460 1,468 1,477 1,477 1,495 1,534

Overall programme in weeks 64 70 66 65 70 70

Savings in nance costs @ 7.75% p.a. (/m 2) +0 +5 +2 +1 +5 +51,460 1,473 1,479 1,478 1,500 1,539

Table 10: Comparison of build costs - Building A and Building B


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Inherent cost benefits of concreteInitial capital cost is not the sole driver for clients, whose main objectiveis almost certainly to achieve optimum value from an overall solution.The additional benefits which can be gained from choosing a concretesolution are:

Fire protectionFire protection is generally not needed for concrete structures as thematerial has inherent fire resistance of up to four hours. This not onlyremoves the time and cost involved in providing added fire protection,but it also potentially enhances property safety, lowers insurancepremiums, increases re-usability of the structure and significantly reducesdown-time for an occupier after a fire.

VibrationThe inherent mass of concrete means that concrete floors generally meetvibration criteria at no extra cost, requiring no extra stiffening. For moreinformation on vibration, refer to A Design Guide to the Footfall-Induced Vibration of Structures available from The Concrete Centre [3].

Durability and adaptabilityConcrete is an inherently durable material, with a potential service liferunning into hundreds of years, if needed. In addition, sustainable constructionnow demands adaptable structures in order that their economic viabilityis enhanced. Flat slabs automatically permit fully adaptable horizontalservices distribution, and design of predetermined soft spots can addfuture-proofing of vertical servicing as well. Ribbed slabs, waffle slabs andprecast flooring solutions are similarly adaptable.

Exposed soffits and thermal massA concrete structure has a high thermal mass. By exposing the soffits,thermal mass can be utilised through fabric energy storage (FES) toreduce initial plant costs, minimise or eliminate the need for air conditioning and substantially reduce the lifetime operational costs of the building. Utilisation of FES permits the designer to create naturallyventilated buildings, giving occupants the chance to control their environment to ensure optimum employee productivity. In addition,suspended ceilings can be reduced or eliminated, giving valuable initialcost and programme benefits and reduced lifetime maintenance costs.For more information on thermal mass, refer toUtilisation of ThermalMass in Non-residential Buildings available from The Concrete Centre [4].

REFERENCES1. Commercial Buildings Cost Model Study,CCIP010, The Concrete Centre, 2007

2. Goodchild, CH, Cost Model Study - A Report on the Comparative Costs of Concrete& Steel Framed Office Buildings, Reinforced Concrete Council and British Cement Association, 1993

3. Wilford,MR and Young P,A Design Guide for Footfall Induced Vibration of Structures,CCIP-016,The Concrete Centre, 2006

4. Utilisation of Thermal Mass in Non-residential Buildings, CCIP-020,The Concrete Centre, 2006

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www.concretecentre.com

All advice or information from The Concrete Centre is intended for use in the UK only by those who will evaluate the significance and limitationsf it t t d t k ibilit f it d li ti N li bilit (i l di g th t f glig ) f l lti g f h d i

Ref:TCC/03/38ISBN:978-1-904818-61-8First published 2008The Concrete Centre 2008

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