Steel-Concrete Composite construction using rolled sections

Sections

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . .3

European standards . . . . . . . . . . . . . . . . .5

Composite beams . . . . . . . . . . . . . . . . . . .8

Shear connection in composite beams . . 10

Design of composite beams . . . . . . . . . . 12

Partially encased composite beams . . . . . 14

Design of partially encased beams . . . . . 16

Verification of the fire resistancefor partially encased beams . . . . . . . . . . 17

Composite columns . . . . . . . . . . . . . . . . 19

Design of composite columns . . . . . . . . . 21

Shear connection in composite columns . 24

Fire resistance of composite columns . . . 26

Construction details . . . . . . . . . . . . . . . . 29

Choice of column type . . . . . . . . . . . . . . 31

‘Pre-installed’ columns . . . . . . . . . . . . . . 32

Connections . . . . . . . . . . . . . . . . . . . . . 35

Structure stability . . . . . . . . . . . . . . . . . . 41

1

Steel-Concrete Composite constructionusing rolled sections

2

City Center Kirchberg, Luxembourg (L)

Figure 1

(a) Without link

(b) With link

Introduction

Steel-concrete composite construction haslong been recognised and used in the formof “traditional” composite beams in buildingsand bridges. In this simple form of construc-tion, the rolled steel section is connected to theconcrete slab using mechanical shear connec-tors at the steel-concrete interface. Becauseof the resistance to longitudinal shear provi-ded by these connectors, the steel and con-crete are linked structurally. The reinforced con-crete slab can therefore be used not only toprovide a horizontal surface in the building, butalso as a compression element in the compo-site section. The presence of the concrete in-creases both the resistance and the rigidity ofthe steel section, which forms the tension ele-ment in the composite section under bending(figure 1).

Steel columns were traditionally often enca-sed in concrete to increase their fire resistance.This type of section was used long before theadoption of true composite columns, for whichthe reinforced concrete encasing the steel sec-tion is assumed to support part of the verticalload (figure 2).

In the1980s it was discovered (or rediscovered)that even a partial encasement in concrete(figure 3) provides a composite column withsubstantial fire resistance. The open form ofsteel H-sections facilitates filling with concretebetween the flanges whilst the steel sectionis laid flat on the ground, prior to lifting intoplace. This eliminates the cost of formwork,and compensates for any overdesign that maybe needed to achieve the highest levels of fireresistance. As a result of numerous researchprojects, reliable methods have been esta-blished for calculating the fire resistance ofcolumns with precast concrete between theflanges.

3

Figure 2 Figure 3

Steel-concrete composite construction

The same technique of partial encasement firstused for columns has been extended to coverbeams in order to increase their fire resistance(figure 4). Although the lower steel flange gra-dually looses resistance as it is exposed to afire, this loss is compensated by the presenceof reinforcement located within the concreteencasement.

Other recent developments include improveddesign methods for composite beams, takinginto consideration continuity at supports(allowing for cracking of the concrete in ten-sion), and partial shear connection (which, byallowing some slip between the steel and con-crete elements, can improve economy).

Composite construction therefore offers consi-derable possibilities faced to those offered bytraditional steel construction, be it in terms offire protection or otherwise to suit particulardesign criteria. Because of the way steelframes are constructed, it is also possible tocombine both composite and non-compositemembers in a single project.

The fire resistance that can be achieved usingcomposite construction has greatly contribu-ted to its success, with the added advantageof being able to retain exposed steel surfacesthat can be used for attachments. The excel-lent ability of composite structures to resistseismic loading is yet another advantage ofthis form of construction.

4

Figure 4

Composite construction with openings in the web for the transmission of the technical devices

European standards

Basic design philosophy

Composite construction has seen rapid adop-tion in countries possessing the necessary stan-dards and design guidance. Methods for eva-luating fire resistance were proposed in the1980s in the form of specific national authori-sations. Subsequently, the appearance of theEurocodes has led to a significant generalisa-tion of design methods, not only for normalservice conditions but also under fire.

The general philosophy adopted for the Euro-codes is to ponderate the loads and forcesapplied to a structure by using factors. Thevalues of these load factors depend on thenature, and variation with time, of particulartypes of load. Each member within a structure,and the structure as a whole, must be checkedfor all potential combinations of loads. In addi-tion, particularly for beams, the designer mustverify that certain criteria are satisfied underthe levels of loading expected during service.These criteria concern deflections, vibration,and cracking of the concrete, which are knownas serviceability limit states.

Eurocode 4 Part 1.1 (ENV 1994-1-1) gives designmethods for composite beams and compositecolumns under normal conditions. Part 1.2(ENV 1994-1-2) gives methods for calculatingthe resistance of these elements under fireloading.

Eurocode 1 (ENV 1991) defines not only theloads to be considered during design, but alsothe safety factors to be considered under both

normal conditions and fire. For an accidentalfire condition the load factor is less than 1.0 formost imposed loads, because it is consideredhighly unlikely that an imposed load of maxi-mum intensity would occur at the same timeas a fire. These standards were completed ineach country by a national application docu-ment for the Eurocode. Requirements for fireresistance also continue to be defined at a na-tional level and, unfortunately, there is somedisparity between different countries.

Quality of materials

Eurocode 4 permits the use of a wide rangeof steel and concrete grades for the materialscombined in a composite member.

The traditional range of steel grades (S235,S275 and S355) is supplemented with higherstrength grades S420 and S460. Steels of thesehigher grades are achieved using the QST pro-cess (HISTAR sections), and are particularly use-ful for members subjected to substantial loads.On the other hand HISTAR steel grades allowa finishing without any preheating nor post-heating during welding.

Concrete should be either grade C20 till C50,with normal or lightweight aggregate. Anycommonly available reinforcement may beused, S500 being the most common grade.

5

Scandia Building, Madrid (E)

Fire resistance: ENV 1994-1-2

Composite sections, with either total or partialconcrete encasement, possess significant fireresistance. However, it is not possible to assessthe fire resistance of a composite member sim-ply be considering temperatures in the steel(as is the case for bare steel sections, whichexperience a more-or-less uniform tempera-ture across the section).

The presence of concrete increases the massand thermal inertia of a member. The varia-tion of temperatures within the body of themember at a given time under fire loading issignificantly non-uniform, in both the steel andconcrete components. This leads to substantialtemperature gradients. The presence of areasnear the core of the section that are relativelycold ensures that the member can remainstable for some time under fire loading.

Part 1.2 of Eurocode 4 gives several methodsfor calculating the fire resistance of a compo-site member :

- use of tables that are essentially based onthe performance achieved in tests

- calculation of the ultimate resistance usinga simplified method based on test data

- numerical modelling using software thathas been sufficiently validated using testresults, such as CEFICOSS, which is usedby Arcelor Sections Commercial.

Both the accuracy of the method, and thescope of its application, increase passing fromthe first to the third of the methods listedabove. The great benefit of software such asCEFICOSS is that the analysis of completestructures, be they flexible or rigid, is a realisticproposition. Fully encased beams and columnsare generally assessed using tables, which areextremely simple to use for these applications.Simple design methods based on test resultsare generally used for partially encased sec-tions.

6

Ecole Nationale des Ponts et Chaussées, Marne-la-Vallée (F)

Office-building, rue Reaumur, Paris (F)

References

Publications giving methods for the verifica-tion of the fire resistance of other compositesections, and for more complex load situa-tions, include the following:

[1] ECCS/CECM - N° 55. “Calculation of thefire resistance of centrally loaded compositesteel-concrete columns exposed to the stan-dard fire.” Edition 1988[2] Report EUR 13309 EN, Schleich, Mathieu,Cajot :”Practical design tools for compositesteel concrete construction elements submittedto ISO-fire considering the interaction betweenaxial load N and bending moment M.”

[3] Hosser, Dorn, El-Nesr : “Entwicklung undAbsicherung praxisgerechter Näherungsver-fahren für die brandschutztechnische Bemes-sung von Verbundbauteilen. Abschlussberichtzum Forschungsprojekt A39 (S24/2/91) derStiftung Stahlanwendungsforschung”. Institutfür Baustoffe, Massivbau und Brandschutz(IBMB), TU Braunschweig, Juni 1993.[4] B. Zhao: “Abaques de dimensionnementpour la résistance au feu des solives de planchernon protégées connectées à des dalles mixtes.”- Revue “Construction métallique” - N° 1 - 1999

7

Composite beams

Beam and slab

Composite beams can be configured in severalways based on rolled steel sections, as shownin Figure 5. The simplest and most commonform is as shown in Figure 5a. It is generallyused for spans between 6 and 16 m, but canbe used to span over 20 m. When necessary,this type of beam can be protected against fireusing an intumescent coating, sprayed fireprotection, or even boxed in using fireproofboards.

The conception of this type of composite beamis substantially linked to the form of reinforcedconcrete slab that is adopted. The slab is ge-nerally cast in-situ using profiled, galvanisedmetal decking as permanent formwork, orsometimes using thin concrete precast slabsas the formwork. Although the resistance ofthe composite beam is relatively independentof the manner of forming the slab, the beamdeflection under the dead weight of the con-crete is significantly affected by the construc-tion sequence. In order to eliminate, or at leastreduce, dead load deflections it is possible to :- prop the beam during casting of the slab;

after hardening of the concrete and remo-val of the props the dead load of the con-crete plus steel is supported by the com-posite beam section. Propping is essentialwhen a system as shown in Figure 5b, usingstub girders, is adopted.

- precamber the steel section during fabrica-tion, by an amount calculated to compen-

sate for deflections during concreting ofthe slab. The precamber may be applied tothe steel section either when cold, using apress, or by controlled local application ofheat.

- provide some continuity of the beam at theend supports.

8

Figure 5

a) Simple composite beam

c) Castellated beams (hexagonal openings)

d) Castellated beam (circular openings)

e) "Stub - Girder"

b) Beam with a reinforcing plate

Car park, Helmond (NL)

Composite construction

using castellated beams

When propping is adopted the loads in theprops may be quite large. The designer/buildershould therefore think carefully before usingprops in a multi-storey building, and must con-sider the rigidity and strength of any lowerlevels that are used to support the props. Theuse of propping becomes less economicalwhen there are significant inter-storey heights.

Unless special measures are taken to controldeflections during concreting, the accuracythat can be practically achieved using pre-cambering is of the order of several centi-metres. However, this should still allow accu-rate positioning of the formwork, and thecorrespondance of holes in adjacent framemembers to be lined up so that connectionscan be made. It is necessary to avoid anyharmful or uncontrolled rotation of the secon-dary beam connections due to the movementof a precambered primary beam during con-creting.

It is clearly necessary to verify that the lateraltorsional buckling resistance of the steel beamis sufficient to support the loads applied duringconcreting, and provide lateral restraint whennecessary. Correctly anchored profiled metaldecking often provides sufficient restraint.

Propping of the decking or precast slabs isneeded when they cannot support the weightof wet concrete and the other constructionloads (for example the weight of the opera-tives) imposed during concreting. This is oftenthe case for spans in excess of 2.5 to 3.0 m.It should also be remembered that the weightof any additional concrete placed due to defor-mation of the steel beam and metal deckingduring concreting (an effect known as ponding)may not always be negligible.

One implication of the various points discus-sed above is that the designer should care-fully consider how the beams and slabs willbe constructed, and should clearly state theassumptions made during the design on theappropriate contract documentation.

9

Composite floor during erection,Espace Léopold, Brussels (B)

Precambering of beams, Car park of the stadium,Luxembourg (L)

Forming the slab with wide span metal decking

Provisory propping of wide steel sheets during concreting

Figure 6: TYPES OF CONNECTORS

a) Headed studs b) Angus fixed on behalf of shot-fired pins

c) Shear connectors (T)

d) Shear connectors (Angle) e) Shear connectors (Brackets) f) Shear connectors (Buckle)

Shear connection in composite beams

The mechanical shear connection between theslab and steel beam is essential for achievingstructural interaction between the two compo-nents under bending. The most common formof connection comprises welded headed shearstuds (Figure 6a), which are attached to thesteel beam using a special welding ‘gun’. Uni-form spacing is desirable to facilitate the cor-rect positioning of the studs, and so that theirpositioning can be checked visually. Severalother types of connector exist as an alterna-tive to welded studs, including angles fixedusing shot-fired pins (Figure 6b). Althoughthese offer a reduced resistance, they avoid

10

Electric welding of headed studs

the need for welding and may therefore beappropriate in certain circumstances. Variousother types of connector may be used, asshown in Figure 6.

The types of connector shown in Figures 6aand 6b are relatively flexible, whereas the othertypes shown in the Figure are rigid. The diffe-rence is significant, because rigid connectorsdo not allow redistribution of the longitudinal

shear force amongst themselves. The ability ofthe more flexible connectors, which are knownas “ductile”, to redistribute the shear allowsthe use of partial shear connection for beamsin buildings.

When possible, shear studs are welded to thesteel beams in the fabrication shop. This canbe done when the decking is not continuousover the beams, or when precast slabs areused. It should be noted that it is not neces-sary to protect either the studs or any surfacesof the steel beam in contact with the concreteagainst paint, given that the design methodtakes no account of bond between the con-crete and steel.

For the thicknesses of decking (and galvani-sing) generally used it is possible to weld thestuds to the beams on site using what is knownas “through-deck welding”. Certain precautionsshould be taken with regard to the conditionsof contact between the various components;excess humidity, unclean surfaces, or the pre-sence of paint (which can be avoided by apply-ing masking tape to the beam before painting)can all affect the integrity of the weld. Despitethese restrictions, through-deck welding of thestuds on site, using appropriate welding equip-ment, is widespread in practice.

On site, as in the fabrication shop, a simplebending check applied to some of the wel-ded studs allows rapid assessment of theweld quality.

Occasionally, in order to avoid site welding ofthe studs, the steel decking is delivered tosite with circular holes cut through it at theshop-welded stud positions. Clearly thisrequires the production of very precise dra-wings, or other appropriate information, anda number of corrections on site are inevi-table.

11

Non continuous metall decking over the beams :the flutes have been closed with a press

Steel decking with circular holes. “Through deck welding” on site

Car parkairport,

Brussels (B)

Figure 7

Collaborating width

(L/4 ≤ distance between the beams)

Steel decking or precast slab

Neutralplasticaxis

Design of composite beams

Resistance at the ultimate limit state

According to Eurocode 4 the resistance of acomposite beam should be verified at the ulti-mate limit state for any cross section that couldbe critical. This is true whether the beam is sim-ply supported or continuous over several sup-ports. Other than for certain relatively complexcases associated with continuity and momentredistribution (which are also covered by thestandard), in general this verification amountsto no more than a simple comparison of theplastic resistance moment and the appliedmoment at one or two critical sections.

For the common case of a beam that is simplysupported at its extremities and subjected touniformly distributed loading, it is sufficient toensure that the ponderated applied momentMsd is less than the ultimate resistance mo-ment Mpl,Rd. This resistance is calculated ac-cording to the traditional rectangular stressblock method, as shown in Figure 7. No ac-

count is taken of the concrete within thedepth of the decking profile, or within thedepth of the dry joint when precast concreteslabs are used as permanent formwork.

Vertical shear forces are assumed to be resis-ted uniquely by the web of the steel section,the ultimate shear resistance of which mustbe greater than the ponderated applied shear.It is necessary to consider interaction betweenbending and vertical shear above the sup-ports of continuous beams, or beneath con-centrated loads, when the applied shear isgreater than 50% of the web capacity.

12

European parliament, Luxembourg (L)

Serviceabilty limit states

To ensure adequate behaviour in service it isnecessary to verify the beam deflections, thecracking of the concrete at the supports, andthe natural frequency of the beam. The de-signer should also verify that the stressesinduced in the section under service loadingdo not cause any local plastification, whichwould invalidate any deflections calculatedusing elastic theory.

The magnitude of the deflections depends onthe construction sequence. Dead loads may besupported by either the composite section, orthe more flexible bare steel section, dependingon whether or not the beams and slabs arepropped during construction. The magnitudeof any precamber to be applied during fabri-cation will depend on the calculated deadload deflections. The rigidity of a compositemember may be calculated according to classicelastic principles ; the effective section of theslab is transformed into an equivalent steelsection using an appropriate modular ratiofor the two materials. The designer must takeinto account creep of the concrete under longterm loading (self weight etc), shrinkage of theconcrete, and possibly the influence of partialshear connection.

Control of crack widths is necessary where theconcrete will be subject to tension, for exam-ple at the internal supports of a continuousbeam. This dictates the adoption of a certainminimum area of longitudinal reinforcementin the slab. In no case should the percentageof reinforcement drop below either 0.4% or0.2%, depending on whether or not the slabis propped during construction.

For most cases when the slab will be subjectto normal “people traffic” design standardsrecommend that the rigidity of the floor issuch that its natural frequency is greater than3 Hz. This check is relatively simple, using aformula which considers the span, the mass,and the rigidity (EI) of the section.

Shear connection

Shear connectors and transverse reinforcementplaced in the slab above the beam transfer thelongitudinal shear force between the steel andconcrete. Any adhesion between the steel andconcrete is not taken into consideration.

13Hôtel des Arts and Mapfre tower(office building), Barcelona (E)

The headed shear studs normally used are duc-tile, which means that they have sufficient de-formation capacity to enable the adoption ofpartial shear connection. The term “partialshear connection” refers to situations in whichthe resistance of the composite beam is gover-ned by the strength of the shear connection.In other words, it is possible to reduce the num-ber of shear connectors (within certain limits)when full shear connection would lead to anexcess in beam capacity, as it is often the case.

Beams provided with shear connectors

Partially encasedcomposite beams

The fire resistance of a traditional compositebeam can be improved considerably by infil-ling the areas between the steel flanges withreinforced concrete (Figure 8). This process is,however, only possible for beam depths greaterthan 180 to 200 mm, which allow the inclusionof appropriate reinforcement (with sufficientcover) in the concrete. Clearly, the weight ofthe structure increases due to the additionalconcrete, which must be allowed for in thedesign. However, this additional weight isgenerally compensated by the increased rigi-dity of the beam, and so does not normallyresult in an increase in the size of steel sec-tion required, when the beam is wide enoughto accept the concrete.

Concrete filling takes place on the groundbefore erection of the beam. The steel beamis laid on well aligned, rigid supports, whichare sufficiently closely spaced to avoid defor-mation of the steel section under the weightof the concrete. Prefabricated reinforcementcages are dropped into the voids between theflanges, positioned, and held in place to en-sure that adequate concrete cover is achieved.If possible the concrete is poured directly from

the mixer truck into the prepared beam,which can be turned over after only a veryshort period to allow concreting of the op-posing chamber.

The process of concreting on the ground re-quires delivery of the finished steel membersapproximately one week before they are duefor erection. It also requires an area that can beserviced by a crane; this area may be either onsite or perhaps in a nearby workshop or similardepot.

The main longitudinal reinforcing bars, whichare placed in the concrete to enhance the fireresistance of the composite section, are com-plemented by other secondary bars. In parti-cular, stirrups are needed to avoid spalling ofthe concrete in a fire and a resulting prema-ture heating of the core of the section at oneprecise location.

The concrete infilling between the flangesmust be mechanically anchored to the web ofthe steel section so that thermal stresses do

14

Concreting of composite beams on the ground

Figure 8

Main reinforcing bar :40 to 60 mm

not cause any break and fall off of the latter.Several solutions are proposed in Eurocode 4;headed studs can be welded to the web, orreinforcing bars that penetrate the web may beadded, or stirrups may be welded to the web(as discussed later).

In theory the steel surfaces in contact with theconcrete are not painted, with the possible ex-

ception of a 3 cm return towards the interiorof the flanges. It should be noted however thatthe presence of paint on the web and studs hasno determinant influence on the behaviour ofthe beam because, as already said, any naturaladhesion between the steel and concrete is notconsidered in the design method.

15

Museum “Museum für Verkehr und Technik”, Berlin (D)

If the presence of the reinforced concreteinfill has not been taken into account forwhen determining the second moment ofarea (I), the designer should be aware thatthe actual deflections will be less than thosepredicted. This will be true in both the finalstate and intermediate states during con-struction, and can have a significant influenceon the magnitude of any precamber (whenspecified). The increased rigidity will also besignificant at any other stage when it is ne-cessary to predict the deflections, for exam-ple when determining the capacity for adjust-ment needed at interfaces with prefabricatedelements such as staircases or cladding panels.

Eurocode 4 (ENV 1994-1-1)Annex G

Tests have shown that the presence of con-crete between the steel flanges not only in-creases the rigidity of a beam, but also itsultimate bending moment resistance and itsvertical shear capacity.

Annex G of Eurocode 4 proposes supplemen-tary rules which take into account the con-crete between the flanges under service con-ditions. The rules are applicable whether ornot there is a participating slab.

The annex proposes a simplified method forcalculating the second moment of area ofthe beam (I), ignoring any concrete in tension.

Normal, relatively weak concrete (C20) is ge-nerally used to infill between the flanges.

16

Design of partiallyencased beams

Design for normal load conditions

Partially encased beams are often designed fornormal load conditions as traditional compo-site beams. The reinforced concrete betweenthe flanges is taken into account as a deadload, but is completely neglected when deter-mining the resistance of the section, and evenwhen calculating deflections.

Although such simplified assumptions areclearly conservative, the basic version of Euro-code 4 gives no alternative rules specificallyfor partially encased beams. The section ofthe reinforcing bars needed is determined byfire resistance requirements rather than nor-mal load conditions.

In reality, the increase in rigidity of the sectiondue to the presence of the concrete and rein-forcement may be considerable. Starting atseveral percent for the smallest practical beams,the increase in rigidity may exceed 20% for thelargest beams in their final condition.

Unfortunately, an accurate calculation of therigidity for use in deflection calculations israther laborious. It is necessary to carry outseveral elastic analyses to cover the variousstages of construction and the load applica-tion sequence. The evolution of the sectionthat is acting structurally, and of the concreteproperties in function of the time, must allbe considered.

Office building of thegeneral contractor

SKANSKA, Göteborg (S)

Figure 10REDUCED TRANSVERSE SECTION

FOR THE NEGATIVE MOMENT CALCULATION

The constraints for calculation in the steel are progressively reduced according to different

temperature zones in which the steel section is divided

Verification of thefire resistance forpartially encasedbeams

Resistance to an ISO standard fire

Eurocode 4 Part 1-2 proposes two methodsfor determining the resistance of a partiallyencased composite beam subject to a stan-dard ISO fire. The first of these, the “tabular”method, requires some resistance calculationsin conjunction with interpolation of tabula-ted values. This method is very conservative,and predicts very high values for the areas ofreinforcement required. Ideally, it should notbe used in preference to the second, “simplecalculation”, method.

It is possible to measure the progressive hea-ting through a section during a fire test.Zones of different temperature can be de-fined for each material, in which the loss ofresistance due to the elevated temperaturecan be evaluated.

The simple calculation method for predictingfire resistance considers the ultimate momentresistance of the section, which is calculatedby dividing the section into different zones.The material properties for each zone aremodified using reduction factors, which de-pend on the average temperature in the zone.These temperatures are determined by con-sidering the section to be exposed to an ISOfire for the required fire resistance period.

The method is equally applicable for both posi-tive moments (Figure 9) and negative momentsat supports (Figure 10). Unfortunately, eventhough simple, hand calculations using thismethod still take some time. However, themethod has been programmed, and softwareis available on request from the TechnicalAssistance department at Arcelor SectionsCommercial.

17

h

Figure 9REDUCED TRANSVERSE SECTION

FOR THE POSITIVE MOMENT CALCULATION

The constraints for calculation in the steel are progressively reduced according to different

temperature zones in which the steel section is divided

18

Fire resistance is assured if the moment resis-tance calculated for the time required (withmaterial strengths reduced to reflect the zonetemperatures at that time) is greater than themoment applied by the combination of loadsappropriate for the accidental fire condition.

Eurocode 4 Part 1.2 allows redistribution ofthe moments in a beam under certain condi-tions, even if the beam has been assumed to besimply supported under normal service loading.In order to comply with reinforced concretedesign standards it is always necessary to haveat least a minimum level of continuity reinfor-cement (anti-crack reinforcement). This rein-forcement will remain cold during a fire, andlimit the rotation capacity of the compositebeam. In order to benefit from a redistributionof moments it is necessary to ensure that the

gap at the ends of the beam satisfies a defi-ned limit (10 to 15 mm according to the situa-tion, which may well be achieved anyway).

In practice some moment redistribution is notneeded in the majority of cases for simplebeams. A minimum of two 12 to 20 mm bars(see Clause 5.3.2 of ENV 1994-1-1) placed atthe bottom of the infill concrete is generallysufficient to achieve 90 or 120 minutes fireresistance for floor beams.

Museum “Landesmuseum“, Mannheim (D)

Isotherms in a partially encasedbeam subjected to an ISO fire

of 90 minutes

19

Composite columns

The types of composite column illustrated inFigure 11 are the most common, being of eithersquare or rectangular cross-section. They arecompared below. Sections that are completelyencased in concrete may also contain two steelmembers placed side by side, with sufficientgap between these members to allow correctfilling with concrete.

Circular sections are also used, primarily tomeet architectural requirements. They maybe formed either using traditional formwork(Figure 12), or by placing the steel memberinside a metallic tube (Figure 13). The formertype is effectively a variation on the more com-mon completely encased rectangular section,with the same advantages and disadvantagesas described below.

Figure 12Circular composite H column encased

in concrete

Figure 13H column encased in concrete inside

a metallic tube

composite columnfilled with concrete

composite column encased in concrete

Figure 11Common forms of composite columns

Bank Bruxelles Lambert, Brussels (B)

Office buildingWinthertur, Barcelona (E)

20

So-called cruciform cross-columns (Figure 14)comprise two steel sections, sometimes iden-tical, one of which is cut into two Ts. The Ts arewelded to the web of the other steel girder. Thistype of column is used when the bucklinglength is substantial in both axes. The steelmembers used for this type of compositesection are generally considerably deeperthan they are wide, with a depth greater than400 mm, or even sometimes 500 mm. Concre-ting on the ground prior to erection is possi-ble, but requires four operations and a fairlycomplex procedure to fix the reinforcement.

Other types of section that combine two steelmembers may also be used (Figure 15). Themain steel girder is reinforced in each the areabetween the flanges by one or more smallersteel sections. The latter are typically H sec-tions, or thick flanged T sections, which arewelded to the web of the main member. Theprovision of this quantity of steel within thebody of the concrete clearly leads to a com-posite column with excellent fire resistancecapabilities.

It is worth noting that the list of compositecolumn section types described above is notexhaustive, and other types can certainly beimagined.

Figure 14Cruciform column filled

with concrete

Figure 15Composite section with

reinforcements welded on the web

Motor car plant Saab, Malmö (S)

reinforcementswith or with

T H

Composite columns with reinforcements welded on the web

21

Design of composite columns

Eurocode 4 proposes a method for the designof composite columns at the ultimate limitstate. The apparent complexity of this method isin fact relatively superficial, and it can be easilyprogrammed. The method may be used for anyof the typical types of section described abovewhen loading is primarily axial. Additional ben-ding moments may be present.

Axial compression

The designer must verify that the axial load inservice, increased by using the appropriate loadfactors, is less than the resistance of the com-posite member. The buckling resistance of themember is a function of the plastic compres-sion load, suitably reduced using a coefficientthat reflects the slenderness of the member(Figure 16).

Figure 16

Sony Center Potsdamerplatz, Berlin (D)

22

Axial compression anduniaxial bending

When the axial load is accompanied by mo-ments about one axis it is necessary to deter-mine the N-M interaction curve for the sectionbent about that axis (Figure 17). The designermust then verify that at the ultimate limit statethe ponderated moment does not exceed themoment resistance limit, which generally in-creases as the level of axial load decreases (sha-ded part of the diagram). The interaction curvecan be determined by calculating numeroussuccessive points, considering the movementof the plastic neutral axis across the section.Alternatively, the curve can be determined re-latively easily by establishing several criticalpoints using the procedures given in Euro-code 4.

Figure 17RESISTANCE TO COMPRESSION AND BENDING

Sony Center Potsdamerplatz, Berlin (D)

23

Axial compression andbiaxial bending

In the case of biaxial bending an N-M inter-action curve must be determined for bendingabout both axes. Corresponding points on thecurves in the y-y and z-z planes are joined by astraight line which defines, along with the axes,a surface inside which the factored momentsabout the two axes must remain (Figure 18).

The reduction coefficient to allow for buck-ling is applied for the axis that is consideredto be critical, which in theory means that eachaxis must be checked successively.

Figure 18CALCULATION OF BIAXIAL COMPRESSION AND BENDING

a. Plane in which a failure is supposed possiblein taking into account the buckling

b. Plane without taking into account the buckling

c. Interaction diagram showing the bending resistance

TAZ, building of a newspaper editor,Berlin (D)

24

Figure 19

transferstuds b= smallest transversal

size of the column

tran

sfer

zon

e

Shear connection incomposite columns

Region of load introduction

Loads are only rarely introduced into thecolumn via a header plate which distributesthem correctly between the steel and con-crete components. Normally, the floor beamsare attached directly to the steel componentof the column, so that part of the load mustbe transferred into the other component (thereinforced concrete). This necessitates provision of adequate trans-fer studs in a transfer zone as described below(Figure 19).

Industrial printing, Lausanne (CH)

Industrial printing, Lausanne (CH)

Finishing of composite columns with supplementary connectors in the transfer zones

a) interlinked stirrups and studs

b) and c) welded stirrups

d) bars passing through holes

e) stirrups and bars

Figure 20

25

Longitudinal shear resistance

Shear transfer is achieved either by frictional oradherence stresses between the contact sur-faces, or by mechanical shear connection whichprevents any significant slip between the steeland concrete.

When relying on friction and adherence it isclearly necessary to avoid any painting of thesteel surfaces. These types of shear transfermay be sufficient when the steel componentis fully encased.

Partially encased sections, with concrete onlypresent between the steel flanges, must in allcases adopt a certain number of mechanicalshear connectors fixed to the web of the steelmember. Without such connection the con-crete may spall away from the web under theaction of thermal stresses during a fire, and in-deed fall off the steel member. The mechanicalshear connection required may be achieved ina number of ways, either using headed studswelded to the web, or welding the stirrupsto the web, or perhaps using bars passingthrough holes in the web and linking the stir-rups on either side (Figure 20).

Rembrandt Tower, Amsterdam (NL)

Composite column

26

Figure 22GENERAL BEHAVIOUR OF A STRUCTURAL ELEMENT

SUBJECTED TO ISO FIRE

Fire resistance ofcomposite columns

Fully encased sections

When the steel member is completely encasedin concrete a composite column possesses avery high resistance to fire. Eurocode 4 Part 1.2proposes a simple table (Figure 21) which allows

the fire resistance of a column to be verified formost practical cases, without the need for cal-culations. If necessary, the documents referen-ced earlier provide any additional informationthat may be required to undertake a more accu-rate verification.

Partially encased sections

The fire resistance of partially encased sectionsis less than that of fully encased sections, be-cause some of the steel surfaces remain ex-posed to the fire.

Figure 22 shows the general behaviour of astructural element subjected to a standard ISOfire. Starting with the failure load at room tem-perature, fire tests under progressively dimini-shing loads demonstrate ever increasing fireresistance capabilities. A curve can be devel-oped for any structural element ; the rate atwhich resistance falls in a fire is greatest whenthe materials used are the most sensitive tofire.

For a level of load corresponding to the com-bination of actions under the fire condition,this resistance curve can be used to predict

RESISTANCE TO STANDARD FIRE

Figure 21ENV 1994-1-2 CALCULATION TABLE

R30 R60 R90 R120 R180 R240min hc and bc 150 180 220 300 350 400min c 40 50 50 75 75 75min us (20) 30 30 40 50 50

ormin hc and bc – 200 250 350 400 –min c – 40 40 50 60 –min us – (20) (20) 30 40 –

Fire test of a composite column

27

the fire resistance period for the element inquestion. The “natural” fire resistance periodfor partially encased composite columns is ge-nerally in excess of 60 minutes.

When the natural fire resistance of a compo-site column is less than that required it is ne-cessary to choose another section which hasa higher resistance curve. The curve must de-monstrate that the required fire resistance (90or 120 minutes) can be achieved under theproposed level of loading in fire.

The procedure described above generally leadsto overdesign of the column under normalservice conditions. However, the excess costremains moderate for a fire resistance of 90minutes (R90) provided a logical choice of asection with thin flanges is made. The requiredoverdesign is preferably achieved by adoptingone or more of the following measures :- increase the design grade of the steel- increase the grade of concrete- increase the amount of reinforcementIf the last of these measures is adopted themaximum percentage of reinforcement canexceed the maximum value of 4% specifiedin the relevant standards, which is appropriatefor normal service conditions. Up to 6% maybe adopted in order to achieve the necessaryfire resistance.

For most current applications in buildings in-creasing the material strengths or the amountof reinforcement is not sufficient to achievetwo hours fire resistance (R120), and it is alsonecessary to increase the size of the steel mem-ber.

Verification of the fire resistance of partially encased columns

Even when the simplified tables proposed inENV 1994-1-2 are used, some calculation isnecessary. It should also be noted that thesimplified design method proposed in Annex Fis more commonly used, having been widelydisseminated in the form of software. As for partially encased beams, this methodconsiders contours of temperature withinthe body of a section after 30, 60, 90 and120 minutes of exposure to an ISO standardfire (Figure 23). The section is divided intozones (Figure 24) in which the mechanicalproperties of the different constituent mate-rials vary as a function of the average tempe-rature in the zone. The collapse load is thencalculated using a process which is essential-ly the same as that used to calculate the col-lapse load under normal service conditions,but considering reduced material strengths.

Figure 24REDUCED SECTION OF A COMPOSITE COLUMN

FOR THE CALCULATION OF THE FIRE RESISTANCE

Figure 23: isotherms in a composite column sub-jected to an ISO fire of 90 minutes

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Figure 25DESIGN OF A COMPOSITE COLUMN

RIGID CORE(Braced structures)

COLUMNSUBJECTEDTO FIRE

STANDARD SERVICE CONDITIONS (b)N = 1.35 G + 1.5 Q (in most of the cases)max 4% of reinforcing barsBuckling length Lcr = L (generally)

VERIFICATION UNDER FIRE (c)N = 1.00 G + 0.5 Q (often)

0.5 - 0.9 according to the category of the building

max 6% reinforcing barsBuckling length Lcr = 0.5 L or 0.7 L

TAZ, printing newspaper office, Berlin (D)

The fire condition may govern design for thistype of column, particularly when fire resis-tance requirements exceed 1 hour. The mea-sures described above to “overdesign” thesection can be adopted. A reduced bucklinglength (Figure 25) can generally be adoptedprovided the floor slabs ensure compartmen-talisation of the fire.

Cruciform sections and sections with reinforcedsteel members

There is no simplified direct method of designfor these types of section. The designer mustuse numerical simulation (CEFICOSS), or inter-polate values from reference tables [2] (thatare based on numerical simulations) using “en-gineering judgment” to draw analogies.

Figure 26CONSTRUCTION DETAILS

Welding of the stirrups to the web

Bars passing through the web to link the reinforcing cages

welding

Studs

29

Construction details

The two basic components of a compositemember - the steel and the reinforced concrete- must clearly respect any application rules re-levant to their individual domains.However, composite construction owes its suc-cess above all to the levels of fire resistancethat can be achieved, and certain construc-tion details are important in order to assurethis fire resistance.

Positioning of the reinforcement

Traditional concrete rules must be respectedwhen considering resistance and cracking re-quirements under normal service conditions.In particular, rules govern minimum diametersfor bars and stirrups, the spacing of stirrups,the configuration of the stirrups as a functionof the spacing between the reinforcing bars incompression, concrete cover requirements etc.

An additional risk must be considered for thefire condition - local spalling of the concretemay occur. Even if this appears to only cause aslight reduction in the area of the effective sec-tion, it is an important phenomenon because itleads to an acceleration in thermal penetra-tion, which may lead to local weakening ofthe member.

As a consequence of the phenomenon descri-bed above, the part of Eurocode 4 (ENV 1994-1-2) dealing with fire proposes several addi-tional rules for reinforcement detailing (Fig-ure 26). In particular, for beams the spacing ofthe stirrups must not exceed 250 mm, andmesh must be added to the exposed facesof the infill concrete, with cover not exceed-ing 35 mm.

Museum“Museum für Verkehr und Technik“, Berlin (D)

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Mechanical shear connection to the web

Mechanical shear connection to the web canbe achieved in different ways, as previously dis-cussed; by welded studs, by welded stirrups, orby passing bars through the web to link thereinforcement cages. For beams with con-crete infilling the maximum spacing of the shearconnectors, and their dimensions, must respectcertain design limits (Figure 26). The same istrue for columns with concrete infilling bet-ween the steel flanges, unless closer spacing isrequired to achieve the necessary shear trans-fer under normal service conditions.

Continuous reinforcementin partially encased columns

The density of continuous reinforcement requi-red in connection regions may make it difficultto accommodate the necessary erection bolts.This problem is eased when the steel section islocally strong enough to resist the applied load-ing, perhaps with the addition of strengtheningplates. In such cases the reinforcement may beinterrupted in these regions, provided that thenecessary force can be transferred from theconcrete to the steel components, and in theother way below the beam.

Web openings in beams

Openings in the webs of partially encasedbeams are achieved in exactly the same wayas for bare steel sections. The openings are generally reinforced usinga short length of tube, or by a surrounding“frame” made from plates. This local web rein-forcement also performs the role of formworkduring the placing of concrete between theflanges (photo).

Provided that the longitudinal reinforcing barsin the concrete remain suitably encased, thepresence of web openings only influences theshear resistance of the section (due to in-creased heat penetration into the web). However,the excess in vertical shear capacity of an en-cased web during a fire is often considerable.An estimate of the reduced capacity next toan opening may be made considering thereduced yield strength of the web steel usinga simplified method proposed in Eurocode 4(ENV 1994-1-2). In principle this method isconservative, because radiation effects are infact reduced when an opening is present.

Clearly it is also possible to “seal” regions werepipes pass through the beams using fire insu-lation after fixing of the pipework.

City Center Kirchberg, Luxembourg (L)Girder with openings in the web

31

Fully encased columns

- Perimeter formwork is required.

- Normally concreted after erection.

- No exposed steel surfaces.

- Preferably based on steel sections with thickflanges (HEM, HEB, HD).

- Reinforcement must be fixed around thesteel member in its final, erected position.

- Relatively small percentages of reinforce-ment are used. Bars should preferably onlybe positioned at the corners of the section.

- The steel member should not be painted.

- A small number of mechanical shear con-nectors are usually sufficient. They are prima-rily needed in regions of load introduction.

- Structural fire resistance is inherently veryhigh.

- On site the full structural resistance of thecomposite column is only achieved follo-wing encasement.

Partially encased columns

- In theory no formwork is required (unlessthere are difficulties associated with lifting,or specific requirements for a very smoothor special textured finish on the concrete).

- Concreted horizontally, on the ground, be-fore erection.

- Steel surfaces remain exposed.

- Preferably based on steel sections with thinflanges, in order to limit the volume of steeldirectly exposed to the fire (HEAA, HEA,HP).

- Reinforcement cages can be prefabricated,and rapidly positioned.

- To achieve fire resistance in excess of onehour it is advisable to adopt the maximumallowable percentage of reinforcement(6% in the fire condition, of which only4% is taken into account for design undernormal service conditions).

- Paint is normally applied to the exposed sur-faces of the steel flanges, generally for pu-rely aesthetic reasons.

- Mechanical shear connectors (studs, or si-milar) are normally needed along the wholelength of the column, to prevent the con-crete falling off during a fire.

- Fire resistance, with possible “over-design”in excess of 60 minutes.

- Because of the over-design needed to satis-fy fire resistance requirements, there is oftenconsiderable over-strength during the cons-truction phase.

Figure 27

Width ≥ 240 mm

Comparison of the two most common solutions

Choice of column type

32

Pre-installedcolumns

Basic principle

“Pre-installed”, or “plunge” columns are fre-quently used in urban locations because theyeliminate a number of problems associatedwith deep excavations on sites adjacent to ex-isting buildings. Each column, of a height equalto the depth of the basement to be construc-ted, is lowered into a bored hole. The foot ofthe column is then embedded in concrete,which is poured into the bottom of the shaft,either at, or below, the final foundation level.A slab is then used to link the individual co-lumns, and the building superstructure is erec-ted on top of this slab. It is then possible toexcavate below the slab to form successivebasement levels whilst the superstructure isbeing erected.

Construction method

To prevent buckling of the columns duringconstruction, the bored holes are filled withgravel, amongst which may be interspersedweak concrete plugs at predefined locations.

The position of the heads of the columns, inboth plan and level, can be controlled relati-vely accurately on site. It is more difficult toadjust the verticality of the columns in theirshafts, and variations on site may be moresubstantial than with more “conventional”construction methods. It is, however, possi-ble to reduce problems of non-verticality usinghorizontal hydraulic pistons which can beplaced near the base of the columns andcontrolled from the surface.

Banque Bruxelles Lambert, Brussels (B)

Office building Woluwe Garden, Brussels (B)

Pre-installed columns

33

Steel members are ideal for this type of ap-plication because they are light and easy tomanipulate. Unfortunately, fire resistance re-quirements for basement locations (typicallyR90 or R120) do not permit the use of steelmembers without additional fire protection,which is not always welcome in locations thatare mostly used for parking. Composite co-lumns are often preferred because of theircompactness, and inherent fire resistance. Thechoice between a fully or a partially encasedcomposite section depends on the conside-rations discussed below.

Fully encased sections

Although a fully encased section may be com-pletely prefabricated before dropping it intoits bored hole, it is more common to utilise thebare steel section during the constructionstage (when the loads are less than in the finalservice condition). The steel member is thenprogressively encased in concrete, to form thecomposite section one level at a time as the ex-cavation progresses and floor slabs are formed.

For this type of application composite co-lumns offer the following benefits and disad-vantages :

- The member to be lifted is relatively light androbust, and need not be painted.

- In principle it is not necessary to over-designthe column relative to the final service loads.However, it is necessary to verify that themember is adequate for each stage duringconstruction. Excessive slenderness, or de-layed encasement of the lowest sections ofthe column, may necessitate the use of a lar-ger steel section. In some instances the baresteel section may be sufficient for the serv-ice loading, in which case the concrete en-casement serves merely as fire protection. It isclearly necessary to evaluate the speed ofconstruction both below and above the

ground in order to determine the differentload combinations and corresponding buck-ling lengths during construction.

- The section is very compact, and can be for-med in different shapes (for example squareor circular). However, excessively wide mem-bers (>40 cm) may prove difficult to arrangeefficiently between parking spaces.

- Formwork can generally be reused severaltimes. By placing the formwork eccentricto the steel member it may be possible tocompensate for any slight misalignment (upto several centimetres) of the latter duringits placing.

- Fixing of the formwork and reinforcement,and placing of the concrete, are not parti-cularly easy operations. In particular, it is ne-cessary to leave openings or ducts in theslabs to permit subsequent placing of con-crete in the lower level of column. Correctfilling and local load transfer capacity mustbe ensured. The need to achieve lapping ofthe longitudinal reinforcing bars means thatthey must extend into the ground below thelevel being concreted, so that they act as“starter bars” at the top of the followinglevel.

Pre-installed columns during erection

Floors

Steel or composite columns used in this typeof construction are compatible with all typesof floor structure. When it is possible to usebeams these may also be composite, eitherpartially encased or fire protected. It is easyto provide connection pieces, or box-outs toallow the subsequent passage of continuousreinforcement. When necessary, such connec-tion details can be provided with a means ofaccommodating any differences in the finallevels of the columns that may occur on site.

The framework formed by the floor beams isused to stabilise the basement walls duringconstruction. Partial encasement of the floorbeams ensures they have a significant buck-ling resistance.

It is also worth remembering that the use ofgalvanised decking as permanent formwork forthe floor slabs practically eliminates the needfor cranage during construction of the base-ment, and that the decking is an ideal com-plement to the steel or composite beams.

34

Kö-Galerie building, Düsseldorf (D)

Kö-Galerie building,Düsseldorf (D)

Partially encased sections

Partially encased sections are concreted atground level prior to being lowered into boredholes. They have the following characteristics :- The composite members are relatively heavy.- It is necessary to identify an area big enough

for concreting to take place. If necessary,substantial lengths of column can be achievedusing several pieces, which are boltedtogether as needed during the lowering ope-ration. The joints are normally located withinthe basement floor slabs, and are thereforeinvisible in the final condition. This meansthat the joints can be made using coverplates and external bolts. Lateral bucklingrestraints should be placed in the shaft,according to the spacing of the joints.

- Design for fire resistance often leads to anover-strength under normal service condi-tions. However, this can be exploited duringconstruction to increase the allowable buck-ling length and possibly permit excavatingof several levels at a time.

- Because the width of the steel sections istypically only approximately 300 to 400 mm,this type of composite column may offer agreater useable floor area between parkingspaces than alternative solutions.

35

Connections

The connections between composite membersare almost always formed between the steelcomponents, and are designed and detailedaccording to the usual rules for steel con-struction.

Their conception is driven by the philosophyof placing the bolts, or lengths of weld, inpositions where they are sheltered from directheat in a fire. Clearly it is also necessary tomaintain sufficient access for bolting or wel-ding during erection of the frame, and toavoid the need for additional fire protectionas much as possible.

As an example, bolts placed within the depthof the concrete slab (plus any finishing screed)will be buried in the mass of concrete andtherefore protected without any need for sup-plementary fire protection in the final condi-tion.

The general considerations described abovehave led to the development of several rela-tively common basic types of composite con-nection, as described below. Figure 28

bolts

Column connection using bracketsa) below the beam b) within the beam

Column connection with brackets

Column with bracketsbefore erection

Beam to column connections

- using a bracket (figure 28) : the bracketmay be placed either below or within thedepth of the beam. Additional bolts areadded within the depth of the slab to aiderection. There is no need to fire protect thebracket provided the upper weld, which isnot exposed directly to the fire, is reinforced,and that the bracket is sufficiently thick.Alternatively, fire protection can be avoidedby adding shear studs to the bracket, andpassing these through holes drilled in thecolumn flange so that they can be embed-ded in the column concrete.

36

- using a web plate (figure 29) : the boltedconnection must be fire protected after erec-tion, either using specific fire protection ma-terials or by embedding the connection inconcrete. The latter operation is facilitatedby oblique cutting of the top flange of thebeam, to allow filling of the cavity duringcasting of the slab.

- using an end plate with “upper” bolts(figure 30) : when possible the bolts shouldbe concentrated within the depth of theslab, or at least a sufficient number of suf-ficient diameter bolts to resist the combi-nation of loads applied in the accidental firecondition. A spacer plate may be used toensure that the connection is free to rotate.

Figure 29Column connection using a web plate

end of concreting

cavity to be filledafter erection

Figure 30

end ofthe

con-creting

end plate with “upper” bolts

bolts

slab

top decking

Column connection using a web plate

37

- using direct support from the column(figure 31) : this type of detail has been usedwith large prefabricated columns, which areinterrupted at each floor level. In order totransfer the loads it is necessary to includethick capping plates and various other largesteel components locally within the slab.

Large prefabricated piles (as found in the base-ments of multi-storey buildings) can be madewith web openings at each floor level to accom-modate the floor beams. It is possible to in-clude a means of adjusting the level of suchopenings to accommodate any variations onsite (Figure 32).

Figure 31

slab

bolts on side of the beam

Figure 32Section B-B

Section A-A

slab

beam

column

Post-office P&T, Saarbrücken (D)

Prefabricated piles with web openings

to accommodate the floor beams

38

Beam to beam connections

- using direct support (figure 33) : this verysimple type of detail results in a relativelydeep floor, which does however providegood flexibility when it comes to thelayout of the services etc. A continuityplate may be welded to the flanges of thesecondary beams after erection. Bolts usedto aid erection can be left in place, withoutfire protection. The upper flanges of theprimary beams, which are not connectedto the slab, may be provided with an insu-lating plate to increase their fire resistanceand thereby reduce the area of reinforce-ment required in the composite section.

- using a web plate (figure 34) : the ends ofthe secondary beams, which are left freefrom concrete during prefabrication, areattached to plates which protrude beyondthe concrete encasement of the primaries.These plates do not interfere with the rein-forcement in the primary beams providedthey do not extend too far towards the lowerflange; the upper bars used to facilitate pre-fabrication of the reinforcement cages cansimply be cut at these locations during pla-cing of the cages. As for beam to columnconnections of this type, it is necessary tofill-in with concrete, or otherwise protect,the region around the bolts after erection.

Figure 33

bolts for thepositionning

Beam to beam connection Post-office building P&T, Saarbrücken (D)

Figure 34

39

- using a bracket (figure 35) : as for thebeam to column detail, it is possible tomake a beam to beam connection using abracket, with an upper fixing to aid erec-tion. However, if the bracket is placed toolow in the primary beam it may interferewith the main reinforcement, which shouldthen be installed in the workshop of thesteel contractor.

Figure 35

erection bolts

Beam to beam connection using brackets

40

- using a nib on the upper flange (figure36) : a thick steel nib may be welded to theupper flange of the secondary beam. Thissimply rests on the primary, with a bolt usedfor location during erection. This very com-mon detail allows all the members to becompletely filled with concrete, and doesnot hinder in any way the reinforcement inthe members.

Figure 36thick steel nib

Beam to beam connection of preconcreted beams using anib on the upper flange

Preconcreted beam using a nib on the upper flange before erection

41

Frame stability

Diaphragm action of the slabs

The monolithic nature, and resulting in-planerigidity of the concrete floor slabs means thatthey can be used to transfer horizontal loads tothe vertical members that provide frame sta-bility. It is however necessary to provide otherways of maintaining the stability of the struc-ture during construction, before the concreteachieves sufficient strength.

Vertical bracing

Although composite members can be used fordiagonal bracing, the nodal connections arenormally rather complicated. There are current-ly several solutions to the problem of provi-ding vertical bracing:- bracing is configured so that it may be left

unprotected against fire because there isat least one bracing system outside the firecompartment.

- the bracing is placed behind a wall, whichprotects it from any fire.

- simple steel bracing is embedded within aconcrete wall following erection, the wallbeing cast between and around the co-lumns.

- the frame is stabilised by attachment to aconcrete element (such as a lift shaft or staircase), or to nodes that are unaffected by fire.Clearly any such elements must be in placeat the time of beginning erection of the steelframe.

Sony Center Potsdamerplatz, Berlin (D)

42

Structural models

The structural model applicable under normalservice conditions may evolve for the fire con-dition, depending on the construction detailsused. Several examples are considered below:

- A beam which is assumed to be simply sup-ported in normal service may be allowed tobenefit from some assumed continuity du-ring a fire. The presence of continuity rein-forcement in the slab above the beam sup-ports will prevent excess rotation of thebeams in a fire.

- When the lower bolts are not fire protec-ted, connections that are assumed to be ri-gid in normal service may either remain rigidor tend towards pinned behaviour during afire, depending on the sense of the appliedmoment. It is clearly necessary that the pro-tected upper bolts are sufficiently strong totransfer the appropriate loads to the columnin the accidental fire condition.

- When the slab is relatively thick (for exam-ple to achieve the required acoustic perfor-mance) and the secondary beams are atrelatively close centres (for example to avoidpropping during construction) it is possibleto consider fire protecting only one joist intwo. It is then necessary to check that theslab can achieve a “double span” underfire loading, and provide the necessaryreinforcement. The protected joist must alsobe checked for the fire condition, conside-ring an appropriate loaded area and loadcombination for this accidental condition.

In summary, it is necessary to verify the resis-tance of the structure for both normal serviceand fire, considering not only different loadsappropriate to each condition, but also poten-tially different structural models.

Expansion joints

Notwithstanding certain requirements for theslab reinforcement being satisfied, buildingswith a surface area in excess of 6000 m2, andup to 120 m in length, have been built usingcomposite construction without any expan-sion joints. It is however more common to res-pect the usual limits for steel structures whenconsidering the frequency of expansion jointsin a composite beam and column frame. Inter-mediate joints may be necessary in the reinfor-ced concrete floor slabs according to concretecode requirements.

Sony Center Potsdamerplatz, Berlin (D)

43

Structural integrity

The structural integrity and robustness of abuilding frame are fundamental requirementswhich are part of the basic philosophy embo-died in the Eurocodes. According to this phi-losophy, an accidental load (explosion, impactetc) must not lead to disproportionate damageof a structure.

One of the measures needed in order to en-sure appropriate integrity is to tie together theframe members horizontally. All the connec-tions are therefore required to possess atleast a minimum resistance to horizontal ten-sile forces. In some countries, such as the UK,the levels of load needed to satisfy this requi-rement have been quantified in an annex tothe National Application Document for theEurocode.

A steel frame normally responds very well tothis fundamental requirement for integrity, be-cause of the inherent tensile strength of theframe members and of traditional connec-tions. For composite construction it may benecessary to verify the suitability of some typesof bracketed connection. In practice the num-ber and size of the bolts used to aid erectionshould be kept at a reasonable level, or lowtensile resistance of some components canbe compensated by detailing additional barsthat are suitably anchored in the slab.

European parliament Espace Léopold, Brussels (B)

Technical Assistance

Arcelor Sections Commercial offers a free advi-sory service for all matters relating to the use ofrolled sections. A team of advisory engineers isat your disposal to answer any questions con-cerning design, fabrication, construction, me-tallurgy, welding, surface treatment and fireprotection. They are ready to collaborate withall those involved in planning, design andconstruction in order to work out optimizedsolutions.

Available brochures

• Structural Shapes Sales Programme• HISTAR - A new generation of rolled sections

for an economical steel construction• Beam-finishing• Car Parks in Structural Steel• Truss Girders• Bridges with Rolled Sections• Composite Beams and Columns• High-rise Buildings

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44

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