Design & Construction of Efficient Concrete Buildings

Design & Construction of Efficient Concrete Buildings

Overview of Presentation Given at IStructE Headquarters Thursday 13th June 2002 by IStructE & The Concrete Society

Christopher Evans JACOBS GIBB

INTRODUCTIONINTRODUCTION

* Design of Concrete Frames.

* Design for Serviceability.

* Fabric Energy Storage.

* Fire Performance.

* High Performance Concrete.

* Discussion & Questions.

DESIGN OF CONCRETE FRAMES (After Charles Goodchild - Reinforced Concrete Council RCC)

DESIGN OF CONCRETE FRAMES (After Charles Goodchild - Reinforced Concrete Council RCC)

* BS8110 - Part 1

* Most commonly used in UK. Conservative design, slow reaction to advances in concrete technology, fire engineering, products, changes in best construction practice.

* BS8110 - Part 2

* Generally used for special structures only, but allows for more stringent design, and less conservative approaches can be utilised, leading to more efficient designs.

* Eurocode 2

* Only in preliminary draft, subject to final ratification. First standard version due for publication in 2003, will full UK ratification expected 2008?

Selecting the right frame optionSelecting the right frame option

* Which criteria governs final design?

* Cost of construction.

* Speed of construction.

* Quality of construction.

* Aesthetics.

* Energy usage / saving.

* Whole life costs.

What clients want (After Latham / Egan)What clients want (After Latham / Egan)

* Cost reductions / value for money.

* Programme predictability.

* Defect free structure.

* Improved teamwork / alliances / partnering.

* Design choice vs. risk following CDM regulations.

* Best endeavours.

What it comes down to What it comes down to

£££

Initial designInitial design

Once the initial choice has been made to

use reinforced concrete and the layout

of the structure has been agreed the designer

may find it useful to use the reference text to

initially size structural members, such as

slabs, beams, columns by using the design

given within.

Initial sizing of R.C. membersInitial sizing of R.C. members

The guide gives typical load span table data for typical dead and occupancy

loading from BS6399, to give a provide the designer with a reasonable set of

details, for example tender, to allow a full and detailed design to be prepared

when final layout, loading etc has been agreed by design team.

DESIGN FOR SERVICEABILITY(After Dr Robert Vollum - Imperial College London)

DESIGN FOR SERVICEABILITY(After Dr Robert Vollum - Imperial College London)

“The depth of floor systems is often governed by the need to control long-term deflections” (After Vollum). This is often true, in as much as a floor slab, beam, etc may require typicalreinforcement to provide additional flexural or shear capacity, it is often a case thatin large spans, additional levels of reinforcement are required to bring the l/d value (BS8110:Part 1) to within prescribed limits. As such, the need to accurately predict the long-term deflection of an element is a major design issue in long span construction, though this has been difficult to do so far, due to the lack of research into long-term deflections in reinforced concrete, and because the deflection is influenced by a number of unknowns at the construction stage, which are often missed as significant by designers, or disregarded.

Factors influencing long-term deflections Factors influencing long-term deflections * Concrete tensile strength.

* Creep and shrinkage co-efficients.

* Construction loads / concrete strength at striking formwork.

* Long-term service loads

* True position of reinforcement.

* True slab thickness

The best detailed research into long-term slab deflections carried out in the UK so far has been undertaken by the Building Research Establishment (BRE) as part of their in-situ concrete building project at Cardington, details of which are well documented and widely available.

Design to reduce deflectionsDesign to reduce deflections* BS8110 limits deflection to span/250, and calculates deflections using full dead and imposed loading as applied to a building. This is conservative, as it mayrequired additional reinforcement, or increasing depth of structural section to increase effective depth.

* EC 2 limits deflection to span/250 also, but calculates deflections using full dead load and ‘permanent’ imposed load factor, e.g. 0.3 0 occupancy for offices. This should be more cost effective as it should reduce the amount of additional reinforcement required and lead to more ‘slender’ members.

* Neither BS8110 or EC2 give guidance on influence of construction loading and the early striking of formwork, and how these could affect long-term deflections.

Testing concrete strengthTesting concrete strength* BS8110 currently allows for concrete cube testing at 7, 14 and 28 days, with the 28 day cube strength required to be equal to or better than design concrete strength. Time consuming, as first result not available for 7 - 10 days, concrete may have gained allowable ‘working’ strength before this time.* EC 2 will allow for concrete cylinder testing, similar to cube testing, but will require numerical adjustment to provide correlation to UK cube results. Again, testing at 7, 14 and 28 days means reduced productivity on site and slow turn around of construction process. * In-situ testing of concrete is being developed, e.g. LOK, and is being tested by BRE as part of St Georges Wharf development. Allows for early assessment of concrete strength, and allows formwork to be stripped upon concrete gaining required working strength, so increasing productivity, and turn around of construction. In correlation with concrete cube / cylinder testing, could dramatically improve speed of construction, and give a more detailed model of the overall strength of a structural member rather than a ‘window sample’ of a batch of concrete, which is ultimately what we are looking for.

Typical LOK test being carried out

on in-situ reinforced concrete slab

Typical LOK inserts placed in reinforced concrete wall and slab construction

Calculation of construction loads and use of propsCalculation of construction loads and use of props

By calculating the probable construction loads, and detailing the correct distribution and type of propping, the BRE propose that the component of long-term deflection caused during the construction process can be reduced significantly and the rate at which floors may be constructed may be increased.

* PROPS support formwork during the fixing of reinforcement and pouring of slab.

* BACKPROPS are installed below the slab immediately supporting the props (which carries its own weight after it is struck first) and distribute some of the load applied to the uppermost slab during construction. Note: More than one level of backprops can be installed, but the likely benefits of this is debatable.

* REPROPS are installed to replace the props when the formwork is stripped and it is considered the slab is incapable of supporting its own weight. Note: This should be considered as a remedial measure and where possible, testing and good planning should negate the needs for reprops

Calculating construction loads and prop distributionCalculating construction loads and prop distributionLoad on prop = S/W + Construction allowance (Say 0.75kN/m2

e.g for 250mm slab Wstrike = 6.75kN/m2

It should be noted that the distribution of props

can be easily modelled using finite element

analysis packages to determine the most

effective layout, but the designer must be

wary as to the amount of pre-stress which

can be put into the props. If conventional

mechanical props are used the amount of

pre-stress may vary significantly between

one installer and the next, and a safe working

load should be allowed for to account for this. Typical Propping Detail

Typical distribution of loading taken

by props in BRE Cardington model

The designer should bear in mind the concrete testing regime to be used during

construction, as this will directly determine when formwork can be stripped, props

installed to support the slab above, and props removed below. If in doubt, seek advice

from specialist concrete technologists, to help plan the construction programme.

FABRIC ENERGY STORAGE (After Dr Jacqueline Glass - Oxford Brookes University)

FABRIC ENERGY STORAGE (After Dr Jacqueline Glass - Oxford Brookes University)

* Sustainability is ‘the need to ensure that development meets the needs of the present without compromising the ability of the future generations to meet their own needs’ (After Glass)

* With around 50% of UK CO2 production associated with the occupancy of

buildings (e.g. heating, lighting and cooling), energy saving is a key issue in meeting the Kyoto protocol targets.

* ‘Fabric Energy Storage is use of thermal capacity in a concrete structure to store/ release heat to reduce indoor temperatures by exposing it to the interior’.

* Useful in buildings where maximum occupancy occurs at times of maximum heat build up (e.g. offices, schools, hospitals, prisons). Removal of air conditioning in typical 32000m2 office building can save up to £185000 p.a.

FES : A range of optionsFES : A range of options

* Lightweight steel construction offers cooling envelope of around 6-10 W/m2. Also, with new Building Regulations Part L, additional insulation will be required to provide greater thermal insulation and air tight construction, thought to add around £5/m2 to building envelope.

* Concrete construction typically offers 15-20 W/m2 of cooling, and with denser, monolithic construction, air tightness and increased thermal insulation is improved.

* Passive FES: Exposed soffits, natural or assisted ventilation + night purging through occupied spaces to ‘refresh’ the concrete, e.g. Toyota UK HQ, Surrey.

* Active FES: Controllable systems of ducted air that consumes 50% less CO2

than air conditioning systems, e.g. Termodeck 25-35 W/m2 cooling capacity.

Practical ApplicationsPractical Applications

Toyota HQ, Surrey

Use of passive FES through

exposed, vaulted concrete

soffits, natural lighting,

natural ventilation and ducted

air vents through pre-cast

floor system .

University of East London, Docklands Campus

Use of Termodeck system in pre-cast concrete

floors, reduces the need for specialist heating

and ventilation systems.

Design Guidance Design Guidance

* Remember 1/5/200 rule:

* Factor 1 : Provide robust, long-lasting structure.* Factor 5-10 : Life cycle energy savings, time required to pay for FES measures.* Factor 200 : Satisfied, comfortable workforce, typically the payroll of staff will be around 200 x initial start up and construction costs, is it worth losing valuable staff? As JacobsGIBB state “people are our biggest asset”

* ‘Fabric Energy Storage Benefits’ BCA Pubn. 97.383.

* PowerGen Project Profile’ BCA Pubn. 97.381

* ‘EcoConcrete’, BCA Pubn 97.361

* Note: Above texts available from RCC: www.rcc-info.org.uk

FIRE PERFORMANCEAfter Dr Colin Bailey - Buildings Research Establishment BRE)

FIRE PERFORMANCEAfter Dr Colin Bailey - Buildings Research Establishment BRE)

* Thermal conductivity is defined as ‘the amount of heat in unit time which passes through a unit area for a unit temperature gradient’, or rather how quickly will it heat up?

* Concrete has a thermal conductivity approximately 50 times lower than steel, such that in a fire, concrete heats up slowly, and by this rational has an inherentfire resistance.

* Following recent world events, can the fireproofing of steel be relied upon to afford the required level of protection required? Also, can the application of site based fire-proofing be relied upon? Concrete offers a fairly good level of re-assurance that the fire protection designed for will be constructed on site, and pre-cast construction provides tighter construction tolerances again.

Fire PerformanceFire Performance

* BS8110 - Conservative - Prescriptive based guidance only.

* Assumes members support full design load during design, very unlikely, occupancy load is mobile. * Cover to reinforcement is based on reinforcing bars losing 50% of strength at 550oC, which as full loading is designed for at this temperature, effectively higher temperatures could be designed for. Also, no account is made of fire control measures, which may effectively reduce the ambient temperature in a fire event to less than 550o C.* Cover limits refer to the minimum concrete cover to ALL reinforcing steel, whichreduces the effective depth, and as such the amount of concrete which can be utilised, leading to greater levels of main bar reinforcement.* No distinction between differing types of aggregate, and improvements in concrete technology, e.g. concrete limited to C40, as no higher concrete strength can be taken under BS8110.

Fire PerformanceFire Performance

* Eurocode 2 - More opportunistic - Performance based guidance allows for possible financial savings.* More realistic Fire Limit State design allowed;

Load Factors EC2 BS8110Dead Load 1.00 1.00Imposed Load (Permanent) 1.00 1.00Imposed Load (Non-permanent) 0.5* 1.0e.g. Office occupancy BS6399:Part 1

MaterialConcrete 1.0 1.3Reinforcement 1.0 1.0

N.B. * Stated as 0.3 in EC2, but UK ratification to require F.O.S. = 0.5

Fire PerformanceFire Performance* In EC 2, two methods of FLS allowed for;

1) 500oC Isotherm Method: Whereby only concrete, and reinforcement which will be subjected to temperature less than 500oC will be allowed to be utilised in design.

2) Zone Method: Whereby a zone of damage is allowed for similar to timberdesign, and reduced sectional area is used in final design. N.B. Similar to BS8110, but allows the designer more flexibility in specifying concrete cover, and so allows greater utilisation of main bar reinforcement.

* EC 2 also allows the designer to use new, high strength concrete grades, andspecify aggregates, over and above the usual C40, allowing up to C100. This will allow the designer to maintain similar structural member sizes throughout a building, utilising higher grade concrete where required, e.g. C100 columns at ground floors, reducing to C40 columns nearer to top of the building. Note, it is recommended that poly-propylene fibre reinforcement is used in high strength concrete members to reduce spalling and maintain integrity in fire event.

HIGH PERFORMANCE CONCRETE(After Dr John Clarke - The Concrete Society)HIGH PERFORMANCE CONCRETE(After Dr John Clarke - The Concrete Society)

* High Strength Concrete (HSC) now available up to C100, commonly used inhigh rise construction in North America, Asia, and Australia. C60 is commonlyused in the UK, but only in the pre-cast industry. * Benefits of HSC include:1) a reduction in size of compression elements, resulting in possible reduction inlongitudinal reinforcement.2) a reduction in time require to gain acceptable working strength, to 3) Reduction in deflections of beams and slabs due to increased elastic modulus, reduction in creep and shrinkage, increase in allowable pre-stress forces, increase in cracking moment due to increased tension strength, and an increased bond strength between the reinforcement and the concrete.4) Allows for reduction in cover to main reinforcement .

Production of HSCProduction of HSC

* Cements: HSC is now available in the UK using all standard forms of readily available Portland Cement, including SR, PFA and GBS, the only precept to remember though is a super-plasticizer would need to be specified in order to ensure the cement to free water ratio was kept to a minimum. * Aggregates: A wide range of aggregates may be used, but it has beensuggested that crushed rock aggregates of size 10 to 20mm have should be used where possible, to ensure a good bond, and that fine sands, and those with high absorption rates should be avoided. * Strength: Normal strength to water / cement ratios design is applicable, except that the ration should be in the range 0.30 - 0.35 or lower. * Workability: Due to the reduced water / cement ratio, typical slumps of 50mm may be encountered, and as such cause problems when placing, but the inclusion of a super-plasicizer should increase slump targets to around 100mm, perfectly suitable for placing structural concrete, by skips or pumps.

Design Using HSC Design Using HSC * Design: The basic principles of BS8110 may be readily adopted, but some amendments are required to account for the increased concrete strength.Guidance on the use of HSC is given in ‘Concrete Society Technical Report 49, Design Guidance for High Strength Concrete’, and is based on amending BS8110: Parts 1 & 2 to allow for the use of HSC. Alternatively guidance on the use of HSC can be found in EC 2.* Construction: With the inclusion of HSC, the greatest asset to construction works will be the relatively short time at which formwork will be able to be struck, so speeding up construction time and turn around in repetitive concrete operations, such as pouring suspended floor slabs, pouring column boxes etc. * Costs: The cost per m3 will be higher than standard concrete mixes, but it should be offset against the reduced tonnage of reinforcement, amount of formwork, and labour costs associated with these activities. Also, due to the increased time to achieve allowable working strengths, the turn around of construction could be increased.

CONSTRUCTION BEST PRACTICE(After Dr Richard Moss - Building Research Establishment BRE)

CONSTRUCTION BEST PRACTICE(After Dr Richard Moss - Building Research Establishment BRE)

* National Structural Concrete Specification (NSCS) for building construction1) Standard Specification

- Section 1; Materials, Workmanship and Construction- Section 2; Falsework and Formwork- Section 3; Reinforcement- Section 4; Concrete and Concreting- Section 5; Pre-cast Concrete- Section 6; Pre-stressed concrete - Section 7; Construction Accuracy

2) Project Specification 3) Guidance Notes

Advantages of NSCSAdvantages of NSCS

* Authorative, allowing level uniformity to developed between designers,contractors and concrete suppliers.* Straightforward, reducing possibility of mis-communication and mistakes.* Performance based, not prescriptive, allowing for more economic designs to be completed to EC 2.* Collaborative not divisive, assisting in the principle of partnering and best endeavours.* Continually under review, allowing for changes in statutory law, methods of design, increased research and changes in technology. * Should assist in providing more economic construction process, making reinforced concrete a more attractive proposition to clients.* Will provide designers with a standard specification, reducing work required to create new specification for each and every project.* Will reduce the risk to contractors, by providing them with a standard document to which they can familiarise and hopefully become comfortable with.

Best Practice GuidesBest Practice Guides

Eight best practice guides are currently available:* Improving Concrete Frame Construction.* Early Age Strength Assessment of Concrete on Site.* Improving Re-bar Information and Supply.* Concreting for Improved Speed and Efficiency.* Early Striking and Improved Back-propping for Efficient Flat Slab Construction.* Rationalisation of Flat Slab Reinforcement.* Pre-fabricated Punching Shear Reinforcement Systems for Reinforced ConcreteFlat Slabs.* Flat Slabs for Efficient Concrete Construction.Two further guides under development:* Ultra high strength novel jointing materials.* Rapid floor construction.

CONCLUSIONSCONCLUSIONS

* BS8110: Part 1 is acceptable, familiar and commonly used in by engineers in the UK, is conservative but acceptable, we should be adopting Part 2 to producemore economic designs, if we are required to use BS8110 contractually.* EC 2 is coming 2003, due to be adopted in 2008, but subject to ratification by UK. It is available to be used now, with National Application document.* FES is set to become a key factor in building design, and with the amendments to Building Regulations Part L, ‘Sustainable Construction’ will become paramount. Jacobs Gibb are ideally suited to co-ordinate, with the ‘in-house’ expertiseutilising services, structural and civil engineers, and architects, and become a leader in the field.* Increased emphasis on fire performance and robustness of building will become a major social factor in the world of tomorrow, focussing on possible terrorist attack, e.g. US Dept of Defence, UK Defence Establishment, as well as major international institutions. A key feature of a relationship based company is to perceive and remove a client’s fears.

* High Strength Concrete is readily available from ready mix suppliers. The utilisation of different concrete grades is allowed, and should be encouraged to allow the rationalisation of structural members throughout a building and keep similar structural sizes throughout, to allow repetition of design, detailing, construction.* HSC allows for early striking of formwork, and so a reduces construction time. If partnering schemes are to work, then contractors will look favourably on ‘sympathetic’ engineers, and savings will be made by all.* NSCS is available and should be adopted, so that a level playing field is brought to concrete construction, to allow for a more standard product as is available with structural steel.* Wherever possible, sufficient time should be allocated so as to allow discussion with contractor, so as to make best use of his experience, to establish how he wishes to carry out his works, focussing on possible problems and addressing them at the design stage.