Shear Capacity of Prestressed Hollow Core Slabs in Slim Floor Constructions

Engineering Structures 31 (2009) 551–559

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Engineering Structures

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Shear capacity of prestressed hollow core slabs in slim floor constructionsJ. Hegger a, T. Roggendorf a,∗, N. Kerkeni ba RWTH Aachen University, Institute of Structural Concrete, Mies-van-der-Rohe-Str. 1, D-52074 Aachen, Germanyb H+P Ingenieure GmbH & Co. KG, Kackertstraße 10, D-52072 Aachen, Germany

a r t i c l e i n f o

Article history:Received 9 May 2008Received in revised form27 June 2008Accepted 3 October 2008Available online 28 November 2008

Keywords:Shear strengthHollow core slabsFlexible supportsPrestressSlim floor constructions

a b s t r a c t

Research in Finland and Germany reveals that the shear strength of hollow core slabs is reducedconsiderably due to transverse stresses when the slabs are bedded on flexible supports (e.g. slenderbeams). The aim of the present investigation is to analyse the load bearing behaviour of hollow core slabssupported on shallow steel beams, so-called slim floor constructions. This paper describes four full-scaletests on two-span floor systems and reference tests on single slabs with rigid supports. The observedshear strengths are compared with predicted failure loads according to international code provisions.Finally, recommendations regarding the design and the construction on flexible supports are given.

© 2008 Elsevier Ltd. All rights reserved.

1. Introduction

A common type of slim floor construction consists of precastprestressed hollow core slabs supported on the lower flange oftransverse shallow steel beams. This construction method enablesslender ceilings without girders (Fig. 1). Due to the fast erection ofstandardised steel frames and the use of precast concrete elements,short construction times and ahigh level of quality canbe achieved.Furthermore, the application of hollow core slabs reduces thedead load of the structure, which positively affects the designof subjacent columns and foundations. After the slabs are set inplace, the joints between the slabs and the beam are reinforcedand grouted with in-situ concrete to obtain diaphragm action.Slim floor constructions are very common in Scandinavia and theNetherlands, and their share in the Germanmarket is continuouslyincreasing. The steel beams are usually arranged in the smallerspan to minimise the deflections. However, remaining deflectionsof the supporting steel beams with a limited bending stiffness leadto transverse stresses significantly reducing the shear capacity ofthe hollow core slabs. The decrease in shear strength may controlthe design prior to flexural failure, and should be considered,especially for slabs with short spans or high loads close to thesupports.Only a limited number of large-scale tests [1–5] determining

the residual failure load is available in the literature. At the Institute

∗ Corresponding author. Tel.: +49 241 80 25170; fax: +49 241 80 22335.E-mail address: [email protected] (T. Roggendorf).

0141-0296/$ – see front matter© 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.engstruct.2008.10.006

of Structural Concrete at RWTH Aachen University, four full-scaletests on continuous two-span floor systems (6 × 10 m2) wereperformed to investigate the load bearing behaviour of prestressedhollow core slabs supported on flexible steel beams. The effectsof several test parameters such as cross section of the hollowcore slabs, bending stiffness of the steel beam, bearing details,horizontal constraints of the slabs and filling of selected hollowcores on the shear capacity were investigated. The observed shearstrengths are compared with international code provisions.

2. Load carrying behaviour and design criteria on flexiblesupports

The first substantial investigations of the load carrying be-haviour of hollow core slabs under different support conditions arepresented in [2]. In full-scale tests, the shear strengths of slabs sup-ported on various beam types (concrete beams with and withoutprestressing, steel beams) were determined. The results indicatethat all types of flexible supports may cause a decrease in shearstrength up to 60%. In addition, longitudinal cracks underneath thewebs of the slabswere observed close to the supports. These cracksmay weaken the transfer of prestress of the strands nearby. Sincethe slabs do not contain any reinforcement except the prestress-ing strands, only limited tensile stresses due to deflections of thesupports can be sustained. The concrete’s tensile strength ismainlyutilised for shear resistance and the transfer of prestress.Fig. 2 illustrates the loads acting on slabs supported on a

flexible steel beam. Shear deformations are the main impact onthe slabs next to the supports of the beam (edge slabs). In tests

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552 J. Hegger et al. / Engineering Structures 31 (2009) 551–559

Fig. 1. Principle of slim floor constructions.

Fig. 2. Stresses of hollow core slabs supported on a flexible steel beam.

from the literature [1–5] with single span beams, shear failure wasalways governed by the edge slabs of a floor. According to [2],test observations and measured displacements indicate that theshear deformations of these slabs control the design. However, theshear deformations cannot solely be explained by the deflectionand curvature of the beam. Even small deflections of l/1000–l/300(where l is the span of the beam) at the failure state may causea considerable decrease in shear strength. Besides the bendingstiffness of the beam, the degree of composite action between theslabs and the beam as well as the cross section of the hollow coreslabs are critical for the design [2].The slabs at midspan of the beam are mainly subjected to

transverse bending moments. Due to the curvature of the beams, aconcentration of the support reaction underneath the outermostwebs may occur. Furthermore, friction between the soffit of theslabs and the beam flange induces additional transverse tensilestresses. The stresses superpose as a result of the support deflectionand the transfer of prestress. Hence, especially for slabswith a highprestressing force, there is an elevated risk of longitudinal cracksincreasing the transfer length.A design model to account for the decrease in shear capacity

on flexible supports which has been adapted to fib-guidelines [6]as well as building codes in Finland [7] and the Netherlands [8]is presented in [9] (based on the test results from [2]). Web shearfailure is assumed when the maximum principle stresses σI in thewebs reach the tensile strength fct of the concrete. The maximumprinciple tensile stresses σI are approximated from the presentstress components in the webs of the edge slabs according to Fig. 3

Fig. 3. Stress components in the webs of an edge slab on flexible supports(according to [2]).

Fig. 4. Considered composite beam to calculate the shear stress τzy in transversedirection (acc. to [9]).

as

σI =σx

2+

√σ 2x

4+ τ 2xz + τ

2zy. (1)

According to themodel, bearing of the slabs on flexible supportsleads to the shear stress component τzy in transverse directiondue to composite action between the supporting beam and theslabs. A composite beam model consisting of the beam, in-situconcrete and the hollow core slabs as illustrated in Fig. 4 is usedto calculate τzy. On both sides of the beam the shaded sectionsof the slabs (upper and lower flange) extending to the width beffare considered. The effective width of the slabs beff takes intoaccount the degree of composite action. Values of beff serve ascalibration factors to adjust calculated failure loads and test results.These values have to be determined experimentally for differentconstruction types. Further information on the model, such asdefinition of the critical section for the design, detailed calculationsof the stress components and effective widths beff for differentbeam types can be taken from [9], while the latest extensions aregiven in [7].According to the Steel Construction Institute [10], the influence

of the support stiffness does not need to be considered if thefactored shear force applied to the slabs after grouting of thejoints and presence of a composite action is less than 35% of thedesign shear strength VRd provided by the manufacturer (for rigidsupports). Advice from the manufacturer is recommended whenpropped constructions are used and hence transverse stresses dueto the dead load of the structure occur or imposed loads aftergrouting exceed 35% of VRd.So far, no standardised design procedure for prestressed hollow

core slabs on flexible supports has been established in Germany.According to [11], the design shear strength VRd should be reducedto 50% when the slabs are supported on beams except forvery stiff concrete girders. An increased bearing depth of theslabs and reinforcement bars in the joints are recommended toenhance anchorage of the prestressing strands. The applicationof prestressed hollow core slabs in Germany is controlled byconstruction approvals since their characteristics substantiallydeviate from the regulations of the German Bulding Code DIN1045-1 [12]. Approvals from several suppliers (e.g. Echo, Betonson

J. Hegger et al. / Engineering Structures 31 (2009) 551–559 553

Fig. 5. Test setup to determine the shear strength on flexible supports.

and Schwoerer) currently allow for the application on flexiblesupports when the shear capacity is decreased and structuralmeasures are taken. The acting shear force may not be greaterthan 0.5 · VRd and the deflection of the supporting beam may notexceed l/300 under service loads. Slabs have to be bedded on aneoprene strip with a thickness of 10 mm. The width of the stripdepends on the pressure in the bearing surface, but has aminimumof 50 mm. Finally, filling of the outermost hollow cores of theslabs with in-situ concrete to a depth of 80 cm including stirrups(∅ 10 mm) connected with the tie rod for diaphragm action isrequired. Revisions of the bearing details and the in-situ concretefilling are currently being discussed.

3. Experimental investigations

3.1. Test setup

To investigate the effects of different support conditions onthe shear strength of prestressed hollow core slabs, four full-scale tests on floor systems consisting of 10 slabs (Fig. 5) as wellas reference tests of single slabs on rigid supports (Fig. 6) wereperformed. In the middle of the two-span floor systems the slabswere supported on a shallow steel beam. A so-called IntegratedFloor Beam (IFB) consisting of a bisected steel section (flange beam)and a broadened lower flange to provide a bearing surface for theslabs was used. According to [13], the load was applied close tothe IFB with a shear slenderness of a/h = 2.5, where a is thedistance of the centre line of the applied load to the middle ofthe bearing surface and h is the slab thickness. The same shearslenderness was used in the reference tests with the aim to obtainweb shear failure instead of flexure failure or flexure–shear failure.In order to prevent torsion of the slabs, the supporting beams atthe ends of the floor system (end beams) were designed to deflectapproximately as much as the IFB.Cracking of the soffit of the slabs could be observed because

the test setup was arranged on a framework at a height of 2 mabove the ground. The loads were applied by tension rods passingthrough the middle hollow core of each slab. Steel sections on topof the floor spread the loads over the total width of the slabs. Theoutline and a sectional view in the longitudinal direction of the testsetup are depicted in Fig. 5. Total length of the slabswas 5.09mandspacing of the centre lines of the supports added up to 4.97 m.

Fig. 6. Test setup to determine the shear strength according to DIN EN 1168 [13].

Fig. 7. Reinforcement of joints and bearing details.

In-situ concrete with an average cylinder strength of 20.8 MPaat the time of testing was used to grout the joints of the floors. Thetop of each middle hollow core was opened to a length of 30 cmfrom the face of the slabs to achieve proper grout of the concretealong the IFB. Plastic plugs provided by the suppliers stopped theconcrete from flowing deeper than 35–50 mm into the remaininghollow cores (Fig. 7). The slabs were tied together with reinforcedconcrete at the end beams.The reinforcement of the in-situ concrete is illustrated in Fig. 7.

One reinforcement bar∅ 10 mmwas placed in each joint betweenthe slabs, as is common in practice. The longitudinal reinforcementwas passed through drilled holes in the web of the IFB. Two bars∅ 8 mmwere arranged in the lateral direction of the slabs on eachside of the IFB and at the end beams. A circumferential tie anchor


Fig. 8. Cross sections of hollow core slabs MV5/265 (top) and VMM-VSD 25(bottom) (mm).

was not applied. It might have had a positive effect on the loadcarrying behaviour due to bonding of the slabs, but may not bepresent in each section of large ground plans in practice.

3.2. Test programme

The main test parameters were cross section of the hollowcore slabs, bending stiffness of the steel beam, bearing details,horizontal constraints of the slabs and filling of selected hollowcores. Table 1 summarises the test programme. Notation of thetests consists of a number for each floor system and the charactersa (slabs no. 1–5) and b (slabs no. 6–10) to distinguish between thetwo spans with different parameters. The aim was to determinethe failure load of each span separately. For this reason, a steelframe was arranged over the slabs where failure first occurred.Simultaneous loading of this framework when the slabs in theopposite span were tested until failure enabled an approximatelysymmetric loading of the IFB. However, damage of the reloadedslabs before failure of the opposite span cannot be precluded,and the steel frame affects the load transfer to the IFB as wellas the interaction between the slabs and the beam. Remainingdifferences in the load bearing behaviour of both spans may givereason to question the results obtained in a second test run afterarrangement of the framework. Hence, the present paper focuseson the results and the conclusions to be drawn from the firstloading of each slab system. E.g., if results from test 2-b arepresented, measurements were made in span b but in the first runof test no. 2.Hollow core slabs from two manufacturers with similar

thicknesses but different cross sections were investigated (Fig. 8).Slab type MV5/265 (Brespa) from DW Systembau features fiveapproximately circular hollow cores with a diameter of 167 mm,while the slab VMM VSD 25 from Echo has eleven oblong-shaped openings. Further substantial differences are theminimumweb width being 45% greater for slab type VMM-VSD 25 andmanufacturing by extrusion and slip forming of the slabs MV5/265and VMM-VSD 25, respectively. Table 2 summarises the slabproperties.Two Integrated Floor Beams with different bending stiffnesses

EIy at similar heights were used to investigate the influence ofvarious deflections (Fig. 9). However, in the first test with slab typeMV/265, the yield strength was reached in the outer compressionfibre of the more slender beam (εs ≈ 2h, steel grade S355 withfyk = 355 MPa) just before web shear failure of the slabs occurred.Following tests were performed with the stiffer beam featuring ahigher steel grade (S460 with fyk = 460 MPa) in order to preventlarge deformations due to plastification. This IFB totally recoveredafter removal of the slabs and the in-situ concrete so that it could be

Fig. 9. IFB with Iy = 35 400 cm4 (left) and Iy = 74 200 cm4(right) (mm).

repeatedly used. Deflection under the dead load of the floor systemwith the more slender beam was compensated by precambering.The stiffer beam was not precambered because it featured twicethe bending stiffness and the deflection due to dead load could beneglected.In addition, different support conditions of the slabs were

investigated in the tests 1 and 2. The effects of a neoprenestrip increasing the friction in the bearing surface on the sheardeformations and the transverse tensile stresses in the lower flangeof the slabs were examined. Fig. 7 (Section 4-4) depicts the bearingdetails of the slabs. On one side the slabs were directly placedon the lower flange of the beam, as is common in Finland. Withregard to the German construction approvals, they were beddedon a 10 mm thick and 35 mmwide neoprene strip with an overlapof 35 mm on the other side. The cementitious grout could flowunderneath the overlapping slab so that a total bearing depthof 70 mm equal to the opposite side with a direct support wasachieved. In tests 3 and 4, all slabs were bedded on neoprene strips(35/10 mm) at the IFB.Since the slabs and the in-situ concrete at the IFB with

increasing deflection of the construction tend to move outwardsover the supports of the beam (Fig. 2), horizontal constraints of theedge slabs were investigated in tests 3 and 4 (one test with eachslab type). A common assembly of the beam–column connectionwith head plates is illustrated in Fig. 1. In the tests, steel plateswerewelded at the supports of the IFB (Fig. 11). Corresponding to headplates in practical constructions, these plates ended at the frontedge of the beam’s compression flange. Thus, anymovement of thein-situ concrete was eliminated and a transverse displacement ofthe slabs was only indirectly restrained due to the standard hollowcore filling with a depth of 35–50 mm (Fig. 7, section 4-4).Both in test 3 and test 4, a horizontal constraint of the slabs

in the transverse direction caused by adjacent floor panels wassimulated in one span by lateral steel beams across the total slablength. Despite the constraint through standard beam–columnconnections, the lower slab flangemoves outwards due to rotationat the supports and curvature of the IFB. Between the edge slabs ofadjacent floors transverse compression stresses in the lower flangemay occur, increasing the shear deformation (Fig. 10).To prevent a vertical load transfer of the lateral steel beams,

friction bearings were arranged between the edge slabs and thebeams. Full load application to the lower slab flange was attainedby a mortar layer with a height of 60 mm from the soffit ofthe slab. An overview and details of the test setup to investigatehorizontal constraints which may occur in common constructionsare presented in Fig. 11. The lateral beams were supported in thevertical and the horizontal direction at the IFB and the end beam.Furthermore, they were connected with tie rods arranged in thetransverse direction of the slabs to increase the horizontal stiffness.The tie rods were slightly prestressed before the tests in orderto forestall significant compression of the friction bearings. Thus,a horizontal constraint of the edge slabs was activated at smalldisplacements.An in-situ concrete filling of the edge slabs outermost hollow

cores to a length of 70–80 cm with the aim to enhance the


Table 1Test programme.

Parameter Test number1-a 1-b 2-a 2-b 3-a 3-b 4-a 4-b

Slab type MV5/265 X X X XSlab type VMM-VSD 25 X X X XIFB with Iy = 35 400 cm4 X XIFB with Iy = 74 200 cm4 X X X X X XSlabs directly placed on beam flange X XSlabs bedded on neoprene strip X X X X X XFree horizontal movement of slabs X X X XHorizontal constraint by steel plates X X X XHorizontal constraint by lateral beams X XEdge slabs with filled hollow cores X X X X

Table 2Characteristics of hollow core slabs.

Unit Slab typeMV5/265 VMM-VSD 25

Concrete grade according to [12] (–) C45/55Thickness h/cross sectional area Ac /moment of inertia Ic (cm/cm2/cm4) 26.5/1685/144700 25/1869/129100Minimum web widthΣbw (cm) ≈33 ≈48Centroid z(1)s (cm) 13.4 12.9Upper prestress layer (grade St 1570/1770) (–) 2 strands 3/8

′′

(0.86 cm2/m) 2wires∅ 5mm (0.33 cm2/m)Lower prestress layer (grade St 1570/1770) (–) 12 strands 0.5

′′

(9.3 cm2/m) 12 strands 0.5′′

(9.3 cm2/m)Edge distance of strands zp,top/zp,bottoma (cm) 3.5/22.4 2.75/20.5Initial prestress σp0,top/σp0,bottom (MPa) 900/900 250/1080Calc. shear strength vRd,ct (fct = fctk;0.05/γc)b,c (kN/m) 61.1 105.6Tensile strength from tests fct,test (MPa) 4.0 (no.1)/4.2 (no.3) 3.5 (no.2)/3.6 (no.4)Calc. shear strength vRm(fct = fct,Test )(3) (kN/m) 229 (no.1)/240 (no.3) 278 (no.2)/285 (no.4)a From top surface of the slab.b With fctk;0.05 = 2.7 MPa (5-% fractile of tensile strength) and γc = 1.8 (safety factor for plain concrete) [12].c According to German construction approvals for rigid supports (vRd,ct given by the suppliers).

Fig. 10. Contact between edge slabs of adjacent floors due to deflection of slim floorconstructions.

resistance against shear deformations was also tested. Concretewas grouted through 30 cm long openings which were arrangedat a distance of 40 cm from the face of the slabs to minimise anydamage close to the support. Fig. 12 illustrates the factory madeopenings of the slabs to grout the concrete at the IFB (hollow corein the middle) and to fill the outer hollow cores of the edge slabs.

3.3. Test results

A main parameter of the tests was the influence of differentbeam deflections on the shear strength of the slabs. Fig. 13shows the load–displacement behaviour of the Integrated FloorBeams until failure of the first slabs. The maximum deflectionat midspan was 56 mm (≈l/100) for the more slender beam in

Fig. 11. Test setup to investigate horizontal constraints.

test 1 and approximately 30 mm (≈l/200) for the stiffer beamin the remaining tests. In the beginning of the tests, load cyclesaccording to [13] were performed. The unique failure mode ofall floor systems was web shear cracking of the edge slabs atshear loads from 158 to 182 kN/m including the dead load of thestructure and the loading equipment.Table 3 summarises the shear strengths on rigid and flexible

supports with the corresponding beam deflections. Based on theload capacity on rigid supports, 60%–68% of the shear strengthwere achieved on flexible supports. A clear influence of the slabs


Table 3Summary of test results.

Testno.

Slab type Shear strength on rigid supportsvs (kN/m)

Shear strength on flexible supportsvbw (kN/m)

vbw/vs(%)

Max. displacement u of IFB (mm)

1-a MV5/265 239 160 67 57.5 (l/104)a1-b 158 66 56.2 (l/107)

2-a VMM-VSD25 266 162 61 31.4 (l/191)2-b 162 34.8 (l/172)b

3-a MV5/265 268 182c 68 31.5 (l /190)3-b

4-a VMM-VSD25 282 168c 60 29.1 (l/206)4-ba Including 4.6 mm deflection after unloading the first test run (failure in span 1-b).b Including 2.6 mm deflection after unloading the first test run (failure in span 2-a).c Failure of edge slabs in both spans occurred simultaneously.

Fig. 12. Openings of the upper flange and filling of hollow cores close to thesupport.

Fig. 13. Load–displacement behaviours in the middle of the IFB.

cross section was not observed since the deviation of the averageproportionate decrease in shear strength for both slab types wasonly about 5%. The concrete’s tensile strength fct was determinedexperimentally from drilled cores with a slenderness of hc/dc = 2(with hc , dc : height and diameter of the samples, respectively)taken from the webs of the slabs. Similar shear strengths of alltested slabs despite the greater web width of slab type VMM-VSD25 are mainly attributable to the smaller tensile strength of theseslabs (Table 2).Despite different stiffnesses and corresponding deflections of

the IFBs, nearly the same ultimate loads were obtained in tests1 and 3 with slab type MV5/265. Besides the IFB, further testparameters have to be consideredwhen comparing the results, butin principle the observation that the decrease in shear strengthis not solely attributable to the deflection of the supports [2] isconfirmed. However, substantial deflections increase the risk oflongitudinal cracking of the slabs supported in the middle of thebeam, as the cracking patterns in Fig. 14 reveal. With greater beam

deflections in the first test setup, cracking of several inner slabswasinitiated under relatively small loads. Due to higher friction in thebearing surface, the majority of cracks occurred in the span wherethe slabs were bedded on a neoprene strip. Though supported onneoprene strips in both spans only few single longitudinal crackswere observed in the remaining tests with the stiffer IFB. Thesmall number of cracks indicates that anchorage of the prestressingstrands is only marginally affected. Measurement of the end slipat selected strands via inductive transducers arranged inside thehollow cores before grouting the concrete showed no great endslip and hence did not reveal a decrease in bond stiffness either.Due to the small curvature of the beam close to the supports, onlyone longitudinal crack (test 1 with slender IFB) was observed atthe edge slabs where web shear failure was initiated. Obviously,longitudinal cracks do not contribute to the decrease in shearcapacity on flexible supports.A 10 mm thick neoprene strip partially compensates for the

potential spacing between the soffit of the inner slabs and the IFBdue to its curvature (Fig. 2). This was revealed bymeasurements ofthe differential vertical slab displacement at the face of the lowerIFB flange. The results may, however, be partially attributable toa shift of the resulting reaction force towards the web of the IFBand lower lateral bending of the supporting flange because of therelatively soft neoprene at the front end of the bearing surface(Fig. 7, bottom). No unfavourable effect of a neoprene strip dueto increased friction was observed when the beam deflection waslimited appropriately. Hence, the applied bearing detail can beused without concern.In tests 1 and 2 without horizontal constraints, a uniform

displacement of the upper and lower edge slab flanges wasmeasured by transducers attached to the flanges of the IFB (Fig. 15,top). However, shear deformations of the steel beam affectingthe recorded data of the transducers occurred. Considering this,effective shear deformations of the edge slabs were in the orderof 0.28 mm.Extensive cracking of the in-situ concrete in the joint along

the IFB was observed under fairly small loads because of its smallreinforcement ratio. Hence, a composite action between the beamand the slabs resulted in the first line only from friction in thebearing surface and was too small to induce measurable sheardeformations of the edge slabswith respect to the IFB. It is assumedthat besides the shear deformations an increased shear forceacting on the edge slabs caused the observed decrease in shearcapacity. Unevenly distributed shear forces with a concentrationtowards the supports of the IFB may occur in the hollow coreslabs (v2 > v1 in Fig. 2) since considerable shear stresses aretransferred over the concrete filled joints between the slabsin the transverse direction. Finite element analyses with three-dimensional continuum models of a slab field are currently beingperformed to further investigate this assumption. In addition, one


Fig. 14. Crack patterns of the soffit of the slabs and the edge slabs in test 1 (left) and test 3 (right).

Fig. 15. Displacements of hollow core slab flangeswithout (top) andwith (bottom)horizontal constraint.

focus of future tests should be adequatemeasurements to examinethe matter experimentally.A comparison of tests 3 and 4 with test 2 (all tests were

conductedwith the stiffer IFB) indicates that horizontal constraintsof the slabs increase the stiffness of the floor system (Fig. 13). Thedegree of composite action and presumably the transfer of shearforces in the transverse direction over the slab joints are enhanced.Furthermore, the average slip between the beamand the edge slabsis decreased by 40% solely due to the steel plates at the supportssimulating head plates of a beam–column connection (comparisonof tests 2-b and 4-b in Fig. 15; transducers H5, H6). The relativedisplacements also reveal a stronger movement of the upper slabflanges and hence a distinct shear deformation. According to Fig. 15

(bottom), the measured shear deformation adds up to 0.3 mm(transducers H5, H6), which additionally has to be increased by theshear deformation of the IFB.Whereas the concrete filling betweenthe head plates follows the rotation of the IFB at its supports, aneffective horizontal constraint of the slabs may only be expectedin the lower part of their cross section due to cracking of thejoint along the beam propagating from the top side of the floor.Constraints underneath the slabs centre line increase the sheardeformation. The lateral steel beams further enhance this effectand accordingly the greatest deformation of the slabs with a valueof 0.6mm(without consideration of the beam’s shear deformation)was measured in test 4-a (Fig. 15, bottom; transducers H3, H4).Despite different shear deformations, the edge slabs of both spansfailed simultaneously in tests 3 and 4, respectively. Furthermore,similar ultimate loads as in the testswithout horizontal constraintsand corresponding smaller shear deformations were achieved.These observations confirm the assumption that a transfer of shearforces from the inner to the edge slabs over the longitudinal jointsconsiderably contributes to the decrease in shear capacity.No increase in shear strength was observed due to filling of

selected hollow cores of the edge slabs in tests 3 and 4. However,slab type MV5/265 with two filled hollow cores (width of hollowcores wc = 167 mm) achieved the same failure load as intest 1 despite greater shear deformations because of the horizontalconstraints. In each span of test 4 with slab type VMM-VSD 25,one edge slab with two and one with four filled hollow cores(wc = 65 mm) was used, respectively (Fig. 12). Web shearfailure was initiated in the edge slabs with fewer modified hollowcores at first, which suggests that the load bearing behaviour maybe positively affected by the concrete filling. The shear strengthwas slightly smaller than in test 2 with the same slab type butwithout horizontal constraints. It is important to mention thatthe in-situ concrete could easily be removed from the innersurface of the hollow cores when the slabs were pulled downafter the tests. The lack of prestressing, subsequent shrinkage anda relatively smooth inner surface caused by the manufacturingprocess obviously prevent an effective bond between the slabsand the in-situ concrete. Hence, a considerable load transfer of theconcrete infill is not to be expected though the resistance againstshear deformations may be enhanced.

4. Comparison of test results with code provisions

German construction approvals provide formulae to determinethe shear strength vRd,ct of hollow core slabs on rigid supports.According to present approvals, 0.5 · vRd,ct may be utilised onflexible supports when the construction details described in


Fig. 16. Hat shaped steel section (left) and delta beam (right).

chapter 2 are accomplished. Comparison between calculated shearstrengths vRm under consideration of the experimental tensilestrength fct,test and γc = 1.0 from Table 2 and observedfailure loads vs in the reference tests from Table 3 shows agood agreement. Hence, the construction approvals are capable ofdetermining the shear strength on rigid supports appropriately,which is also described in [14]. However, the compulsory structuralmeasures on flexible supports should be reevaluated underconsideration of the present test results and a general limitationof the utilised shear strength to 0.5 · vRd,ct may be conservative insome cases.Tests from [2] were evaluated in [9] to determine values for

the effective width beff of the slabs when supported on differentbeam types. The Finnish design model could then be used tocalculate the shear strength on the considered (flexible) beamtypes with adjusted beff -values. Tested steel sections in [2] werea so-called delta beam and a hat shaped beam (Fig. 16). For abeam span of 6 m as in the present tests, beff is given as 106 mmand 165 mm for the hat shaped section and the delta beam,respectively. Greater effectivewidths consider among other effectsa stronger composite action between the slabs and the beam.Obviously, a high degree of composite action occurs with a deltabeam since the in-situ concrete can flow through theweb openingsinto the section. It may be expected that the investigated IFB toa certain extent encloses the in-situ concrete between its flangesand hence features a stronger composite action than a hat shapedsection. This leads to the assumption that beff determined fromthe own tests should range between 106–165 mm. Correspondingto [9], beff -values of 73 mm, 154 mm, 113 mm and 145 mm arecalculated for tests 1–4, respectively. Compared with slab typeMV5-265, relatively high effective widths are determined for slabtype VMM-VSD 25 (tests 2 and 4), which cannot be explained bythe similar proportionate decrease in shear strength of both slabsand indicates that the effects of different cross sections are notconsidered appropriately in the model. Test 1 leads to a rathersmall beff -value, which may be due to extensive cracking of the in-situ concrete and a correspondingly weak composite action underthe great deflections of the more slender IFB. The evaluation wassensitive to several parameters and the calculated values feature aconsiderable scatter for different tests. However, the design modelwas applied to construction parameters other than tested in [2]and with a mean value of beff = 137 mm (beam span l = 6 m)determined from tests 2, 3 and 4 provides appropriate results forthe shear strength of hollow core slabs supported on IntegratedFloor Beams. Further information on the test evaluation is givenin [15].

5. Summary and conclusions

The load bearing behaviour of slim floor constructions, inparticular the shear strength of prestressed hollow core slabs,was investigated in four full-scale tests. Main parameters werethe cross section of the hollow core slabs, bending stiffness ofthe supporting steel beam, bearing details, horizontal constraintsof the slabs and filling of selected hollow cores. The following

conclusions for the design of slim floor constructions and hollowcore slabs on flexible supports in general can be drawn from thetests results:(1) For beam deflections from l/100–l/200 at failure no effect

of the support stiffness on the slabs shear strength was observed.Considering tests from [2] and [5] with even smaller deflectionsbut a similar decrease in shear strength, this observation leads tothe implication that besides a limitation of the beam deflection,further measures are necessary to considerably increase the shearstrength. It is assumed that on the other hand, large deformationsdue to plastification of the supporting beam will cause prematurefailure of the slabs in any case.(2) When the deflections are limited to l/300 under service

loads, only marginal longitudinal cracking of the inner slabs dueto transverse tensile stresses in the lower flange occurs. At theultimate limit state only few cracks not affecting the overallbearing capacity of the floor will develop. Shear strength ofthe edge slabs which control the design was not affected bylongitudinal cracks in any test.(3) It is recommended to support the slabs on a 10 mm

thick neoprene strip to adjust dimensional tolerances and aconcentration of the bearing pressure underneath the outer websof the slabs. When the deflection of the supporting beam is limitedappropriately, it is expected that the load bearing behaviour is notaffected by higher friction in the bearing surface. Minimumwidthsof 35 mm for the neoprene strip and 70 mm for the total bearingdepth are sufficient according to the present tests.(4) An in-situ concrete filling of selected hollow cores did not

increase the slabs shear strength but the fracture pattern of onetest with different numbers of filled hollow cores indicates thatthe resistance against shear deformationsmay be enhanced. Due topoor bond between the slabs and in-situ concrete, no considerableshear transfer of the filling was observed.(5) A reasonable decrease in the design shear strength of

hollow core slabs on flexible supports is required to ensure asufficient safety level. Test results indicate that 60%–70% of theshear strength on rigid supports can be utilised under appropriateboundary conditions corresponding to those in the tests. On theother hand, even the utilisation of 50% may be nonconservativefor long beam spans or unfavourable cross sections and interactionproperties between the beam and the slabs among others.(6) Only a marginal effect of the slabs cross section on the shear

strength was observed for the tested slab types. The ultimate loadwas controlled by the concrete tensile strength of the slabs.(7) Interaction between the slabs and the IFB leads to transverse

stresses and shear deformations of the edge slabs. However, thepresent tests indicate that the decrease in shear strength is notsolely attributable to this effect. Considerable shear forces aretransferred from the inner to the outer slabs over the longitudinaljoints, gradually increasing the shear load of the slabs towards thesupport of the IFB. The interaction of both effectsmay be the reasonfor the total decrease in shear strength.(8) Evaluation of the test results shows that the German

construction approvals provide appropriate formulae to determinethe shear strength of hollow core slabs on rigid supports. However,to generally utilise 50% of the calculated values on flexible supportsmay not be appropriate. The Finnish design model can be usedto determine the shear strength on flexible supports. The resultsfeature a certain scatter but a reasonable mean value to adjust themodel to IFBs was determined.Finite Element analyses to further evaluate the mechanical

behaviour of the edge slabs are currently being performed atthe Institute of Structural Concrete at RWTH Aachen University.The effects of different construction parameters on the sheardeformations and corresponding shear strengths due to interactionof longitudinal and lateral load transfer were evaluated. Furtherexperimental and numerical results are given in [15].


Acknowledgements

The present research program (AiF-No. 14203) was supportedby the funds of the Federal Ministry of Economy of Germany(BMWi) via the Arbeitsgemeinschaft industrieller Forschungsvere-inigungen ‘‘Otto von Guericke’’ e.V. (AiF) and the German CementWorks Association (VDZ). The authors express thanks to the BMWi,AiF andVDZ for their financial support, aswell as to the Bundesver-band Spannbeton-Fertigdecken e.V. and ArcelorMittal Com.Sect.S.A. for the contribution of test materials. They are also grateful forthe helpful suggestions of the suppliers and the committee accom-panying the project.

References

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[2] Pajari M, Koukkari H. Shear resistance of PHC slabs supported on beams I:Tests. J. Struct. Eng. 1998;1051–61.

[3] Bode H, Heppes O. Flachdecken mit integrierten Stahlträgern. Forschungs-bericht Deutscher Ausschuss für Stahlbau DASt. Universität Kaiserslautern.Fachgebiet Stahlbau; 2000.

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Spannbetonhohlplatten auf IFB-Trägern. Prüfbericht Nr. 01-02p, Institut fürStahlbau und Werkstoffmechanik. Technische Universität Darmstadt; 2001.

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[6] Guide to Good Practice by Fib Commission 6. Special design considerations forprecast prestressed hollow-core floors; 2000.

[7] Code Card No 18: Design of hollow core slabs supported on beams. Englishedition. update; 2007.

[8] CUR/BmS-Aanbeveling 104: Vloeren van kanaalplaten met geintegreerdeliggers; 2005.

[9] Pajari M. Shear restistance of PHC slabs supported on beams II: Analysis. J.Struct. Eng. 1998;1062–73.

[10] Hicks SJ, Lawson RM. Design of composite beams using precast concrete slabs.The steel construction institute. Publication number SCI P287; 2003.

[11] Hegger J. Bemessung und Konstruktion von vorgespannten Decken imHochbau. In: Der Prüfingenieur; 2003. p. 19–28.

[12] Normenausschuss Bauwesen im DIN Deutsches Institut für Normung e.V. DIN1045-1. Tragwerke aus Beton. Stahlbeton und Spannbeton. Teil 1: Bemessungund Konstruktion; 2001.

[13] Normenausschuss Bauwesen im DIN Deutsches Institut für Normung e.V.DIN EN 1168, Betonfertigteile–Hohlplatten. Deutsche Fassung EN 1168: 2005;2005.

[14] Hegger J, Kerkeni N, Doser HP. Gutachten zur Umstellung der Zulassungenvon Spannbeton-Hohlplatten auf das Normenkonzept nachDIN 1045-1: 2001;2003.

[15] Hegger J, Roggendorf T. Einfluss der biegeweichen Lagerung auf dasTragverhalten von Slim-Floor-Konstruktionen. AiF-Forschungsvorhaben Nr.14203/N1. InstitutsberichtNr. 184/2007; Lehrstuhl und Institut fürMassivbau.RWTH Aachen; 2007.

Documents

Shear Capacity of Prestressed Hollow Core Slabs in Slim Floor Constructions