CONCRETE STRUCTURES05

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CONCRETESTRUCTURES05JANUARY2005

The Concrete Centre is the central marketdevelopment organisation for the £5 billion UKconcrete sector. The Centre works in the interests ofall those involved in concrete design andconstruction. It focuses on design and constructionmethods, education and training, research, newproduct and process development and theperformance of concrete in practice. A free nationalhelpline for all concrete-related queries is availableon 0700 4 500 500.

The Concrete Centre works closely with other well-established cement and concrete bodies in the UK.For more information see: www.concretecentre.com

front cover image Client: Salvation ArmyProject: 101 Queen Victoria StreetArchitect: Sheppard RobsonEngineer: Arup

CONTENTS2 Resources for designers

3 Versatile concrete

4 Schools for the future

5 Thermal mass and structural design

6 Hybrid concrete construction

8 Eurocode 2

10 Finite element analysis

11 Concrete cores

12 Structural assistance

The concrete sector is leading the constructionindustry in the introduction of Eurocodes. TheConcrete Centre is making the followingresources available in 2006 to enable designersto follow this lead.

Publications and software

• Concise Eurocode 2 – CCIP-005All the essential information needed to design simple frames to Eurocode 2

• Worked examples – CCIP-007Element design to Eurocode 2

• Spreadsheets and User Guide – CCIP-004Update of the RCC’s RC Spreadsheets toEurocode 2 (and to BS8110 Amendment 3)

• ‘How to’ extractsFree extracts from CCIP-006 explaining howto design concrete structures to Eurocode 2:

1. Introduction

2. Getting Started

3. Slabs

4. Beams

5. Columns

6. Foundations

7. Flat slabs

8. Deflections

Websitewww.eurocode2.info

CALcreteComputer aided learning of Eurocode 2 (and BS8110 )

CPDs45 minute talks by The Concrete Centre staffthrough Societies, Institutions and byarrangement in offices.

TrainingOne and two-day courses available

Lecture material5 basic PowerPoint files dealing with flexure,basis of design, shear, deflection, detailing

For further information on the above contact the free nationalconcrete helpline on tel: 0700 4 500 500, visitwww.concretecentre.com or www.eurocode2.info

RESOURCES FOR DESIGNERS

HOW TO DESIGNCONCRETE STRUCTURESUSING EUROCODE 2This cement and concrete industry publication (CCIP),published by The Concrete Centre, aims to make thetransition to Eurocode 2 as easy as possible bydrawing together in one place key information andcommentary required for the design of typical concreteelements - flat slabs, beams, etc… For furtherinformation of Eurocode 2 see pages 8-9.

The publication, CCIP-006, can be pre-ordered from ourfree national helpline, 0700 4 500 500. Alternativelyextracts will be available in forthcoming issues ofStructural Engineer.

Concrete offers the structural engineer a widerange of material choices and constructionforms to choose from to meet designconstraints and performance requirements.

These constraints and requirements vary foreach building, but concrete warrantsconsideration in all sectors.

Construction techniquesIn the residential sector, flat slab constructionoffers the thinnest possible structural solutionand, hence, minimises cladding costs.Increasingly these slabs are being post-tensioned making them even thinner. (Typically50-75mm less than conventional flat slabs). Forstudent accommodation and hotels, tunnel formconstruction and precast crosswall are fast tobuild. They take advantage of the cellulararchitecture by also making the separating wallsthe structure, thereby reducing the period toerect internal partitions. Both tunnel form andcrosswall can be arranged with openings fortwo and three bedroom apartments.

Hospitals and laboratories are the most heavilyserviced buildings - the flat soffits of flat slabsprovide infinite flexibility during design and,more importantly, operation for servicesdistribution. Flat slabs are also the mosteconomical at meeting vibration criteria.

In the retail and schools sector adaptability is animportant design issue, but it means differentthings in each sector. In retail the ability to meettenant demands means being able toaccommodate large voids (e.g. escalators) andhigh imposed loads (e.g. partitions). Hybridconcrete construction, utilizing the best of in-situand precast concrete, can offer this flexibility, butsome design teams opt for in-situ slabs withjudicious over provision of reinforcement andincorporation of knockout panels.

In the schools sector, the adaptability required isgenerally on classroom size and often leads todesigns with columns with in-situ slab solutions(flat slab, ribbed slab or one-way slab) or precastfloor planks on beams. However, crosswallsolutions with large openings (75% of classroomwidth) have been used to provide flexibility ofjoining classrooms (refer to article p.4 ).

Long span solutions offered by post tensioningmake concrete an option to be considered for allcommercial projects. 12m x 12m post-tensionedflat slabs are economical, as demonstrated bycurrent London projects. For longer spans up to18m, one-way post-tensioned slabs on post-tensioned band beams provide an office solutionthat avoids the constraint of integrating servicesand structure.

Other concrete construction techniques maysuit particular projects. Precast concrete

cladding can be more than simply cladding.Cladding panels with factory fitted insulationand decorative finish are being produced onautomated carousels. They can be made asload bearing elements – supporting theperimeter of floor structures.

Performance as standardThe changing regulatory environment is directlyaffecting other members of the design team,but indirectly affecting the structural engineerand his/her solutions.

Changes in acoustics regulations (Part E) formaliseswhat structural engineers have always known –concrete’s mass makes it ideal for acousticperformance. The required levels of soundreduction can be met with a minimum of finisheswhen concrete structural solutions are adopted.

Vibration criteria for hospitals and laboratoriesoften dictate design choices. An Arup report1

concludes that concrete meets vibration criteriaat little or no extra cost, whereas other solutionsrequire significant additional mass and cost.Once again structural engineers have alwaysknown this and continue to adopt concrete.

Thermal mass is primarily the domain of thearchitect and services engineer. The changingEnergy use in buildings regulations (Part L) willlead to thermal mass being utilised more widelyand the necessary exposure of concretesurfaces will impact on structural design choicesand wider use of concrete (refer to p.5 ).

Inherent fire resistance has and remains a keyadvantage of concrete structures. Whilst lifesafety requirements may minimise fire resistanceperiods, there is a drive for property safety andthe ability to survive complete burn-out2. Thesurvival of the concrete structure of the Madridtower to complete burn-out is testament toconcrete’s inherent fire resistance. This shouldperhaps be a key driver in schools where arsonrates are high and self insurance is common.

BRE, along with the Association of BuildingInsurers is currently working on Loss PreventionStandard LPS 20:20: a document thathighlights, amongst other things, durability,design life, flooding resistance and damageresistance. In areas such as these, concrete’sinherent properties make it ideal for socialhousing and public buildings.

Material ChoicesThe range of concretes available offer thestructural engineer wide choices. The latestdevelopments in ultra high strength concrete withcompressive strengths up to C160/C200 andtensile strengths of up to 50 MPa (utilising steelfibres), have been applied to structures overseaswith inspiring results (see picture). Which engineerwill be the first to use this material in the UK?

TimeThe material choices and construction formsavailable with concrete enable time constraintsto be met. If overall construction period iscritical, precast offers the option of pre-manufacture and quick erection. For short lead-in time in-situ concrete offers the best solution.In terms of overall construction period, concreteoffers the fastest solution as follow on tradesand cladding can closely follow a concreteframe to ensure the earliest completion.

CostA check list of items to consider whenassessing cost and value of a structural frameis available from www.concretecentre.com.This highlights the importance of considering allissues, not frame cost alone when choosing astructural solution. Concept.xls is asophisticated new spreadsheet for theconceptual design of reinforced concreteframes. It should be run on every new projectto compare frame choices. Up to date rates toimport into Concept.xls are available fordownload from www.concretecentre.com.

1 Hospital Floor Vibration Study Arup 20042 Safety in Tall Buildings and other buildings with large

occupancy IStructE 2002

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VERSATILE CONCRETE:ALL OPTIONS AVAILABLE FOR YOUR NEXT PROJECT

Ultra-high strengthconcrete spiral staircase,Tuborg Nord, Copenhagen

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The design and construction of schools canplay a major role in the effectiveness ofteaching. This quality of provision is thefocus of the Government’s Building Schoolsfor the Future campaign (BSF).

BSF is the biggest single governmentinvestment in improving school buildings forover 50 years. The aim is to rebuild or renewevery secondary school in England over a 10 –15 year period. This year the Chancellorannounced that he was extending the Schoolsfor the Future initiative into the primary sector.Brown said that he was planning to invest atotal of £9.4bn over the next five years.

There are a number of key design issues forprimary and secondary schools. These areoutlined in the Building Bulletins BB99 and BB98and the ‘Key Design Guidance for Schools:access to information for school design’ fromthe Department for Education and Skills.

Key design issues for schools for the future include:

• Inspirational design to positively influence thebehaviour and enthusiasm of pupils and staff.

• Flexibility and adaptability for short and longterm changes.

• Comfort with regards to lighting, heating,ventilation, humidity and acoustics and useof thermal mass and night cooling.

• Safety and security

• Sustainability

• Minimum BREEAM rating of ‘very good’

• Fire resistance

• Robustness in terms of reducedmaintenance resulting from robust wallfinishes for internal and external walls andceilings

• Implementation in terms of timing andphasing of construction, cost to becalculated on a life-cycle cost basis,buildability and off-site constructionopportunities

Structural considerations include the imposedloading for a typical classroom being 3.0 kN/m2

plus partitions. The acoustic design can lead to

the use of relatively heavy partitions and anallowance of 2.5 kN/m2 for partitions is likely tobe a minimum.

The room areas lead to typical spans of 7.5mto 8.0m for classrooms and 16.5m to 17.5mfor the larger sports and main hall areas.

One of the key design issues is to ensure shortterm and long term flexibility and adaptabilityconcerning the size and layout of the rooms.There is usually a cost associated withproviding a structure that will allow the mostflexibility and adaptability. Short term flexibilitycould be achieved by movable partitions.However, movable partitions are expensive andalso have to satisfy the acoustic requirementsof BB93. An alternative way to provide flexibilityis to build larger classrooms, in this way thepremium paid for adaptability has an instantreturn by way of more space. Adaptability mayrequire the structure to be built in largestructural sections of say three classrooms of60 m2 that can be changed to one of 70 m2

and one of 110 m2. As always, the client mustdecide how much flexibility he can afford upfront, and if the funds will be available to takeadvantage of it in the future.

Precast concrete walls, concrete twinwall andmasonry crosswall construction with precastconcrete floor units are all ideal structuraloptions for classrooms. If maximum futureadaptability is required then an in-situ flat slabconstruction would be appropriate. The soffitsof the concrete floors can be exposed to usethe slabs’ thermal mass together with nightcooling to reduce the need for energy intensiveair-conditioning. This is a significantsustainability benefit and will help to satisfy theBREEAM schools assessment and reduce thewhole life cost of the project.

Concrete’s heavyweight mass means that it caneasily meet acoustic insulation requirements andits inherent fire resistance means that it is likelyto exceed the minimum fire requirements at noextra cost. The resulting repair cost followingany fire will thus be reduced. Using precastconcrete on its own or with in-situ concrete, (i.e.hybrid concrete construction), provides robustexposed finishes and can give a shorter andmore risk free construction programme.

The area of a room depends on the use and type of school, primary or secondary. Guidance onappropriate room areas is given in BB98 and BB99. For initial design some typical classroom sizes are:

Primary school room Area m2 Secondary school room Area m2

Nursery and Reception 63 Typical classroom 59Infants and Juniors 57 Science laboratory 85Main Hall 180 Textiles and 3D art rooms 106Music Classroom 68 Main Hall 294

SCHOOLS FORTHE FUTURE

Guidance for designInformation on BSF is at www.bsf.gov.uk andthis site contains links to other school websites. Partnership for Schools (PfS) is a newbody, jointly managed by the DfES andPartnerships UK, with the participation of PublicPrivate Partnerships Programme (4ps), to co-ordinate the national delivery of the BSFprogramme. PfS has been created to be thedelivery vehicle for the BSF programme andrefers for design information to the TeacherNetweb site, www.teachernet.gov.uk .

At the TeacherNet web site the DfES document“Key Design Guidance for Schools: access toinformation for school design”, updated August2004, can be downloaded. This guide refers tovarious Building Bulletins for school design andthese can be downloaded. Some of the maindesign Building Bulletins are:

• BB98: Briefing Framework for SecondarySchool Projects.

• Briefing Framework for Primary SchoolProjects. First Draft.

• BB87: Guidelines for Environmental Design inSchools. 2nd Edition (May 2003)

• BB93: Acoustic Design of Schools

• BB7: Fire - is due in 2005.

• BB95: Schools for the Future

The constructional standards BB87 and BB93are used by Building Control Bodies as thenormal means of assessing compliance withthe Building Regulations for Schools. BuildingRegulation Approved Documents in support ofPart F, 1995: Ventilation and Part L2, 2002:Fuel and Power both quote BB87. Theconstructional standard for acoustics is nowBB93 and this is quoted in AD E 2003.

The government is keen to ensure thatsustainability is at the top of the agenda for allthose involved in the construction of schools.DfES requires that new primary school projectscosting over £500k and new secondary schoolscosting over £2m achieve a BREEAM rating ofat least ‘very good’. Details of the BREEAMSchools environmental assessment can befound at the Building Research Establishmentweb site www.breeam.org

In June 2003 the Government appointed elevendesign teams to develop exemplar designs forschools fit for the twenty-first century. Thedesigns examined both primary and secondaryschools, including a 5 to 18 all-through school,on a range of sites. The exemplar designs andcomments on them can found in “ExemplarDesigns Compendium”. This can bedownloaded at www.teachernet.gov.uk

The potential of concrete’s thermal mass torealise fabric energy storage (FES) solutionsfor improving the energy efficiency ofbuildings and so reduce their carbonemissions is being increasingly recognised.Whilst this is primarily the domain of thearchitect and services engineers, thestructural engineer needs to accommodatethe subsequent structural requirements.

The UK Climate Impacts Programme predictsthat by the 2080s, annual temperatures for theUK may increase by up to 3.5ºC. In London,peak summertime temperatures are likely toincrease by around 7ºC, taking a warm summerday to over 40ºC. This will have a considerableimpact on the internal temperatures within thebuildings that we are designing and buildingtoday, especially those that do not exposethermal mass. Growing concern over climatechange, increasing energy prices, and changesto the Building Regulations are increasingpressure on designers and clients to realise thepotential of Fabric Energy Storage (FES). FES isthe ability of heavyweight materials, such asconcrete, to absorb and store heat.

Typically, using concrete’s FES can result ininternal temperatures being some 8ºC coolerthan peak external summertime temperatures.The provision of a cooler and more comfortableinternal environmental would have a positiveeffect on the workplace.

The minimum impact on the structural engineeris the exposure of the floor structure. This willaffect the specification of surfaces and can bea driver for the use of precast concrete due toits high standard of exposed soffit finish. It mayalso act as a driver towards using a cofferedslab, which with its additional surface area canincrease cooling by 10 to 20%.

In addition the cooling strategy may require airmovement into atria or façade chimneys. Thismay influence the structural solution, forexample, the avoidance of downstand beamswhich otherwise interrupt the airflow. The mostintegrated designs will include air or watermovement through the floor structure. This willaffect the design and construction of the end ofthe floor units where there is structural supportand entry/exit ducts. Established systems, suchas TermoDeck, have standard solutions for this.

The optimal thickness of concrete floors tomaximise FES is greater than 250mm. Thisrelies on exposure of both sides of the slabs.The top surface of the slabs can be ‘exposed’by passing air through a floor plenum andhaving floor diffusers.

A FES system using exposed concrete soffitsprovides a cooling capacity of up to 25W/m2 offloor area. Active FES systems, using ducted air

within the concrete slab, can provide cooling of upto 40W/m2. Alternatively, water can be used in anactive concrete system as an effective cooling (orheating) medium. Embedded pipes in floor slabscan achieve a cooling capacity of 80W/m2. Thesefigures compare with typical heat gains in officedues to equipment, lighting and people of 20 to30 W/m2 and solar gains of up to double this.

FES is by no means a new technology, indeedthe principle is the basis behind the constructionof ancient civilisations throughout theMediterranean. With Britain predicted to havetemperatures more associated with SouthernFrance, FES must become a key feature in thedesign of new buildings. The high thermal massof concrete can contribute positively and costeffectively to a significant reduction in the energyconsumption of buildings by minimising the needfor air-conditioning. Best of all, this is aperformance advantage that you get for freefrom concrete construction.

A report from The Concrete Centre provides anintroduction to a range of FES solutions forbuildings appropriate for the designprofessional, including natural ventilation withexposed soffits, underfloor ventilation withexposed soffits, permeable ceilings, hollowcoreslabs with mechanical ventilation, and water-cooled slabs. Copies of ‘Thermal Mass: AConcrete Solution for the Changing Climate’are available free of charge from The ConcreteCentre, tel: 0700 4 500 500, or visitwww.concretecentre.com

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Exposed concrete is a major featureof the environmentally acclaimed

Jubilee Library, Brighton.

THERMAL MASS ANDSTRUCTURAL DESIGN

CASE STUDYExposed concrete is a major feature of theenvironmentally acclaimed Jubilee Library inBrighton. The central space of the library isconstructed from a reinforced concrete tablesupported by a series of eight tree-likeconcrete columns with fins that support anexposed ceiling soffit. The thermal sinkprovided by this exposed concrete plays animportant role in the building’s FES approach.Incorporated within the ceilings of the roomson either side of the central atrium are 1200 x260mm TermoDeck precast hollowcore slabs.During the summer, air is pumped through theTermoDeck to cool temperatures inside thebuilding. In addition to the exposed concrete,three five-metre tall wind towers on the roofdraw warm air up and out of the building.

The library has a conventional air-conditioningchiller unit for use when outside temperaturesexceed 30ºC, but even during the peaktemperatures experienced in 2005, the unitwas not needed. The overall cooling effectachieved by FES and the associated passivecooling system is estimated to be some 5ºClower than the ambient external temperature.

Precast columns and edge beams with cast in-situ floor slab

Advantages:• Columns can be erected quickly

• Precast quality finish for columns

• Precast edge beam houses post tension anchorages, slab edgereinforcement and cladding fixings. Avoids need for slab edge shuttering.

• Post-tensioning minimises materials handling, steel fixing and striking times

• More flexibility for late changes

Example – Chiswick Park, LondonUtilising post-tensioned in-situ slabs with precast columns and edgeunits ensured the most efficient structural solution was specified whichsuccessfully met the fast-track programme.

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Hybrid concrete construction (HCC), in which in-situ and precastconcrete are combined for maximum benefit. The drivers for this includecontractors seeking ways of speeding construction and architectsdemanding precast quality soffits to utilise concrete’s thermal mass.

• Hybrid concrete construction provides:

• Faster construction

• Improved safety on site

• Cost effective construction

• Simple, buildable structures

• Excellent fire performance

• Sustainability benefits associated with high thermal mass

• Exceptional acoustic performance

Whatever the drivers, engineers have many hybrid concrete options tochoose from. Those presented here are representative of current UK practice.It is not intended to be exhaustive but to reflect the spectrum of possibilities.

Key:precast in-situ

HYBRID CONCRETEPrecast twin wall and lattice girder soffit slab with in-situ infill and topping

Advantages:• Precast quality finish for walls and soffits

• No formwork for vertical structures

• Structural connection between wall and slabs is by standardreinforced concrete detail, is inherently robust and for basements canbe made watertight

• No permanent sealing at connections between precast units

• Flexible for casting-in items

Example – Hilton Hotel, More LondonThe walls have two skins of precast concrete including all of the engineer'sreinforcement requirements and a central cavity which is filled with in-situconcrete at site. Floor units are 50mm thick with 150mm topping andtypically span 4.2m. The hotel floors (21 bedrooms) including all mainstructural walls, floors, lift and stair risers were completed in 5-day cycles.

Hilton Hotel, More London

Photo: Courtesy of JohnDoyle Construction

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‘Hybrid concrete construction: combining precast and in-situ’is a 12 page publication that provides further information and casestudies. ‘Best practice guidance for hybrid concrete construction’,explains the principles of hybrid concrete construction with detailed‘process maps’. Copies are available from The Concrete Bookshop,tel: 0700 4 500 500, or visit: www.concretebookshop.com

CONSTRUCTIONPrecast columns and floor units with cast in-situ beams

Advantages:• Vertical structure can be erected quickly,

no formwork required

• Precast floor structure can be erected quickly, no formwork required

• Precast quality finish for columns and soffits

• Structural connection between precast elements is via standard reinforced concrete details

Example – Homer Road, SolihullHCC was used to create a moment frame. The structure comprisesprecast columns, precast beam shells with in-situ infill and floor units.These are all exposed. The soffits of the concrete floor slabs areexposed so that temperature and ventilation strategy can exploit thepotential of the concrete’s thermal mass for fabric energy storage.

Cast in-situ columns and beams with precast floor units

Advantages:• Precast floor structure can be erected quickly

• Precast quality finish for soffits

• In-situ can account for site irregularities

Example – Whitefriars, CanterburyThe use of HCC reduced construction costs and time. In the multi-storeycar park, hollowcore floor slabs on in-situ beams provided 16 x 9.6mbays. Use of hollowcore for the ground floor, with perimeter in-situconnection to the secant piled walls allowed top-down construction withno further slab falsework.

Cast in-situ columns and floor toppings with precast beams and floor units

Advantages:• Precast flooring can be erected quickly

• Precast beams support precast floor planks minimising floor propping

• Precast quality finish for soffits

• Formwork for in-situ columns can be used to prop precast beams

• Structural connection between precast elements is via standardreinforced concrete details

• In-situ topping to beam permits beams to be continuous over columns

Example – Home Office, LondonIn order to meet tight programme deadlines and the demand for a highquality, durable structural frame, a hybrid solution which comprisedprecast floor planks and precast beams with in-situ core walls, columnsand slab topping was chosen

Photo: Courtesy ofFoggo Associates

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The introduction and implementation of thenew Eurocodes is a significant event for theUK construction industry. BS EN 1992,Eurocode 2: Design of Concrete Structureswill affect all concrete design once thecurrent British Standards, BS 8110 forDesign of Reinforced Concrete Structures,BS8007 Design of Concrete Structures forRetaining Aqueous Liquids and BS5400Steel and Concrete Bridge Design havebeen withdrawn. This is due to happen by2010, but BS 8110 may be withdrawn asearly as January 2008.

Ultimately Eurocode 2 will become the onedesign code for all concrete structures in theUK and Europe. It will bring reinforced concretedesign up-to-date. Whilst Parts 1-1 and 1-2have been published, we await Part 2 Bridgesand Part 3 Liquid Retaining Structures and allthe National Annexes (NAs). NAs give specificrules for the use of Eurocode 2 in a specificcountry. The UK annexes for Parts 1-1 and 1-2should be available for use from January 2006.Once these are published it will then bepossible to use Eurocode 2.

Some European Standards will not beavailable, but as the background document tothe UK NA explains, the intention is that during

the interim period, where not all ENs areavailable or are covered by UK NAs, relevantcurrent British Standards can be used.Examples include wind loads, design offoundations and couplers.

The design process will not change as a resultof using Eurocode 2. Eurocode 2 is laid out todeal with phenomena rather than elements.There are also specific rules dealing withbeams, slabs, flat slabs, columns, walls, deepbeams, foundations, tying systems and precastconcrete. In the long term, it is anticipated thatEurocode 2 will result in more economicstructures so conceptual design done to, say,BS8100 may confidently be taken through todetail design using Eurocode 2.

A range of resources is being made availablevia The Concrete Centre, to help with thetransition from BS8110 to Eurocode 2.

A dedicated website, www.eurocode2.info, hasnow been launched. This provides advice andassistance on the introduction, interpretationand implementation of Eurocode 2. In addition,there is latest news concerning Eurocode 2,detailed analysis and examination of the codeplus free downloads and a FAQ section. Aseries of seminars and courses on Eurocode 2

throughout the UK has begun and a series ofguides under the banner ‘How to DesignConcrete Structures using Eurocode 2’ will bepublished. These will be distributed free inrelevant publications once the UK NationalAnnex has been finalised.

The guides aim to make the transition as easyas possible by drawing together the keyinformation and commentary necessary for thedesign of typical concrete elements, such asslabs, beams, columns etc. The ConcreteCentre will also publish a Concise Eurocode 2(CCIP-005) that brings together information forbuilding structures, spreadsheets for design toEurocode 2 and a book of worked examples.Publications on civil engineering subjects suchas integral bridges worked examples will follow.

The UK construction industry faces a majorchallenge with the replacement of BritishStandards by Eurocodes. The Concrete Centreis making available a range of resources that willassist with the interpretation and use ofEurocode 2 and associated Eurocodes. Withthese resources, design offices can startintroducing Eurocodes through concrete design.If you require information or assistance relatingto Eurocoded 2, callour free national helpline on 0700 4 500 500.

EUROCODE 2

EUROCODE 2BENEFITSLearning to use the new Eurocodes will requiretime and effort, so what are the benefits?

• Eurocode 2 should result in more economicconcrete structures

• Eurocode 2 is less restrictive than British Standards

• Eurocode 2 is extensive andcomprehensive

• The new Eurocodes are claimed to be the most technically advanced codes in the world

• In Europe, all public works must allow theEurocodes to be used for structural design.

• Use of the Eurocodes will provide moreopportunity for designers to workthroughout Europe and for Europeans towork in the UK

• The Eurocodes are logical and organised to avoid repetition.

The 1st January 2006 will see the implementationof revised British Standards dealing with thereinforcement of concrete. Many of the revisionsare the result of falling into line with therequirements of European Standards. Otherrevisions reflect contemporary good practice.

In summary, reinforced material standards areas follows:

• BS EN10080:2005 Weldable steel for thereinforcement of concrete

It will be implemented in the UK in late 2005.it gives no actual specification or figures, thatis left to the National Standards.

• BS 4449:2005 Weldable reinforcing steel

This will be implemented in January 2006

• BS 4482: 2005 Steel wire for thereinforcement of concrete products

This will be implemented in January 2006

• BS 4483:2005 steel fabric for thereinforcement of concrete

This will be implemented in January 2006

For a full description of the changes visit:www.eurocode2.info

BS 8666:2005 Scheduling, dimensioning,bending and cutting of steel reinforcement.The main change is that there will be far moreshape codes available in BS 8666:2005. 34shape codes (plus shape code 99) will now beavailable, compared to the 13 in the 2000version. Old favourites have returned, including:

• Single leg link, SC22 (the old SC85 to BS 4466:1989

• Two-legged link, SC47 (the old SC77)

• Torsion links, SC63

• Chairs, SC98

New shapes have been introduced in order tostop the plethora of ‘standard’ shape code 99sthat have emerged from different sources.Fabricators have been reporting up to 10%shape code 99s going through. That is a lot of

bars for the detailer to draw, the fabricator toprocess (redraw, bend, check) and the fixer torecognise. So the decision was made toincrease the available number of shape codes inBS 8666. Shape code 99s will not becompletely eradicated, but the hope is that therewill be considerably less – to everyone’s benefit.

The notation of steel reinforcement has had to change with the changes to BS 4449 andrequirements of BS EN 1992-1-1. The newnotation is set out in table 1. Grade 250 hasdisappeared, it does not comply with BS EN1992-1-1 nor does it have any economic orbend advantage. ‘H’ indicates 500 grade,hence will replace ‘T’ in T16, T25 etc. A, B andC indicates 500 grade with the ductility of (Agt

percentage elongation at maximum force) of atleast 2.5%, 5.0% or 7.5% respectively.Reinforcement to BS 4482 is no longerallowed, it does not conform to therequirements of BS EN 1992-1-1.

It has been recognised that many schedulesare by agreement, prepared, sent and used aselectronic data. A new section gives guidanceon the preparation of such files – essentiallythey should follow the requirements of thepaper-based versions.

The requirements of BS 4483: 2005 havecaused the withdrawal of standard fabrics A98and B196 and changes to fabrics C503, C385and C283 where 5mm bars have beenreplaced with 6mm bars.

It should be noted that the IStructE/ConcreteSociety detailing manual has been updated andshould be available in late 2005. It embraces

the changes in BS 8666: 2005. The manual willreflect current good practice.

ImplementationBS 10080: 2005 is due to be implemented atthe end of November 2005. BS 4449: 2005,BS 4482:2005 and BS 4483: 2005 will beimplemented on 1st January 2006.

The revisions will cause some major changes inthe reinforcement supply chain. But designers,specifiers and contractors will need to heedsome of the changes too. UK CARES(www.UKCARES.co.uk) have already beeneducating fabricators and mills about theimminent changes.

BS 8666: 2005 is intended to be used withmaterial to BS 4449 (1997 or 2005 versions) andBS 4483. It will be implemented on 1st January2006 at the same time as BS 4449: 2005, BS4482: 2005 and BS 4483: 2005. It is presumedthat existing projects will continue to use theolder standards and that only new 20006designs to BS 8110 or Eurocode 2 will embracethe new reinforcement standards. It will thereforebe some time before schedules to BS 8666:2005 feed through the system. It should be clearfrom the bottom of the bending schedule whichversion of BS 8666 is being used.

Users will be assured to know that material to BS4449: 2005 will satisfy BS 4449: 1997. However,designers must not use the B500, 500Mpa to BS4449: 2005 with the y factors in BS 8110: 1997.

The British Association of Reinforcement intendto release laminated versions of the revisedshape codes in time for the changeover.

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Table 1 – Notation of steel reinforcement

Type of steel reinforcement Notation

Grade B500A, Grade B500B or Grade B500C conforming to BS 4449:2005 HGrade B500A conforming to BS4449:2005 AGrade B500B or Grade B500C conforming to BS 4449:2005 BGrade B500C conforming to BS 4449:2005 CA specified grade and type of ribbed stainless steel conforming to BS 6744:2001 SReinforcement of a type not included in the above list having material properties X

NOTE: in the Grade description B500A, etc ‘B’ indicates reinforcing steel

NEW REINFORCEMENT STANDARDS

Amendment 3 to BS 8110:1997 was issued forpublic comment in 2005. While the amendment islarge and affects many clauses, the actual changesessentially revolve around two new standards.

BS 8500Recommendations for durability will beremoved from BS 8110:1997 and replacedwith references to exposure classification anddurability in BS 8500. There are also changesin terminology, eg the dual cylinder/cubestrength notation.

Nominal cover – maximum covers from BS8110:1997, bond etc and from BS 8500 for

durability etc + design fixing tolerances (whichis taken to be 10mm unless the fabrication issubject to a quality assurance system in whichcase may be reduced to 5mm.

The The Concrete Centre’s ‘How to use BS8500 with BS 8110’ is available fromwww.concretecentre.com to help usersdetermine required concrete and covers.

BS 4449:2005UK industry has agreed to adopt Grade 500high yield steel for reinforcement. The 500 MPastrength may be considered as being acharacteristic strength and until such time as

there is sufficient data available to the newstandard in BS 8110, it has been consideredwise to increase the material factor to 1.15.This necessitates many changes, especially1/1.05 = 0.95 to 1/1.15 = 0.87 in manyformulae. Engineers will note that 460/1.05 verynearly equals 500/1.15.

Other changes include: a new tying provisionthat requires two bottom bars to pass throughthe tops of columns; changes to the anchorageprovisions for precast floor, stair and roofmembranes; reference to model specificationfor bonded and unbonded post-tensioned flatslabs and updated references.

BS 8110:1997 : AMENDMENT 3

Finite Element (FE) analysis, a powerfulcomputer method, has become anincreasingly popular method for analysingflat slab concrete structures. However, thereare some pitfalls to avoid, that often catchout the unwary, which are discussed below.

A common myth is that FE will give you lowerbending moments and deflections than wouldbe obtained by traditional methods. In fact, acomparative study was carried out by Jonesand Morrison1 and this demonstrated that usingFE methods for a rectangular grid gives similarresults to other analysis methods including yieldline and equivalent frame analysis. Therefore,for simple structures there is no benefit in usingFE analysis, and hand methods or specialisedsoftware are probably more time-efficient.

FE software is now relatively simple to use, butthe engineer should still understand what thesoftware is doing on his/her behalf and whatdefault parameters have been assumed in thepackage, particularly for deflection calculations.

A linear elastic FE analysis can be used byassuming reinforced concrete is an elasticisotropic material, as is generally assumed forhand analysis. It is suitable for carrying out adesign at the ultimate limit state (ULS) and theserviceability limit state (SLS) can be checkedby using one ‘deemed to satisfy’ span-to-depthratios in BS 8110, or by considering the long-term creep, shrinkage and cracking effects.

Ultimate Limit StateAt the ULS the key points to watch are:1. Columns should be included in the analysis

model or column stiffness should beconsidered in the analysis by calculating arotational spring stiffness in X and Ydirections at each support.

2. The ‘twisting moment’ Mxy needs to beapportioned to direction of the reinforcement(usually X and Y). The most widely usedmethod in the UK is ‘Wood-Armer moments’2,3,although there are other methods.

3. The maximum moment that can betransferred between the slab and the columnshould be checked against code provisions.

4. High peak moments are usually generated inconcentrated zones over the supports. Inreality, reinforced concrete will crack andyield under these peak stresses, which willbe distributed to adjacent areas. For design,these peaks should be averaged over adesign strip.

5. The design moments can be taken at theface of the column.

Serviceability Limit StateIt is acceptable to check deflection criteriausing linear analysis by determining thecracked stiffness of the slab (which isapproximately 1/2 the gross stiffness).However, since many programs calculate theslab stiffness from the depth of the slab, it isusual to adjust the elastic modulus to achievethe same result. The gross depth should beused in the analysis to ensure that the correcttorsional constant is used.

A long term elastic modulus must be used,which will generally be in the range 1/3 to 1/2of the short term value. When this is combinedwith the adjustment for the cracked sectionproperties, the elastic modulus used in theanalysis should be in the range 1/6 to 1/4 ofthe short term value.

Where an estimate of deflection is required thena non-linear analysis is essential. The usershould fully understand how the software iscarrying out the deflection calculations, sincethere is a large variety of methods employedwithin the different software packages. Evenwith the most sophisticated analysis, theestimate of deflection is reliant on the accuracyof the elastic modulus value used. This variesdue to many factors including age and durationof loading and properties of the aggregatesused. It is advisable to give a suitable caveatwith any estimate of deflection which otherparties will be relying on. Further advice on SLScan be found in Concrete Society report TR584.

ValidationBefore any analysis is carried out usingcomputer software, it is always good practiceto carry out some simple hand calculations toverify the results. It is particularly important todo this when using FE, and not treat thecomputer as a ‘black box’. A check should becarried out to confirm that the bendingmoments are reasonable. Also, the total loadon the slab should be calculated andcompared against the sum of the reactionsfrom the model. Always include any handchecks in your calculations.

The use of FE analysis is certain to increase inthe future. The practising engineer shouldunderstand how to correctly model concrete,know the limitations of the software and beable to correctly interpret the results. He mustalso ensure that sufficient validation checks arecarried out to ensure that the output is correct.

1. Jones A.E.K. & Morrison J. Flat slab design past presentand future, Structures and Buildings, April 2005

2. Wood RH. The reinforcement of slabs in accordance with apre-determined field of moments, Concrete, February 1968,pp69-76

3. Armer GST. Correspondence. Concrete, April 1968, pp319-320

4. Concrete Society. TR58 Deflections in concrete slabs andbeams Concrete Society 2005

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FINITE ELEMENT ANALYSIS OFCONCRETE FLAT SLABS

Deflection Contour Display courtesy of CSC Orion

The design and construction of largeoccupancy buildings continues to be subjectto review in the UK as the repercussions ofWorld Trade Center are taken intoconsideration. The NIST ‘National Institute ofStandards and Technology’ report into theWTC disaster was issued, following a periodof public comment, in September 2005 andhas thirty recommendations.

Two of these relate to core construction (see box).These closely match guidance already issued inthe UK. The Institution of Structural Engineersconvened a working group which prepared‘Safety in Tall Buildings and other buildings withlarge occupancy’ July 2002 (see box).

The guidance in both reports is that in additionto providing a fire resistant escape route, theroute must be robust. It must be hardened toensure it remains fire resistant and functionalwhen subjected to an event.

The provision of robust and hardened escaperoutes is most effectively done by usingconcrete. Since 9/11 there has already been a shift in design of tall buildings by majorconsultancies to using concrete cores. After all, rather than have to clad a skeletal frameand have to detail cladding to resist an event,the inherent robustness of a concrete wall willprovide both the structure and the cladding fora robust enclosure.

Constructing concrete coresConcrete cores can be constructed usingprecast concrete, in-situ concrete or hybridconcrete construction.

Precast cores are commonly used in 100%precast buildings such as crosswall residentialdevelopments or precast car parks, but arealso used elsewhere. They can be formed from2D panels or 3D units comprising ‘L’ elementsor closed cell elements. In all cases theconnections are the key to the solution. Hybridconcrete cores using precast twinwall with in-situ, infill are likely to become more common asthe use of precast twinwall increases generally(see page 6). In-situ cores remain the mostcommon perhaps because of the very highlevel of innovation in terms of speed, health andsafety and adaptability in recent years.

The three major in-situ methods of formingcores are jumpform, climbform and slipform.Whilst different formwork suppliers haveproprietary products, they fall into these threecategories. Jumpform and climbform havediscrete lifts after which the formwork is raisedfor the subsequent lift. They are raised by towercrane in the case of jumpform or are ‘selfclimbing’ in the case of climbform. Thetolerance of construction is limited by settingout rather than the formwork as this isfabricated to 1 or 2mm. The top of each lift canbe correctly set out each time. Typically higherstrengths of concrete (C50/60) are used toenable a quick succession of pours, often onelift per 24 hour cycle. Slipform systems areconstructed to equivalent tight tolerances.Based on a normal shift, slipform has littlespeed advantage. However, if 24 hour workingis possible slipform offers greater speeds ofconstruction. Slipform systems can operateeffectively on C32/40 mixes carefully designedwith appropriate retardants.

CONCRETE CORESNIST* recommendations relatingdirectly to core construction:

18 - Design of occupant friendly evacuationpaths that maintain functionality inforeseeable emergencies.

21 - Fire protected and structurally hardenedelevators

(NIST recommendations relating toprogressive collapse (1) and lateral stiffness(3) also have implications for coreconstruction, as do those relating to fireresistance).

IStructE** recommendations relatingto core construction:

7.4.1.2“Shafts containing escape routes need tohave sufficient structural robustness andintegrity so that there is only a small risk ofthem becoming impassable by occupantsduring an extreme event”.

7.4.1.3“It may be that properly designed andprotected lifts can be used for evacuationgenerally”.

*www.nist.gov/public_affairs/ncst/sept2005_meeting/sunderNCSTAC(2)091205%20final.pdf

** Safety in Tall Buildings and other Buildings with LargeOccupancy

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Lancefield Quay, Glasgow.

Photo: PERI Ltd

STRUCTURAL ASSISTANCEThe Concrete Centre has an ongoing programme of initiatives aimed at assisting all members of the professional andproject team. Those aimed at structural engineers include:

In-house CPD presentations

Eurocode 2An introduction to Eurocode 2, how the codesare structured and how they are appliedtogether with an examination of thepublications available to help designers.

Specifying appropriate and durableconcrete mixesExamination of how, following therecommendations of the new British StandardBS8500 (which supersedes BS 5328), tospecify the appropriate concrete.

Post-tensioned slabsIntroduction to the technique of post-tensioningand presentation of design parameters to allowconsideration of post-tensioning at the earlystages of project design.

Cellular construction for residential buildingsIntroduction to tunnel form construction andprecast crosswall structures that examines thedesign advantages and architecturalconstraints.

Hybrid concrete construction (HCC)HCC exploits the benefits of both precast andin-situ concrete. The CPD presentation explainsthe options available and demonstrates how togain the best out of HCC by considering theapproach at the early stages of procurement.

Software/Design Tools

Concept.xlsConcept.xls is a sophisticated newspreadsheet for the conceptual design ofreinforced concrete frames. Concept should berun on every new project to compare framechoices. It is quick and easy to use.Concept.xls is available on CD or as a free trialdownload. For this or registration forcommercial use and future updates visitwww.concretecentre.com.

RC-SpreadsheetsThe design spreadsheets help with the rapidproduction of clear and accurate designcalculations for reinforced concrete elements.RC-Spreadsheets are available on CD or as afree trial download. For this or registration forcommercial use and future updates visitwww.concretecentre.com.

CALcreteCALcrete is a comprehensive suite of 16computer-aided e-learning modules on concretematerials, design and construction. The suitecontains up to 20 days worth of essential CPDlearning material and is fully updated to the latestversion of Eurocode 2, January 2004. Trialdownloads are available from visiting the FrameBuildings section on www.concretecentre.com.

Practical yield line designA world leading pre-scheme design handbookthat helps designers choose the most effectivebuilding frame solution from in-situ, precast,hybrid and prestressed options. The handbookresults from the research carried out at theEuropean Concrete Building Project atCardington. Yield line design is a robust andproven design technique. For further details tel:0700 4 500 500 or visitwww.concretecentre.com.

Publications

National Structural ConcreteSpecification, Edition 3The National Specification has been producedby consultants, clients and main contractors aswell as specialist frame contractors. It is avaluable tool to promote better understandingand efficiency. For further details: tel: 0700 4500 500.

Best Practice Guidance for HybridConcrete ConstructionDefinitive procurement guidance and case studieson how to realise the benefits of hybrid concreteconstruction where the construction techniques ofprecast and in-situ concrete are married tocomplement each other. For further details tel: 07004 500 500 or visit www.concretecentre.com.

Best Practice GuidesA new series of case studies based on the StGeorge Wharf development in Vauxhall, London,where the recommendations and innovationsresulting from the European Concrete BuildingProject were trialled. Innovations includereinforcement rationalisation, advanceddeflection prediction techniques and the use ofspecial concretes. For free copies tel: 0700 4500 500 or visit www.concretecentre.com.

High Performance Hospitals usingConcrete Frames and CladdingGood hospital design can provide more efficientfacilities and a better environment for both staffand patients. Concrete construction provides greatopportunities for the project team to meet theneeds of the client by improving the function, valueand whole life performance of the facility often atno or little additional cost. For free copies tel: 07004 500 500 or visit www.concretecentre.com.

Thermal Mass: A Concrete Solution forthe Changing ClimateExploiting the thermal mass of concrete as part ofa Fabric Energy Storage (FES) solution canprovide an effective means of maintaining acomfortable internal environment while reducingor even avoiding the need for energy intensive air-conditioning. This publication outlines theapplication of FES techniques using cast in-situand precast floor slabs. For free copies tel: 07004 500 500 or visit www.concretecentre.com.

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Tel: +44 (0) 1276 606800E-mail: info@concretecentre.com

Internet: www.concretecentre.com

Photo: Falcon WharfCourtesy of Getjar