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1 Introduction The last century has seen the increasing need to provide space using multi-storey solutions. In the early days steel framing supporting floors of insitu or precast concrete was a popular solution. However in the post 1945 war years the knowledge, understanding, and familiarity by designers of reinforced concrete meant that concrete became the preferred solution for the next 35 years. By the late 1970s a mood developed to provide a more efficient and more design-conscious construction process than had been experienced hitherto, and this occurred during a time of change within the world of commerce with the advent of information technology. Whilst some buildings had incorporated air conditioning, it was the increasing use of IT and the building services needs this generated, that created the widespread use of false floors and false ceilings that sandwiched and masked the structure. Prior to this reinforced concrete of flush soffit construction that could receive plaster and decoration direct to the soffit had predominated. With more intense servicing generally disposed over the whole floor plate and false ceilings to hide this, the opportunity to provide downstanding construction giving more depth and leverage and hence economy returned. From the early 1980s we entered the era of the composite steel frame supporting concrete slabs reinforced by the surface bonded steel decking sheet rather than the embedment of rod reinforcement. The provision of insitu through-deck shear studding provided further significant economy to the steel beams and produced a construction process which totally eliminated shuttering and propping, ie construction efficiency through avoidance of material and man-hours spent on the temporary works previously needed to create the permanent project. This process of offsite prefabrication of steel parts assembled at site produces a quality solution, more dimensionally accurate, with reduced programme risk, which with the addition of the insitu poured concrete slab provides a plate diaphragm connecting the whole together without a loss of unity and robustness so often experienced with other prefabricated frame answers. On maturity of the over-deck concrete, an easily created composite section results to both the slab and supporting beams, the efficiency of which is a significant improvement on steel frames formed of independently designed parts or frames formed of insitu cast reinforced concrete. The resulting construction depth is less than that of traditional steel frames, and in structural terms we have obtained the greatest strength to weight ratio currently available, thus providing architectural opportunities to design more adventurous buildings of greater interest in their form, and the potential to give economic designs providing large internal clear span space. To use a motor transport analogy, modern composite steel has crossed the line where the payload (load applied onto structure) now exceeds, for whatever span we choose, the weight of the transport vehicle (structural dead load). This marriage is a massive move forward in structural efficiency, and achieves the best of both worlds from concrete and steel. 2 Form and Concept The ever increasing desire to seek, in the widest sense, designs and products of improved quality at better value requires an understanding of where significant worth lies in the total design and construction process. It is a fact that the greatest opportunity to achieve value for money is at inception, but some of this value at this earliest stage is subjective, particularly the value attributable to aesthetics. There are, however, some guiding rules that, if matters of planning and aesthetics allow them to be followed, will inherently help greatly in providing best value. It should not be forgotten that this greatest achievable value starts with the determination of the form and concept, and that this opportunity to achieve best value diminishes as we progress from inception to the final "nuts and bolts". This is not to say that best value should be ignored at the latter "nuts and bolts" stage; rather, that the ultimate best value is the sum of applying best value culture to all stages, but accepting that quantum of return diminishes through the process. Distilling this to the major points, we must acknowledge that once a market quality of the floor space has been established this is a "given", and obtaining best value is not a tampering with component quality and the cost thereof, but maximising value from the variables that produce the desired area of floor space at the given quality. The façade skin enclosing the building is a major cost component and examined idealistically we can enclose the greatest area (ignoring the lesser practicability of circles) by a square plan form, hence this form yields 43 CHAPTER 4 Multi-Storey Buildings By James H Mathys, Director, Waterman Group Plc

Multistorey General By James H Mathys

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Page 1: Multistorey General By James H Mathys

1 Introduction

The last century has seen the increasing need toprovide space using multi-storey solutions. In the earlydays steel framing supporting floors of insitu or precastconcrete was a popular solution. However in the post1945 war years the knowledge, understanding, andfamiliarity by designers of reinforced concrete meantthat concrete became the preferred solution for the next35 years.

By the late 1970s a mood developed to provide a moreefficient and more design-conscious constructionprocess than had been experienced hitherto, and thisoccurred during a time of change within the world ofcommerce with the advent of information technology.Whilst some buildings had incorporated airconditioning, it was the increasing use of IT and thebuilding services needs this generated, that created thewidespread use of false floors and false ceilings thatsandwiched and masked the structure. Prior to thisreinforced concrete of flush soffit construction thatcould receive plaster and decoration direct to the soffithad predominated. With more intense servicinggenerally disposed over the whole floor plate and falseceilings to hide this, the opportunity to providedownstanding construction giving more depth andleverage and hence economy returned.

From the early 1980s we entered the era of thecomposite steel frame supporting concrete slabsreinforced by the surface bonded steel decking sheetrather than the embedment of rod reinforcement. Theprovision of insitu through-deck shear studdingprovided further significant economy to the steel beamsand produced a construction process which totallyeliminated shuttering and propping, ie constructionefficiency through avoidance of material and man-hoursspent on the temporary works previously needed tocreate the permanent project. This process of offsiteprefabrication of steel parts assembled at site producesa quality solution, more dimensionally accurate, withreduced programme risk, which with the addition of theinsitu poured concrete slab provides a plate diaphragmconnecting the whole together without a loss of unityand robustness so often experienced with otherprefabricated frame answers.

On maturity of the over-deck concrete, an easilycreated composite section results to both the slab andsupporting beams, the efficiency of which is asignificant improvement on steel frames formed ofindependently designed parts or frames formed of

insitu cast reinforced concrete. The resultingconstruction depth is less than that of traditional steelframes, and in structural terms we have obtained thegreatest strength to weight ratio currently available,thus providing architectural opportunities to designmore adventurous buildings of greater interest in theirform, and the potential to give economic designsproviding large internal clear span space. To use amotor transport analogy, modern composite steel hascrossed the line where the payload (load applied ontostructure) now exceeds, for whatever span we choose,the weight of the transport vehicle (structural deadload). This marriage is a massive move forward instructural efficiency, and achieves the best of bothworlds from concrete and steel.

2 Form and Concept

The ever increasing desire to seek, in the widest sense,designs and products of improved quality at bettervalue requires an understanding of where significantworth lies in the total design and construction process.

It is a fact that the greatest opportunity to achieve valuefor money is at inception, but some of this value at thisearliest stage is subjective, particularly the valueattributable to aesthetics. There are, however, someguiding rules that, if matters of planning and aestheticsallow them to be followed, will inherently help greatly inproviding best value. It should not be forgotten that thisgreatest achievable value starts with the determinationof the form and concept, and that this opportunity toachieve best value diminishes as we progress frominception to the final "nuts and bolts". This is not to saythat best value should be ignored at the latter "nuts andbolts" stage; rather, that the ultimate best value is thesum of applying best value culture to all stages, butaccepting that quantum of return diminishes throughthe process.

Distilling this to the major points, we must acknowledgethat once a market quality of the floor space has beenestablished this is a "given", and obtaining best value isnot a tampering with component quality and the costthereof, but maximising value from the variables that produce the desired area of floor space at thegiven quality.

The façade skin enclosing the building is a major costcomponent and examined idealistically we can enclosethe greatest area (ignoring the lesser practicability ofcircles) by a square plan form, hence this form yields

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CHAPTER 4Multi-Storey Buildings

By James H Mathys, Director, Waterman Group Plc

Page 2: Multistorey General By James H Mathys

best cladding value, ie most floor area with leastcladding, giving the least cost per unit area of floorattributable to the external skin. For typical UK buildingheights, and for efficiencies of servicing, verticaltransportation, and the structure itself, the most efficientbuilding form tends to be a cube. Such floor spacehowever will have large distances between walls, ie itwill be deep space. Conversely, at the other extreme along thin building will have the worst proportion of skincost on the floor area, but has the quality of shallowwell-lit space. Adjusting the optimum cube toincorporate an internal atrium with lower cost non-weathering walls adds internal daylight and gives aracetrack of floor space to the desired floor spacewidth. This is not an advocation that all buildingsshould be cubical – but to have a realisation of theoptimum when designing, as shown in the illustration.

Ideal cubic form to give least wall cost to floor space created.

Building height is a cost variable that is determined bydesigners by the sum of all the individual componentsthat contribute to the floor-to-floor dimension, namely:the desired finished floor-to-ceiling dimension, thedepth of access floor for cable distribution, the desiredcolumn spacings and hence beam depths togetherwith the dimension needed for building services andfalse ceilings. All these should be chosen to achievethe minimum depth required to achieve the individualfunction with efficiency acknowledging the fact that anunnecessarily loose dimension will add to the storeyheight and to the cost of every vertical component ofthe building, ie structure, walls, cables, pipes, lifts,cladding, etc.

Time is a major cost component of a project and hastwo aspects:

Building life span

The resulting building should be of a concept thatis sustainable not only in its material durabilityneeded to achieve the design life, but in its design

style and dimensions to be a visually pleasing anduseful building as far as is predictable throughoutits design life, ie avoid repeating prematurerebuilding as occurred with 1960s buildings in the1990s attributable to dimensional limitations for re-use. A less than 30 year life is not normallyeconomically or environmentally sustainable.

Construction period

During design every component before beingspecified should be assessed for site constructiontime to assist in optimising the build rate overall.Care should be taken not to prejudice thebuilding’s overall life span by adopting solutionsthat simply maximise the build rate, nor should thedesign for speed of one trade component be at theexpense of another such that total time iscompromised.

Much assistance in optimising the constructionperiod is achieved by disengaging interfacesbetween parts, particularly at design stage, eg withsteel frames and prefabricated cladding a designercan avoid the time critical interfaces for claddingattachment by fixing cladding to the slab concreterather than perimeter steel. Cladding fixinginformation to concrete can be required manymonths later than cladding fixed to steel wheresuch information is required in time for steelfabrication. Firm and final cladding information isoften only available much later than when thesteelwork information has been finalised.

3 Foundations

The structural frame and foundations thereof aretypically in the range of 15 to 25% of the cost of thewhole building. However the safe and serviceableperformance of this is essential to the wholeinvestment. Malfunction of the non-structural 75 to85% may be inconvenient and disruptive but anymalfunction of the lesser cost structural and foundationcomponent affects the whole investment. Foundationstherefore carry everything, they are the most intenselyloaded part of the structure, and almost impossible toinspect later, yet they are the part that transfers thebuilding load to natural strata, the strength and vagariesof which are outside our control. It is therefore of primeimportance that foundations are conceived anddesigned by experienced professionals, and installedby competent trade contractors, to ensure that theytransfer building loads to the strata without risk offailure; and that the inevitable minor movements uponload application are of a magnitude that permits thebuilding to function without distress.

For multi-storey UK buildings, the common foundationtypes will be piles, pads (reinforced [RC] or massconcrete), RC strips, and rafts.

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External weathering wallcosts are expensive

components

Internal non-weathering wallcosts can be good value inproviding internal daylight;

permitting use of overallcube proportions

and mitigating “deep space”

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Pads and strips tend to be used on higher bearing strataor where loads are low; rafts where soils are low incapacity, or where other constraints exist or areproposed, such as underlying tunnels or services. Pileshave now improved in value and are frequently used formulti-storey buildings, particularly where moderninstallation techniques using single large diameter pilesfacilitate the incorporation of the first lift of steel with pileconstruction. Whilst this is most efficient for constructionprogress, it does require careful design and planning.The big gain in adopting this "plunged column" techniqueis the opportunity given to commence steel erectionsoonest, and leave behind all the usually more complexconstruction associated with the ground floor andsubstructure. This technique can be particularly usefulwhere buildings have two, three or more basementlevels, enabling the frame to progress upward with someor all bulk excavation yet to occur, but careful planning ofthe logistics of "muck away", ventilation, etc must beundertaken to optimise the time benefit (hence cost)against the aggravation of excavating through theproceeding permanent works installation. It usuallyoccurs that maximising uninterrupted "blue sky"excavation is best, with only the residual amount ofexcavation left to be "top down". Benefits in controllingsoil heave in deep basement construction are alsoworthwhile with the "top down" techniques.

Thus, whilst on a pure cost of material basis, pads andstrips are often more economic, the increasingpopularity of piling is due to the advantages that can begained by single piles with plunged columns enablingsignificant overlapping of substructure and frameconstruction to save on time cost, which is so oftenmore worthwhile than adopting conventionalfoundations requiring traditional bottom up construction.The following photographs, taken on the same day,show the degree of construction overlap achievable.

Three photos of 59-67 Gresham St on the same day

Deck bundles delivered to 6th; 5th floor decked; 3rdconcreted. Cladding to 3rd floor.

Sub-basement blinded, piles to be trimmed for stanchions tosupport first basement perimeter bays yet to be built.

5th Floor decking laid prior to concreting (3rd concreted).

4 Substructure

Multi-storey buildings usually being in high land costmetropolitan areas often extend downward for thesame reason of maximising value on a given land plot.Whilst such substructures often house functions thatwould otherwise take up more valuable above groundspace, ie storage, plantrooms, and parking, they canuse atria (refer to concept and form) to bring daylightinto a below ground level and increase the floor spacethat produces revenue, (as in Standard Life's building at10 Gresham Street, London shown in Foster andPartners’ illustration).

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Natural DaylightThrough Stepped

Atrium

Daylight YieldsRentable Space Below Ground

Street Level

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Substructures must be considerate of adjoiningbuildings and the sensitivity of these to movement,particularly if the adjoining buildings are to be undercutby the proposed excavation. Ground water levels are amajor consideration, and it is necessary to determinethe style of the perimeter retention system whether it istemporary or permanent, for cutting off the waterseepage both to facilitate construction and to avoiddewatering consequences on neighbouring buildings.Substructures need to be watertight, and variousqualities of watertightness are defined in BS 8102 forthe various intended uses of the space.

The dimensional relationship of the multi-storey frameover the substructure needs careful consideration,particularly around the building perimeter, as it is naturaleconomics that the building above ground will extendto the site boundary but details of below groundperimeter retention may determine that the steel framebelow ground is inset. Alternatively, such geometricdetails may restrict the style of perimeter retention tosystems that are capable of carrying the perimetercolumn loads, ie bored pile walls or diaphragm walls. Ifthis type of solution is adopted the differentialmovement between perimeter columns sitting on acontinuous embedded wall to that of the columns tothe first inward grid needs to be limited as part of thedesign analysis. The dimensional positioning of all partsof the substructure needs to allow for the opportunity toraise the steel frame independent of the more complexand slower construction rate of the substructureconstruction to gain the big economies of time cost.Alternatively it is often possible to design out complexretention systems and avoid boundary underpinning byadopting inwardly stepping profiles to the substructureboundaries as illustrated.

Three level basement at 55 Bishopsgate showing steppedretention system to Bishopsgate elevation.

The costs of excavation, dewatering and perimeterretention systems are significant, and designers shouldconceive the structure at ground floor and below tominimise the depth of the substructure. Whilst the mainsteel frame above ground may enjoy the benefits oflarge spans giving the column free highly servicedspace afforded by composite steel solutions, the oncost below ground of going deeper than necessarymust be appraised. The writer has found that the sub-division of a large span superstructure is an appropriatebest value answer for the lesser serviced substructure.Doing this produces construction depths to the groundfloor and other below ground suspended floors of farless depth than on the superstructure. Such depthsavings can easily exceed half the construction depthof that for the superstructure by using "Slimflor" or RCflat slab construction and on a two level substructurethis can save from the three construction layers about1000 mm on the bulk excavation, depth of retainingwalls etc and will mitigate dewatering or even avoidentering the groundwater.

5 Superstructure Floor Construction

Before determining the form of construction to beadopted for a particular project the designer has a dutyto his client to review all appropriate options. This isbest achieved by producing a formal "Options Report"stating the various pros and cons of each option for theproposed development.

The key points to consider are:

• The site constraints and the architectural solution tothese.

• The building users' needs and the required floorspans.

• Economy – Which option can produce the optimumcost and value?

• The building services intent and how best toincorporate them into the structure.

• Construction time – What are the time differencesbetween options and what are their different risks tothe certainty of timely delivery?

• Building height constraints due to Rights to Light,historic view lines, and general planning limitations.

• Foundation complexities that may be eased orexacerbated by the various options.

• Detail aspects; ie is a particular option a "natural"solution to a detailed requirement.

Every building is different and there will be differentcases where steel or concrete frames are the "natural"answer. However, the low dead weight of moderncomposite steel frames giving a high strength to weightratio makes steel advantageous on many of these key

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point headings. This lesser weight building answer isalso often a useful attribute on sites of archaeologicalinterest where the intensity and size of the foundationscan be shown to be the least intrusive as aconsequence of the light frame.

In its simplest form the composite frame comprisesuniversal columns supporting plain UBs traditionally laidout in the primary and secondary manner. These areoverlaid with an indented profiled metal deck throughwhich shear studs are welded through the deck to thetop flange of the beams on site. As a process thisachieves great construction efficiency as it totallyeliminates all the temporary works associated withother solutions by the steel itself carrying all the wetweight of concrete during construction in a simple non-composite mode. Upon concrete maturity thecomposite mode develops to both the deck slab andsupporting beams to carry the service design load ofthe building. The simplicity of simply-supported designpermits the simplest of fastenings to occur at endconnections and results in only static loads beingtransmitted to columns and foundations, ie the totalstatic load of the whole building is that carried bycolumns and foundations whilst an RC design thatnaturally benefits from design continuity the load oncolumns and foundations has to respect patternloading such that loads on columns must include a loadfor elastic continuity, such that the design load carriedon columns and foundations then exceeds the load ofthe total building. This compounds the fact that mostcommonly-used concrete solutions are heavier thanthose of composite steel.

Steel solutions have evolved from the simplest UBsolutions, with services in a separate zone below thebeam soffit, to larger span solutions with iterationsinvolving tapered beams, stub girders, lattice girders,Cellform beams, and Fabsec beams. The ability toprovide economic asymmetric beams with infinitevariety of depth and breadth with any pattern of webholes for service penetration has come about by thefully automated computerised fabrication processesnow available, and has given the ability to provideexciting and adventurous composite solutions in anaffordable way.

The earliest tightly-gridded composite steel buildingsproduced shallow beam depths, precluding penetrationsof sufficient size to accommodate service ducting. Theinterest and growth of larger spans has been popular inimproving the quality and utilisation of the floor spacebecause of increased flexibility. By adopting larger spansthe fewer number of deeper members gives space formore frequent and larger penetration opportunities thatallow integration of services for the base build design (asillustrated), and for future upgrades throughout thedesign life of the building.

10 Gresham Street has 18 m clear span floors throughout,with all service penetrations through the beams with largeover-provision for change.

As well as being more service friendly, the larger spansolutions significantly reduce the number of parts,producing savings on drawing office and fabricationcosts, with the on-site benefit of fewer connections,less crane hook and general erection time to fix. Theextra cost is in the raw steel component, and whilstthese larger spans usually cost a little more, the costmitigations derived from fewer parts often yields largerspans as a best value answer.

The past 15 years has seen the adoption in commercialbuildings of lower floor loadings from the historic marketnorm of 4 to 5 kN/m2 + 1 kN/m2 for demountablepartitions to 3 to 3.5 kN/m2 + 1kN/m2 for partitions. Theprinciple has been to adopt these lower loadings over95% of the floor plate whilst significantly increasing thecapacity of the remaining 5% at structurally convenientlocations which produces a more usable answer at lesscost. This design brief imposed load reduction hasyielded steel answers leaner than before, ie less metal, buttherefore of less stiffness, which together with theincreasing spans, has made the floor construction moresusceptible to vibration, in particular that from footfallvibration from people within the building. Guidance ondesign for vibration control is given in SCI documents andBS 6472 and a good understanding of these is animportant aspect of frame design, particularly whenbeams are penetrated by large rectangular holes allowingsecondary influences on bending, deflection and vibrationto occur. The writer has found that very large spans of 18m and more are not necessarily the most vulnerable andattributes this to the larger area of floor and hence massto be accelerated, and judges that spans in the range of10.5 m to 16.5 m to be more vulnerable.

Fire protection of steel floor construction has evolvedfrom concrete encasement, through fibrous sprays,quilts, dry board to the now affordable paintedintumescent coatings, which have the advantage ofbeing applied at the fabrication works. All of these areessentially insulants to defer the time at which the steeltemperature reaches a critical level affecting structuralperformance. The unprotected steel deck, which iseffectively the tensile reinforcement of the slab, avoidsthe need for applied protection as these slabs are

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justified by design of the concrete embedded meshreinforcement acting in catenary at the ultimate fire stageand this behaviour has been verified by fire tests. Theaim of such fire protection as required by the BuildingRegulations is to ensure that the building “….shall bedesigned and constructed so that in the event of fire, itsstability will be maintained for a reasonable period”.Such time periods normally range from 30 to 120minutes and depend on building use, height of building,and whether or not the building has sprinklers.

6 Building Services Integration

As few buildings can afford the height (planning andcost) associated with keeping separate zones forstructure and services within the ceiling space, thepassing of services through the web of the steelwork isa workable compact answer that creates an intimateinterface between the frame and services that isfundamental to structural design. Alternativelyoverlapping zones can occur by using haunch endedcomposite beams or tapered composite beams, andagain such service positioning and size is fundamentalto the structural design.

Typical storey height components for currentcommercial buildings

Raised floor zone 125 to 180 mm (more for financial tradingfloors, often 300 mm)

Structure/services zone 700 to 1100 mm

Ceiling & lighting zone 90 to 150 mm

Clear height 2600 to 3000 mm

Total storey height 3515 to 4430 mm

As illustrated below, the structural zone in the abovemust allow for tolerances to both steel and concreteand for deflections that occur prior to installing theceiling and false floors.

Typicalstoreyheightcomponentsfor currentcommercialbuilding.

Services that need to be incorporated involve cabling,piping, air ducts and the outlet boxes to these, all ofwhich are easily suspended from the profiled metaldeck having an in-built fixing facility to the soffit to avoidon-site drilling for such service suspension.

The most common generic beam/service solutions areas follows:

i) The simplest and most economic in fabricationterms but is far from the most economic overallbuilding solution as the summation of the depthsrequired for structure and service zones producesthe greatest floor-to-floor height solution for thebuilding. It is however the least constrained,interface-free building services solution.

Standard UB with separate services zone.

ii) A plain composite UB with service holes providedthrough the web gives a significant saving on floor-to-floor heights. For economy of the steel section,large rectangular holes should avoid being locatedat beam ends and mid-span. Adding suchpenetrations to standard sections may requirestrengthening to the hole perimeter, and alimitation exists on number of holes economicallyavailable.

Standard UB with integrated services through web.

iii) A composite lattice girder with top and bottomchords cut from UBs or UCs can provide answerslow in steel weight that provide a high proportion ofvoid space for penetration by services. Triangularspaces are good for circular ducting, piping, wiringand large rectangular ducts can be accommodatedin a central rectangular panel provided this isstructurally analysed for unsymmetrical loading, ie"Vierendeel" bending. Using inverted tee topbooms increases further the penetrable space, butadds further fabrication complexity. Lattice girdersare high in fabrication costs.

Lattice Girder with large central rectangular

services aperture.

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iv) Fabricated tapered beams can provide anoverlapping zone solution mixing some benefits ofavoiding strict integration of services and structurewhilst avoiding total summation of zones in thefloor sandwich. Its disadvantage is that thedefined service routes on plan may not be idealand structurally there is a reduction in the inherentstiffness to that of other solutions as the ends"tend" towards a true pin.

Fabricated Tapered beam with overlapping zones.

v) The stub girder which is essentially a composite"Vierendeel" girder with very stiff uprights isinherently very sympathetic to penetration byintense servicing by providing large rectangularvoids across the span. In its most efficient formthe stub girder is used as a primary memberreceiving point loads from the secondary beamssupporting the slab. Best economy is achieved bythe opportunity given to design the secondaries ascontinuous over the bottom boom and theengineer choosing where to splice these, usuallybetween the support and point of contra flexure.Stub girders also have the benefit of being asystem that produces two structural zones thatcan be mimicked by the services to avoid clashes.

Stub Girder with inherent large rectangular apertures.

vi) "Cellform" beams have been enabled byautomated cutting and welding processes. Theyare formed from cutting circular profiles in thewebs of rolled UB and UC sections cut to halfdepth and re-welded to provide both symmetricaland asymmetrical sections penetrated throughoutby circular holes. Good for multiple circular ductpenetration, but not for single large area air ducts.

Cellform Beam for many smaller circular

services penetrations.

vii) "Fabsec" beams have again been made possibleby automated cutting and welding processes.They are made as any size of I beam from threeplates incorporating any specified configuration ofholes at an affordable cost. This solution is theideal method to integrate services through thestructure and its affordability enables over-provision of service hole penetrations from thebase build requirement to suit changes requiredthrough the building life. Ideal for large spancomposite designs because of the limitless rangeof asymmetry available, but care to be exercised innot providing too many large rectangular holes,particularly toward supports, as deflection andvibration susceptibility is increased due to this.This solution can be regarded as the industrialisedequivalent of lattice girders with improvedusefulness for service integration.

Fabsec beam providing many mixed size penetrations.

7 Lateral Stability

The modern steel frame described largely achieves itsquality and economy by virtue of the composite natureof the floor structure, which by definition requires asimply supported approach to beam design. Thisproduces a frame low in inherent stiffness at the beamto column joints such that stiffness to resist lateral loadshas to be provided elsewhere.

All buildings must have a structurally defined route bywhich applied lateral loads, primarily wind, pass downthe frame to terra firma, with this impact on designincreasing with building height. On low rise and multi-storey buildings to circa 40 storeys, such stabilitysystems have little architectural impact, but for high risebuildings often the most efficient solution is to providesuch systems where greatest leverage is available, ie onthe façade skin, and is often a driver to the architecture.For most UK multi-storey buildings the best solution isto provide stiff diagonally braced steel elements withinthe core walls to the circulation, toilets and serviceareas which occur on every building. These stiffelements are essentially upward cantilevers from thefoundation to which all the upper floor plates areattached to gain stability. To be most effective, suchstability towers should be symmetrically placed withinthe building floor plate, but positioning cores is oftendriven more by other functions than structural stability,and the engineer has to frequently work with cores ofdissimilar dimension unsymmetrically placed on thefloor plate. When this occurs it is important torecognise the consequence of differential stiffness onecore to another, and whilst the lateral load may beadequately resisted, the result of differential deflection

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causes torsion on the frame as illustrated. Use of theperimeter beam and column framework around thebuilding is a useful way of controlling such racking andcontributing to stiffness.

Floor plan of 10 Gresham Street showing totally column free18 m wide space with almost perfect distribution of stiffnessto resist lateral forces whilst providing well distributed entryand exit points to the users.

Eccentricity of lateral load action and reaction and perimetercontrol of resultant racking.

The mechanism of wind reaching the braced cores is:

• wind applied to cladding;

• cladding reacting on floor plates;

• floor plates reacting on bracing.

There is a need for cores to provide vertical circulation(lifts and stairs) for people and for all the vertical servicerisers supplying the floors, hence there is significantpenetration of the floor plate within cores often adjacent

to walls. It is important therefore to check thatadequate connection is maintained between the floorplate diaphragm transferring the lateral load and thebraced wall that ultimately resists this.

Portalising the frame with rigid joint beam to columnconnections is an alternative way to provide thestiffness to resist lateral loads but this is less efficient inmaterial use than braced walls, and it imposes fixity onthe plain beam and column sections, increasing thebeam sizes that would otherwise have been sizedcompositely as well as adding complexity to theconnections. It can occur however that architecturerequires a close centre perimeter column arrangementof say 3 m centres, and it would then be opportune toportalise this to provide a stiff perimeter to resist lateralloads. It is always important to keep an open mind andexamine the best opportunities arising from eachdesign at concept stage. Whichever solution isadopted it is important to design the stability system tolimit the lateral drift of the building to avoid highconsequential cost on other components like non-structural walls and façade cladding where movementjoints need limiting for both aesthetic and functionalreasons. Limits in common use are 1/300 x storeyheight and an overall maximum at roof level of 50 to 75mm, but this is variable dependent on the individualbuilding location and circumstances.

Whilst the above refers to steel stability systems, therehas been a return in recent years, particularly in centrecore buildings, to providing the stability stiffness byusing reinforced concrete cores constructed by slip orjump form techniques. Whilst these can sometimes beadvantageous, a careful appraisal of the time costtaken by devoting the site initially to reinforced concreteconstruction which later supports the steel is important,particularly when compared to the rapid progressopportunities available from raising steel from singlepiles with plunged stanchions, as previously notedunder 'Foundations' and 'Substructure'.

8 Robustness

As well as providing safe and sound solutions for theapplied design loads whilst being mindful of the use ofsociety's resources, the engineer is to conceive asolution that has the assembly of parts well integratedone to the other, such that they are united in providinga robust whole.

In London in 1968, the partial collapse of the multi-storey residential building known as Ronan Pointillustrated the need and importance of an integratedand united whole. In response to post-war housingneeds, the building was formed from a standardisedprefabricated building system using precast concreteelements. A domestic gas explosion from a kitchen onthe sixteenth floor produced an explosive force that

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blew out the enclosing load-bearing external wall panelsupporting the floor above, together with other similarload-bearing panels up to roof level. The collapseprogressed to roof level and the weight and impact ofthis material on the floors below the explosion floorcaused them to collapse also, and the event saw thetotal collapse of the corner of the building betweenground and roof.

The consequence of a relatively small event was adisproportionate collapse, which was identified inhaving its root cause in a lack of unity between theprecast walls and floors at their junction. This resultedin amendments to Building Regulations and Codes toensure that a sensible degree of unity and robustnessof the whole is achieved. Obviously such regulatoryrequirements are limited to a sensible degree ofprotection and with the advent of terrorist bombings,there may be some building users that seek more thanthe statutory minimum.

Distilling the requirements of the regulations theyessentially require that the robustness be achieved bythe provision of ties designed to prescribed forces tounite all the structural frame parts together. Taking asan example a simple compression member like astanchion, the requirement is that at a splice there is thecapability to achieve a prescribed tension such that if acolumn storey was removed, those above could hangfrom the network of beams and columns withoutcollapse, albeit with significant distortion and catenaryaction of the remaining parts. Where an unusual designmakes such tying difficult or impossible to achieve,there is an alternative, more rigorous route to design theelements and assembly to prove adequacy insustaining prescribed forces.

The current generation of composite steel framesinherently provide much of what is required in respect ofties. In the vertical members the bolts used in standardsplices often suffice, and where they do not, there islittle economic impact from providing more. On thehorizontal floor plate, the beam end connection boltsagain usually provide the prescribed ties. Furthermore,the mechanical connection of the steel deck throughshear studs and the mesh reinforced overdeck slabprovide a superb unification of all the steel partsthrough the in situ poured concrete slab diaphragm.Whilst currently less common, any designs usingprecast concrete floors do not have the benefit of the insitu diaphragm and tie forces are best achieved at thesteel to steel connections. Fortunately the frequency ofgas explosions like Ronan Point, which was seminal informulating the UK's prevention of progressive collapserequirements, is rare. Unfortunately the threat andreality of terrorist explosions is increasing and thecurrent rules have served the UK well; other countriesand international codes are now making similar tyingrules mandatory. The writer notes that withresponsibilities on buildings in close proximity to the

Bishopsgate bomb, it was noticeable that thecomposite steel buildings fared relatively well, andattributed this to the benefits afforded by a light and wellunified whole, and the malleability of steel to functionrelatively well with large distortions, compared to thebrittleness of concrete.

9 Cladding

Whilst by definition cladding is a non-structuralcomponent on multi-storey framed buildings, it doeshave an impact on several important aspects ofconceptual and detailed structural design. The externalwall is the important element that keeps wind andweather from the users within and the fabric forming thebuilding. It can range from being a heavy componentof brick or stone to lightweight metal and glass, and onmulti-rise framed buildings is usually carried at eachfloor of the frame, either bearing on the floor below orhanging from the floor above.

Conceptually the best overall economy is usuallyachieved where the cladding cloaks the buildingoutboard of the whole frame, thus avoiding detailinginterfaces from complexities of shape between thecladding and structure, thus protecting both the peoplewithin and the frame supporting them. Exceptions doexist and good economies of frame and cladding havebeen achieved on buildings where the frame isexpressed totally outboard of the façade with simplerepetitive penetrations of the structure through thefaçade, as illustrated.

Mid City Place, Holborn with fabricated external box columnswith rigid box beam spigots through the cladding splicedinternally to floor beam.

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Fleet Place, Holborn with simple rectangular spigots penetratingcladding with joint to floor beams inside the cladding.

It has evolved that the most useful planning grid formost commercial buildings is 1.5 m and the structuralgrids respect this at multiples of 1.5 m, with floor beamsusually set out at 3 m centres, and columns commonlyranging from 6 m to 9 m on the façade and 9 m to 18m internally. Potential sub-division of the internal spaceby partitions to give cellular office space is mostcommon at 3 m and 4.5 m. It follows that the perimeteroffices that abut the façade cladding create the needfor the façade mullion module to also be considerate ofthe 1.5 m planning module such that potential partitionlines neatly abut this. Cladding, whatever the externalfaçade material, is now commonly prefabricated anddesigned at a multiple of 1.5 m. It is the writer's viewthat the best quality cladding system is where panelsextend the full bay between column grids. Doing sominimises the number of site sealed weathering jointsand locates them where minimal vertical structuralmovement exists. As illustrated, panels of 9 m widthare often used and have been found to be readilytransportable from European suppliers.

10 Gresham Street 9 m wide 4 m high finished claddingdelivered from Germany. Rigid units bearing on slab adjacentthe stanchions.

However easier transport logistics, and economies ofcladding manufacture can mean that smallercomponent systems are more affordable. "Stick"systems are one such alternative, where a 9 m façadebay would be enclosed with 6 No 1.5 m wide "sticks",each bearing on the perimeter structure with each stickhaving differential vertical movement to its neighbourdue to structural deflection. In so choosing, thedesigner and client must balance the difference in costbetween these systems and the value including designlife between them.

Whatever system is chosen it is imperative that all thedesign movements expected from the structure areprovided to the cladding company; the majordeformations being:

• deflection of perimeter beams

• shortening of perimeter columns

• lateral movement from horizontal loads

Design of the perimeter structure should respect thefact that it is far more economic, more aestheticallypleasing, and less risk to long term claddingperformance to stiffen the supporting structure ratherthan attempt to design the cladding to absorb largemovements. It may also be appropriate that a moreeconomic or more movement prone cladding systemrequires a stiffer structural perimeter than a full-baypanel solution.

On a fast running project, attachment of the cladding tothe perimeter structure can be a difficult informationsupply interface where preparation of steel fabricationdrawings can be many months in advance of detailedinformation being available from the cladding supplier.Many interface difficulties can be avoided in theconstruction industry, and in this case a decision todesign and fix the cladding to the concrete slab buysseveral months of design and detail time for the cladderand avoids this critical steel-to-cladding interface. Themanner by which the cladding bears on the structurecan be most important in avoiding torsion being appliedto the perimeter beams. The basic principles of theattachment should be set at scheme design and thecladding load should be applied concentrically to theunderlying edge beams.

10 Buildability

Buildability is a word having many meanings,depending on whom you talk to! Designers should onlydesign that which can be built, and with today'sknowledge virtually anything can be, but often at somedetrimental cost to the important aims we subscribe to,namely: quality, cost, programme, and safety.

What should really be sought is the best of the aboveaims by "thinking smart" in how design may beconstructed with inherent ease. The modern

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construction site is now very much an assembly placefor prefabricated parts, and this should continue to befurther maximised. The modern composite steel framedesign is very much an example of this in the manner bywhich it totally avoids the need of on-site construction tobuild wasteful temporary works involving man hours andmaterials to achieve the permanent product. Byconsidering the earlier headings in this chapter, the ideasand methods described go a long way towards makingour multi-storey buildings more "buildable" than theypreviously were.

Examples include:

• Frames that avoid materials and man hoursconstructing temporary works.

• Stanchions placed by plunging into piles to start steelerection soonest, and bypass the more complex,time-consuming substructure.

• Use of three storey column sticks for fast upwardprogress.

• Larger spans giving fewer parts for an improvedpiece count.

• The simplest connections associated with simplysupported designs.

• The provision of the permanent staircases at the leadof steel erection for all the workforce to gain safe andsound access to their workplace.

• Steelwork painted at works with intumescent fireprotective paint to eliminate fire protection as aprogramme item in the site works.

• Steel beam solutions like "Fabsec" giving asymmetricanswers with a generous provision of service holesavoiding a precise tailor-made answer.

• Solutions that avoid transfer structures.

• Best degree of repetition possible by grid planningand rationalisation.

• Leave decking across floor service holes afterconcreting to be removed at start of servicesinstallation.

• Designing for more realistic lower imposed loadings.

• Minimising by design the depth of sub-structures.

The above improves quality and safety, and enables theframe to reach roof soonest to commence work on roofplant and lift motor rooms at the earliest opportunity.

Having reached the roof soonest, the buildabilitycontinues with full-bay panelised cladding fixed ontothe slab from operatives working safely and efficientlyinside the building, whilst below ground the conceptualsubstructure framing has minimised the depth toformation of the lowest basement.

What further improvements can be made? The writersuggests that for each element being specified andbefore committing the answer for that element,designers consider whether or not they have achievedthe collective optimum of quality, cost, programme, andsafety; can it be created off site for further improvement;and can it be further refined to improve buildability at site.

In summary, a well conceived modern, high strength-to-weight ratio, composite steel frame gives in thewidest sense a quality answer enabling affordableadventurous design forms to be built. Adopting well-thought-out construction processes into the designconcept achieves build costs that are hard to beat. Theoff-site fabrication with well-conceived, simple on-siteassembly permits an unmatchable construction ratewith less exposure to risks – both time and safety.

This chapter has been written as a generic andphilosophical approach to design where the writerconsiders the greatest worth lies, rather than beinganalytical and figurative where much guidance currentlyexists. Whilst much of the content is derived from the“cut and thrust” of designing commercial buildings itapplies equally to other building uses, ie hotels,hospitals, residential buildings etc.

High quality achieved at least cost and delivered quicklyand safely is the result, and will always be the demandfrom our clients, and we should continue to strive forfurther improvements on today's best.

“It is not cheaper things that we want to possess butexpensive things that cost a lot less”.

John Ruskin 1819-1900

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