Engineering Structures 32 (2010) 3866–3878

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Engineering Structures

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Structural safety and serviceability evaluations of prestressed concrete hybridbridge girders with corrugated or steel truss web membersKwang-Hoe Jung a,b, Jong-Won Yi b, Jang-Ho Jay Kim a,∗

a School of Civil and Environmental Engineering, Yonsei University, 134 Shinchon-dong, Seodaemun-gu, 120-794, South Koreab Institute of Construction Technology, Hyundai Engineering and Construction Co. Ltd., 102-4, Mabuk-dong, Giheung-gu, Yongin-si, Gyounggi-do 446-716, South Korea

a r t i c l e i n f o

Article history:Received 26 August 2009Received in revised form25 August 2010Accepted 26 August 2010Available online 28 September 2010

Keywords:Hybrid girderCorrugated steel webSteel truss webFlexural capacityStructural safetyShear capacityServiceability

a b s t r a c t

Prestressed concrete box girders have been regarded as the most favorable medium span (30–50 m)concrete girder type in many countries, but they have a crucial limitation compared to steel girders inthat a single span length cannot be extended over 50 m due to its relatively heavy self-weight. As aresult of this restriction, the majority of medium span girder bridges constructed in Korea have beensteel box girder types. In the 20th century, numerous attempts have beenmade to improve the structuralefficiencies of prestressed concrete box girders using concrete–steel hybrid subcomponents to reducethe weight of the superstructure. However, the behaviors of hybrid bridge girders with various steelweb types and connection joints have caused safety and serviceability problems. Therefore, in order tofully understand the behaviors of steel web girders and the effects of steel web connection joints, a staticloading test was conducted on five prestressed concrete hybrid girders with steel web members. Resultcomparisons for structural safety and serviceability were also performed. The five girder specimens weretwo hybrid girders with corrugated steel webs and three hybrid girders with steel truss webs. The studyresults showed that the serviceability issues such as cracking load and deflection and the safety issuessuch as stiffness and ultimate load capacity can be improved by modifying the steel web members andconnection joints of concrete slabs and tendons. The study results are discussed in detail.

© 2010 Elsevier Ltd. All rights reserved.

1. Introduction

In the 20th century, prestressed box girder bridges with steeltruss webs such as the Kinogawa Bridge, the Sarutagawa Bridge,the Tomoegawa Bridge, and the Shitsumi Ohashi Bridge, as wellas ones with corrugated steel webs, have been constructed inJapan [1–3]. There have been multiple attempts in Korea toreplace the concrete webs of prestressed concrete box girderswith steel web members. Fig. 1 shows Ilsun Bridge, the firstprestressed concrete hybrid girder bridge with corrugated steelwebs constructed in Korea, which is the world’s longest (totallength of 801 m) and widest (21.2 m with a tri-cellular section)bridge using steel webs [4,5]. The bridge has a total of 14 spans(50m, ten at 60m, 50m, two at 50.5m)with 12 spans having beenerected using an incremental launchingmethod and the remainingtwo spans using a full staging method. Fig. 2 shows the ShinchunBridge currently being constructed in Korea, a prestressed concretehybrid girder bridge with steel truss web members. This bridge

∗ Corresponding author. Tel.: +82 2 2123 5802, +82 2 2123 2798; fax: +82 2 3641001, +82 2 364 5300.

E-mail addresses: jkh@hdec.co.kr (K.-H. Jung), jwonyi@hdec.co.kr (J.-W. Yi),jjhkim@yonsei.ac.kr (J.-H.J. Kim).

0141-0296/$ – see front matter© 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.engstruct.2010.08.029

design was chosen due to its low superstructure height (3.2 m),long single span length (80 m), and attractive appearance that canbe merged into its surrounding environment. It has a total of fivespans (two at 60 m, three at 80 m) that will be erected usingtemporary bents and cranes in order to fully utilize its light weightadvantage.

These two bridges have a similar structural concept in that theyreplaced traditional concrete web with steel web, transformingthe bridge into a hybrid type. Due to the drastic transformation ofconcrete girder concepts in these bridges, research on these bridgetypes was necessary and has been performed [6–12]. In order toimplement this new bridge type in real world construction, HICT(Hyundai Institute of Construction Technology) has undertaken amega research project to verify and improve PSC hybrid girdersusing steel webs. This paper describes the research results on thesafety and serviceability issues concerning the behaviors of hybridgirders according to the steel web type and connection method.

2. Static loading test setup

2.1. Test specimens

In this study, in order to compare the flexural behavior ofeach hybrid girder according to steel web type and connection

K.-H. Jung et al. / Engineering Structures 32 (2010) 3866–3878 3867

Notations

The following symbols are used in this paper:

Ap Area of the prestressing tendonsAs Area of the tensile reinforcementsA′s Area of the compressed reinforcements

Atr Area of the truss membersa Depth of the equivalent compressive regionb Width of the compressive regiond Distance from the extreme compression fiber to the

centroid of the tensile reinforcementd′ Distance from the extreme compression fiber to the

centroid of the compressive reinforcementdo Diameter of the hole in the perfobonddp Distance from the extreme compression fiber to the

centroid of the tendonsds Diameter of the studdtr Diameter of the truss memberd1 Distance from the neutral axis to the centroid of the

upper slabd2 Distance from the neutral axis to the center of the

lower slabfca Allowable axial compressive stress of the truss

memberfck Compressive strength of the concretefps Stress of the prestressing tendonfpu Ultimate tensile strength of the prestressing ten-

donsfy Yield strength of the reinforcementsftr_max Maximum local stress at the connection jointHs Height of the studItr Second moment of inertia of the truss memberMtr Maximummoment of the truss membersMu Ultimate momentM1 Bending moment at the upper slabM2 Bending moment at the lower slabNua Allowable ultimate strength for the tensile forceN1 Axial force at the upper slabN2 Axial force at the lower slabn Number of studs or perfobondsPtr Maximum axial force of the truss membersQa Allowable shear strength of a stud or perfobond per

one unitQu Ultimate shear strength per hole in the perfobondto Thickness of the perfobondVH Horizontal shear forceVua Allowable ultimate strength of the shear forceβ1 Coefficient determined by the concrete strengthρ Ratio of the tensile and compressive reinforcementsρ ′ Ratio of the compressive reinforcementsρp Ratio of the prestressing tendonsγp Coefficient determinedbyprestressing tendon typesτd Design shear stress of the corrugated steel webτcr,G Critical global design buckling stressτcr,I Critical interactive design buckling stressτcr,L Critical local design buckling stressφ = 0.85 Strength reduction factor for the flexural memberφo Diameter of the cross reinforcement in perfobondφMn Nominal moment of the cross sectionφNn Design strength for tensile forceφVn Design strength for shear force

Fig. 1. Ilsun Bridge.

Fig. 2. Shinchun Bridge.

method, static loading tests were performed on five prestressedconcrete hybrid girders with various steel webs. These five hybridgirders were composed of two hybrid girders with corrugatedsteel webs (FHC, PHC) and three hybrid girders with steel trusswebs (FHT, GHT, EHT), as shown in Table 1. As shown in Fig. 3(a),FHC was a conventional connection type with a connectionsystem that had shear studs welded onto the flange plate in thelongitudinal direction, while PHC had perfobond shear connectorsinstead of shear studs, as shown in Fig. 3(b). FHT had the sameconnection system as FHC, but its web consisted of steel pipesinstead of corrugated steel webs, as shown in Fig. 3(c). GHThad a connection unit with a gusset plate and shear studs,which were welded onto the base plate as shown in Fig. 3(d).Experimental study results on flexural and shear capacities ofGHT have been published recently [13,14]. EHT had embeddedconnection systems composed of connection plates and a steelrod without a longitudinal flange plate, as shown in Fig. 3(e). Asexpected in EHT, the axial forces of the truss members are directlytransferred to the concrete slabs without any shear connectersof studs or perfobonds. This hinge connection greatly facilitatedconstruction implementations in which no welding procedureswere required and truss angle adjustment was possible. Throughthe experiments, the EHT construction process and structuralperformance were verified. The detailed comparison of loadtransfer mechanisms and design concepts for the three connectionsystems are explained in Sections 3.4 and 4.1.2.

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Table 1Test specimens.

Web structure Index Connection type Specimens

Corrugated steel FHC Flange plate + studs

PHC Flange plate + perfobonds

Truss membersFHT Flange plate + studs

GHT Gusset plate + studs

EHT Hinge connection + steel rod

HC: Hybrid girder with corrugated steel web.HT: Hybrid girder with truss web members.F: Flange plate with studs. P: Flange plate with perfobonds.G: Gusset plate with studs. E: Embedded type connection.

Table 2Dimensions of the specimens (Units: mm).

GirderLength 6350Height 1300Width 500

Concrete slab Height 250

Corrugated steel web

Horizontal panel 200Inclined panel 186Inclined angle 53.7°Thickness 6

Steel truss web Diameter 165Thickness 9

2.2. Dimensions and properties

All specimens were constructed with the same dimensions.Table 2 presents the main dimensions of the specimens, and Fig. 4presents the cross sections of the hybrid girders with corrugatedsteel and steel truss webs. The lengths, heights, and widths ofthe hybrid girders were 6350 mm, 1300 mm, and 500 mm,respectively. Also, the heights of the upper slab and lower slabwereboth 250 mm. In the case of the corrugated steel web, the lengthsof the horizontal and inclined panels were 200 mm and 186 mm,respectively, with an inclined angle of 53.7°, and the thickness ofthe corrugated steel web was 6 mm. In the case of the steel trussweb, the diameter and thickness of the circular truss section were165 mm and 9 mm, respectively.

Table 3 presents the material properties of all specimens. Theconcrete was mixed using OPC (Ordinary Portland Cement) andcoarse aggregate with a maximum size of 19 mm and an expected

Table 3Material properties of the specimens.

Material Type Strength (MPa)

Concrete OPC Design compressive strength: 40Steel truss pipe SM490 Allowable tensile strength: 190Corrugated steel SS400 Allowable tensile strength: 140Reinforcement SD40 Ultimate tensile strength: 400

Tendon SWPC7B, 15.2 mm Ultimate tensile strength: 1900Yield tensile strength: 1600

28 day compressive strength of 40 MPa. All of the steel used tomanufacture the corrugated steelweb and connection systemswasSS400, which has an allowable tensile strength of 140 MPa and amanufacturer specified minimum yield strength of 240 MPa. Thesteel truss webs were manufactured using SM490, which has anallowable tensile strength of 190MPa and amanufacturer specifiedminimumyield strength of 320MPa. All of the reinforcements usedwere SD40, with amanufacturer specifiedminimumyield strengthof 400 MPa. Each specimen has two SWPC7B-type prestressingtendons in the lower slab. The ultimate strength and diameter ofthe SWPC7B tendon were 1900 MPa and 15.2 mm, respectively.

2.3. Loading system

Fig. 5 shows the loading system for the three-point bendingtest used in this study. A 2500 kN hydraulic actuator was usedfor loading, where the design ultimate load for all specimenscalculated from the flexural designwas approximately 917 kN. Thelength of the actuator and the heights of all of the specimens wereabout 2.7 m and 1.3 m, respectively, where sufficient clearance

K.-H. Jung et al. / Engineering Structures 32 (2010) 3866–3878 3869

(a) Flange plate type hybrid girder with corrugated steel web (FHC). (b) Flange plate and perfobond type hybrid girder with corrugated steel web(PHC).

(c) Flange plate type hybrid girder with truss web members (FHT). (d) Gusset type hybrid girder with truss web members (GHT).

(e) Embedded type hybrid girder with truss web members (EHT).

Fig. 3. Test specimens (Units: mm).

(a) Corrugated steel web. (b) Steel truss web.

Fig. 4. Cross sections of the hybrid girders (Units: mm).

under the specimen was provided to install data acquisitiondevices such as LVDT and support bases, making the total height ofthe loading frame greater than 4.5m. In this test setup, long loading

frames with lengths greater than 4.5 m were needed. Therefore,it was essential that these frames should have sufficiently highstiffness to ensure safety during testing. In this test, two pulling

3870 K.-H. Jung et al. / Engineering Structures 32 (2010) 3866–3878

Fig. 5. Loading test system and measurement points (Units: mm).

actuators and four guide columns were used in the loading systemto ensure sufficient loading capacity and frame safety. As shown inFig. 5, two actuators were fixed to a loading frame and connectedby a cross beam. When the pulling forces of the actuators wereinitiated, the load was applied to the specimen by way of thiscross beam. The maximum pulling capacity of each actuator wasapproximately 1600 kN, equivalent to a total loading capacity ofapproximately 3200 kN.

3. Design of the hybrid girder

3.1. Basic design concept

The hybrid girder was composed of upper and lower concreteslabs and steel web members. A basic design concept of hybridgirders allowsupper and lower concrete slabs to resist only flexuralstress and the steel web to resist only shear stress. Also, anotherimportant design conceptwas that the connection systembetweenthe concrete slabs and steel members did not yield until theconcrete slabs and steel members failed. Therefore, the designprocedure of the hybrid girder could be divided into three partsconsisting of flexural design, shear design, and connection design.

3.2. Flexural design

Since the flexural design of the hybrid girder is related to thedesigns of the upper and lower concrete slabs, the dimensionsof the concrete slabs and the reinforcement sizes including theprestressing tendons should be determined based on the requiredflexural capacity of the girder. The ultimate moment Mu can beobtained through frame analysis; however, in the case of the steeltruss web, an additional calculation using the following equationis needed after frame analysis in order to calculate the ultimatemoment, as shown in Fig. 6:

Mu = M1 + M2 + N1d1 + N2d2 (1)

where M1 and M2 are the bending moments at the upper andlower slabs, N1 and N2 are the axial forces at the upper and lower

centroid of upper slab

centroid of lower slab

centroid of beam

M1

d1

d2

Mu

N1

M2

N2

Fig. 6. Ultimate moment of a hybrid girder with truss web members.

slabs, and d1 and d2 are the distances from the neutral axis tothe centroids of the axial forces at the upper and lower slabs,respectively.

The nominal moment of the cross section of a hybrid girderφMn can be calculated using the following equation, whichis commonly used to calculate the nominal moment for aconventional prestressed concrete section.

φMn = φApfps

dp −

a2

+ Asfy

d −

a2

+ A′

sfy(d − d′)

(2)

where φ = 0.85 is the strength reduction factor; As and A′s are the

areas of the tensile and compressive reinforcements, respectively;Ap is the area of the tendons; d and d′ are the distances fromthe extreme compression fiber to the centroids of the tensile andcompressive reinforcements, respectively; dp is the distance fromthe extreme compression fiber to the centroid of the tendons; andfy is the yield strength of the reinforcements. The stress of theprestressing tendon fps is given by:

fps = fpu

[1 −

γp

β1

[ρp

fpufck

+

ddp

[ρ

fyfck

− ρ ′

fyfck

]]](3)

where ρ and ρ ′ are the ratios of the tensile and compressivereinforcements, respectively; ρp is the ratio of the tendons; fpu is

K.-H. Jung et al. / Engineering Structures 32 (2010) 3866–3878 3871

Table 4γp and β1 .

Index Condition Value

γp

fpy/fpu ≥ 0.90 0.28fpy/fpu ≥ 0.85 0.40fpy/fpu ≥ 0.80 0.55

β1fck < 28 MPa 0.85fck ≥ 28 MPa 0.85−0.007(fck−28)

the ultimate tensile strength of the tendons; fck is the compressivestrength of the concrete; γp and β1 are the values determinedusing Table 4, which is based on the concrete structure designspecification in Korea [15].

The depth of equivalent compressive region a is given by:

a =Apfps + Asfy − A′

sfy0.85fckb

(4)

where b is the width of the compressive region.Finally, it is required that the nominal moment of the cross

section of the hybrid girder φMn must always be larger thanthe ultimate moment Mu. According to this procedure, thenominal flexural strength of each specimen was calculated to beapproximately 1375 kN m, and the expected ultimate load in thethree-point bending test was approximately 917 kN.

3.3. Shear design

Since the shear design of the hybrid girder is related to thesteel members of the web, the dimensions and thicknesses of thesteel tube members should be designed to have sufficient strengthsuch that the shear mode of failure is eliminated. In the case ofthe corrugated steel web, the local buckling mode, global bucklingmode, and interactive buckling mode should be determined. Thedesign shear stress of the corrugated steel web τd was obtainedby taking the minimum value of the critical interactive designbuckling stress τcr,I , critical global design buckling stress τcr,G, andcritical local design buckling stress τcr,L,

τd = min(τcr,I , τcr,G, τcr,L). (5)

Table 5 presents the design shear stress of the corrugated steelwebused in this study. Each value is calculated using the equationsproposed by JSCE [16], Abbas et al. [17], EI-Metwally [18], and Yiet al. [19]. As shown in Table 5, it is clear that the design shearstress of the corrugated steel web τd is always greater than themaximum shear stress in a real corrugated web section τ , which iscalculated using a design load of 917 kN, showing that thememberis sufficiently safe with regard to the shear stress.

In the case of the steel truss web, the maximum local stressesconcentrated at the ends of the truss members should be smallerthan the allowable compressive stress of the truss members. Inthe truss member, the maximum axial forces and local momentsalways occur at the joints, since the real joints of truss membersand concrete slabs are not hinged. As such, the maximum localstress ftr_max can be given by the following equation:

ftr_max =PtrAtr

+Mtr

Itr

dtr2

(6)

where Ptr and Mtr are the maximum axial force and the momentof the truss members, respectively; and dtr, Atr and Itr are thediameter, area, and secondmoment of inertia of the trussmember,respectively. Finally, it should be required that the maximum localstress ftr_max is always smaller than the allowable axial compressivestress fca proposed by the design specification. The same framemodel containing a fixed connection condition has been usedin the design procedure of hybrid truss beams regardless of the

connection type. Since it is difficult to numerically define thestrength of a joint, the basic design assumption of the strengthof the connection system must be greater than the strengthsof both the concrete slabs and the truss members. From theanalysis results of the frame model with a fixed connectioncondition, the maximum axial force and bending moment of thetruss members were determined. When the expected ultimateload of 917 kN was applied to the specimen, the maximumaxial force and bending moment of the truss members were416.7 kN and 9.17 kN m, respectively. Therefore, the maximumlocal stress of the truss members can be calculated as 150.7 MPa,which is smaller than the allowable axial compressive stress of190 MPa.

3.4. Connection design

The connection designs of the concrete slabs and steel webscan bemodified according to the connection system. However, thefundamental design requirements of the connection system arethat it must not yield before the failures of the concrete slabs andtruss members and must be able to continuously resist appliedhorizontal shear forces at the connection joint. In particular, in thecase of the steel truss, the tensile forces at the connection partsmust be maintained since the joints of the truss members alwayshave to resist tensile and compressive forces as well as horizontalshear forces.

FHC and FHT have a connection system in which the studs arewelded onto the flange plate, but PHC has a connection system inwhich perfobonds are welded onto the flange plate. In these twoconnection systems, since the studs or perfobonds on the flangeplate are the only member parts resisting the horizontal shearforces, the number of studs or perfobonds n can be calculated usingthe following equation:

n ≥VH

Qa(7)

where VH is the horizontal shear force and Qa is the allowableshear strength of the stud or perfobond per one unit. The allowableshear strength of the stud can be easily determined by applyingEq. (8), required by the concrete structure design specification inKorea [15].

Qa = 9.5d2sfck, if Hs/ds ≥ 5.5 (8a)

Qa = 1.74dsHs

fck, if Hs/ds < 5.5 (8b)

where ds and Hs are the diameter and height of the stud,respectively. As shown in Fig. 7(a), the horizontal shear designforce per one stud is about 29 kN when ds is 22 mm and Hs is150 mm.

The ultimate shear strength per hole of perfobond has beenproposed by several researchers. In this study, Eqs. (9a) and (9b),which were used in the design of perfobond shear strength for theTanigawa Bridge [20], are selected. Depending on whether or notthe perfobond has cross reinforcements, Eqs. (9a) or (9b) can beapplied, respectively.

Qu = 3.38d2o(to/do)1/2fck − 39.0 × 103 (9a)

Qu = 1.45(d2o − φ2

o )fck + φ2o fy

− 26.1 × 103 (9b)

where Qu is the ultimate shear strength per hole of the perfobond,to is the thickness of the perfobond, do and φo are the diameters ofthe holes in the perfobond and cross reinforcements, respectively,and fy and fck are the yield strength of the reinforcements and thecompressive strength of the concrete, respectively. As shown inFig. 7(b), the horizontal shear force capacity per one perfobond is

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Table 5Design bucking stress of the corrugated steel web.

Index Local buckling stress (τcr,L) (MPa) Global buckling stress (τcr,G) (MPa) Interactive bucklingstress (τcr,G)

Design bucklingstress (τd)

Max. shearstress (τ )

Safety(τd/τ )

Elastic Inelastic Elastic Inelastic

JSCE 95.5 95.3 8436 104.6 – 80.0 10.4 7.69Abbas 954.9 80.0 7795 80.0 56.6 56.6 10.4 5.44EI-Metwally 954.8 – 8426 – 79.8 79.7 10.4 7.66Yi et al. 91.2 60.7 763.3 81.5 60.7 10.4 5.83

Fig. 7. Connection details (Units: mm).

about 621 kN when do and to are 50 mm and 8 mm, respectively,and φo is 13 mm.

GHT has a connection system in which studs are welded to adiscontinuous base plate which is connected to a vertical gussetplate. This setup causes the local moment at the joint to occurat the base plate, as shown in Fig. 8(b). Based on this design, thefailure mode of GHT is a tensile failure mode similar to that of aconcrete spalling failure in the anchor system due to the tensileforce. Similarly, the local moment can occur at the joint in FHT;however, since FHT has a continuous flange plate resisting thislocal moment, as shown in Fig. 8(a), the joint can sufficiently resistthe local moment. However, if the local moment does not occur atthe joint in EHT, the horizontal shear force is directly transferred tothe truss members in the concrete slab, as shown in Fig. 8(c). In thedesign ofGHT, the following force requirement equation originatedfrom the anchor design concept of the concrete structure designspecification in Korea [15], in which a tensile failure as well as ashear failure is checked.

Nua

φNn

2

+

Vua

φVn

2

≤ 1.0 (10)

where Nua and Vua are the allowable ultimate strengths for thetension and shear forces, and φNn and φVn are the design strengthsfor the tension and shear forces, respectively.

If the local moment does occur at the joint of EHT, a hingeconnection system composed of connection plates and a steel rodas shown in Fig. 7(c), then the connection plates and steel rodsare designed to prevent yielding until the ultimate and serviceablestates are reached as required by this study.

Table 6Yielding and ultimate loads (Units: kN).

Index FHC PHC FHT GHT EHT

Yielding load 932.0 863.0 924.5 595.9 794.2Ultimate load 1206.0 1054.0 1089.2 883.7 1060.9

4. Structural safety

4.1. Flexural capacity

Many experimental studies on the flexural behavior of steel–concrete composite beamshave beenperformedby Larbi et al. [21],Bouazaoui et al. [22], Zhang and Fu [23], and Kim and Jeong [24].In this study, to verify the flexural capacity of the hybrid girders,the tested specimens’ yielding loads and the ultimate load werecompared as shown in Table 6. The ultimate loads of all specimensexcept for GHT were larger than the design ultimate load of917 kN. The structural safety requirement for each specimen canbe defined as the actual ultimate load thatmust be greater than thedesign ultimate load. From the structural safety point of view, GHThad insufficient flexural capacity, whereas the other specimenshad sufficient flexural capacities. In particular, in the longitudinalflange plates of FHC and FHT, their yielding loadswere even greaterthan the design ultimate load of 917 kN. This enhanced safetycapacity shows that the longitudinal flange plate plays a vital rolein significantly improving the member flexural capacity.

4.1.1. Behaviors of FHC and PHCIn order to look more deeply into the flexural behavior of each

specimen, the load–displacement relationships obtained from theexperiment were analyzed. Fig. 9 presents the load–displacement

K.-H. Jung et al. / Engineering Structures 32 (2010) 3866–3878 3873

(a) FHT. (b) GHT.

(c) EHT.

Fig. 8. Load transfer mechanisms at the connection joint (FHT, GHT, EHT).

Fig. 9. Load–displacement relationships in FHC and PHC.

relationships of two hybrid girderswith corrugated steelwebs. Theinitial stiffnesses of the two hybrid girders with corrugated steelwebs (FHC, PHC) were almost the same in the linear elastic region(from 0 to about 800 kN). The figure also shows that the ultimatestrength of FHC was higher than that of PHC, and the behaviors ofthe two specimens after the yielding state were different, based onthe connection system.

The actual level of a composite action in a steel–concretecomposite structure is generally categorized into either a perfectcomposite action, a partial composite action, or a non-compositeaction. A perfect composite action is one in which an interfacebetween the steel and concrete surfaces is fully fixed and no slip isto occur at the interface until its ultimate state is reached. A non-composite action has no friction between the steel and concrete,resulting in a large slip at the interface. A partial composite action

is one in which a minute slip occurs at the interface between thesteel and concrete surfaces, where an ultimate strength of a partialcomposite structure can be slightly less than that of a perfectcomposite structure [25–27]. Consequently, in order to guaranteea perfect composite action in the specimen, the shear connectorsshould be able to resist the total horizontal shear force of 7980 kN,which is equivalent to installing over 275 studs (29 kN/ea) or 13perfobonds (62l kN/ea).

FHC has 276 studs, each with a horizontal shear design forceof about 29 kN. This means that the total resisting shear forceis about 8004 kN. However, PHC has 36 perfobonds, each witha horizontal shear force capacity of about 621 kN, so the totalresisting shear force capacity for PHC is about 22,370 kN. Thisvalue for PHC is approximately 2.8 times greater than that ofFHC. The results show that FHC has a sufficient number of shearstuds, thereby illustrating a perfect composite behavior and anapproximately 14% greater ultimate load compared to that of PHC.Fig. 10 shows that the concrete between the perfobonds in the PHCspalled after the yielding statewas reached. This failure behavior isdue to the fact that PHC had discontinuously arranged perfobonds,as shown in Fig. 3(b). If the studs are continuously arranged inthe longitudinal direction, as was the case in FHC, the failurebehavior would have been prevented. From these test results, itcan be concluded that the arrangement and the number of shearconnectors are extremely important parameters that affect theflexural behaviors of hybrid girders. Fig. 11 presents the measuredreinforcement strains in the upper and lower slabs. The resultsclearly indicate that the reinforcement strain is similar to theload–displacement curve shown in Fig. 9, but the strain incrementof the tensile reinforcement in the lower slab is greater than thatof the compressive reinforcements in the upper slab.

3874 K.-H. Jung et al. / Engineering Structures 32 (2010) 3866–3878

Fig. 10. Concrete spalling at the yielding state in PHC.

Fig. 11. Measured strains of the compressive and tensile rebars in FHC and PHC.

Fig. 12. Load–displacement relationships in EHT, FHT, and GHT.

4.1.2. Behaviors of FHT, GHT and EHTFig. 12 presents the load–displacement relationships of three

hybrid girders with steel truss webs. The initial stiffnesses ofthe three hybrid girders with a steel truss web (FHT, GHT, EHT)were similar in the linear elastic region, but their behaviors weredifferent in the nonlinear inelastic region. The figure also showsthat the yield and ultimate strengths of FHT are higher than thoseof GHT and EHT, and that the yield and ultimate strengths of EHTare higher than those of GHT. Also, the behaviors after yielding aredifferent according to the connection system. From these results, itis safe to conclude that GHT has the lowest flexural capacity amongthe three types of hybrid girders with a steel truss web. FHT has276 studs in continuous regular spacing, which gives a horizontal

Fig. 13. Measured strains of the compressive and tensile rebars in FHT, GHT, andEHT.

shear design force per one stud of about 29 kN and a total resistingshear force of about 8004 kN. However, GHT has 80 studs in adiscontinuous regular spacing, which gives a total resisting shearforce of about 2320 kN. Therefore, it can be expected that FHT hasa sufficient number of shear studs to impart a perfect compositebehavior, while GHT does not have sufficient studs to result in apartial composite behavior, where its yield and ultimate strengthsare less than the design strength.

As shown in Fig. 8, FHT and GHT have a longitudinal flange plateand a base plate attached with studs, respectively. So axial forcesof the truss members are indirectly transferred to the concreteslabs byway of longitudinal steel plates and stud shear connectors.Therefore, the centroid of the cross section does not coincidewith the middle point of the concrete slab height, causing aneccentricity to occur between the centroid of the slab and the crosspoint of the two truss axes. This eccentricity inevitably generatesa local bending moment at the connection joint. However, EHThas an embedded hinge connection system where truss membersare connected to each other in the concrete slab without anylongitudinal steel plates and stud shear connectors. Therefore, theaxial force of the truss members is directly transferred to theconcrete slab. Since the centroid of the cross section coincideswith the middle point of the concrete slab height, no eccentricitybetween the centroid of a concrete slab and the cross point ofthe two truss axes occurs and no local bending moment occurs atthe connection joint. Consequently, it can be concluded that theEHT tested in this study can be a useful connection system forhybrid truss girders, providing sufficient structural safety withoutrequiring any shear connectors or welding during construction.From these results, it can be clearly concluded that hybrid girderscould have different initial stiffnesses according to the connectionsystem used, despite having the same web members.

Fig. 13 presents the measured reinforcement strains inthe upper and lower slabs. The results clearly indicate thatthe behavior of the reinforcement strain is similar to theload–displacement curve shown in Fig. 12, but the strain incrementof the tensile reinforcement in the lower slab is greater than that ofthe compressive reinforcements in the upper slab. Fig. 14 presentsthe strains in the cross section of each hybrid girder (FHT, GHT,EHT) according to a load increment of 200 kN. Fig. 15 presentsthe strains in the cross section of the three hybrid girders (FHT,GHT, EHT) at the critical loading points of 200, 400, and 800 kN. Asshown in Fig. 13, the reinforcement of FHT in the upper slab didnot reach the yield state at the load of 1000 kN, but those of GHTand EHT in the upper slab reached the yield state at loads of 800 kNand 1000 kN, respectively. The reinforcements of FHT and EHT inthe lower slab reached yield states at loads of 400 kN and 600 kN,respectively. However, the reinforcements in GHT in the upperslab reached yield state at a load greater than 400 kN. The results

K.-H. Jung et al. / Engineering Structures 32 (2010) 3866–3878 3875

(a) FHT. (b) GHT.

(c) EHT.

Fig. 14. Strain variation in the cross section.

indicate that FHT and EHT show more stable strain incrementsin the cross section compared to that of GHT. It can be clearlyconcluded that a longitudinal member such as the flange plate inFHT or a direct connection system such as the hinge connection inEHT can be used to obtain sufficient flexural capacity and structuralsafety in hybrid girders with steel truss members.

4.1.3. Comparison of FHC and FHTFig. 16 presents the load–displacement relationships of hybrid

girders with corrugated steel web and steel truss web members.Both FHC and FHT have the same connection system composed ofa flange plate and shear studs in the longitudinal direction, but FHChas corrugated steel webs and FHT has steel truss webs. As shownin Fig. 16, the initial stiffness of FHC is higher than that of FHT inthe linear elastic region, and the yield and ultimate strengths ofFHC are also higher than those of FHT in the nonlinear region.

Fig. 16 also presents the 3-D nonlinear analysis results of thetwo types of hybrid girders using both a perfectly plasticmodel anda tri-linear plastic model for prestressing tendons. This nonlinearanalysis was performed using the commercial FEM programDIANA. The concrete was modeled by a solid element (HX24L),and the steel members such as corrugated steel tubes or trussmembers were modeled by a shell element (Q8MEM), as shownin Fig. 17. The prestressing tendon was modeled as an embeddedbar element, and the Drucker–Prager and the von-Mises plasticitymodels were applied to the concrete and steel, respectively. Thesmeared crack model was also applied to the concrete solidelements. From the analysis, it was found that the stiffness of theanalysis results was higher than that of the experimental results.The difference between the analytical and experimental resultswas due to the partial composite behaviors of the connectionsystems in the real specimens, while the analyses assumed that

they had perfect composite behaviors. Fig. 18 also presents themeasured reinforcement strains in the upper and lower slabs. Theresults clearly indicate that the behavior of reinforcement strain issimilar to the load–displacement curve shown in Fig. 16, but thestrain increment of the tensile reinforcement in the lower slab islarger than that of the compressive reinforcements in the upperslab. From the test results, it can be inferred that FHC has a higherflexural capacity than FHT, despite having the same connectionsystem. This capacity difference is due to FHC having a typicalregular cross section, while FHT has an opened irregular crosssection in the longitudinal direction. This inference is discussedbelow in terms of the shear capacity.

4.2. Shear capacity

As shown in Fig. 16, FHC had a higher flexural capacity than thatof FHT despite having the same connection system. The differencein the capacitywas due to FHChaving a typical regular cross sectionwhile FHT has an open irregular cross section in the longitudinaldirection. This result trendmay also be related to the shear capacityof each specimen. However, it is difficult to directly compare theshear capacities of hybrid girders with corrugated steel webs (FHC,PHC) to those of steel truss webs (FHT, GHT, EHT), since theyhave different load transmission paths. The applied load transfersdirectly to the support through the corrugated steel web in HCgirders, but the applied load transfers indirectly to the support byway of truss members in HT girders.

Fig. 19 presents the principal strains of the corrugated steelweb (FHC, PHC). The maximum principal strain was less thanapproximately 600 micro strains (equivalent to 120 MPa) andremained in the linear elastic state as shown in Fig. 18 because thedesign buckling stress of the corrugated steel web in this studywas

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(a) 200 kN. (b) 400 kN.

(c) 800 kN.

Fig. 15. Comparison of strain variations in the cross sections of FHT, GHT, and EHT.

Fig. 16. Load–displacement relationships in FHC and FHT.

highly conservative and had a higher safety factor (more than 5.44times), as shown in Table 5. Fig. 20 presents the maximum strainsof the steel truss members (FHT, GHT, EHT). Since the maximumlocal stress of truss members used in the design was 150.7 MPaand the allowable axial compressive stress was 190 MPa, thesafety factor for truss buckling was 1.26. Also, the maximum strainof a truss was less than about 1500 micro strains without localbuckling. In this study, each hybrid girder was designed such thatit had sufficient shear capacity until flexural failure occurred. Fromthe test results, the failure modes of all specimens showed typicalflexural failure modes regardless of web type. Also, the maximumstrain of the steel webs remained in the linear elastic region untilall specimens reached their ultimate states. Consequently, it canbe concluded that the structural safeties of the shear capacities fortwo types of hybrid girders are guaranteed, and that the design ofthe corrugated steel web is more conservative than is that of thesteel truss web.

Table 7Cracking load of EHT, FHT, and GHT (Units: kN).

Location FHT GHT EHT

Upper slab Top 1026.9 789.1 840.4Bottom 1086.2 713.6 591.7

Lower slab Top 634.2 335.3 333.9Bottom 145.2 234.4 291.7

5. Serviceability

5.1. Cracking loads

In order to determine the cracking loads and crack patternsfor all specimens, the cracks formed on all concrete surfaces werevisually checked at every 10 kN load increment during the loadingtest. Table 7 presents the cracking loads of the hybrid girders withsteel truss webs (EHT, GHT, FHT) according to member location,and Table 8 shows the crack patterns. Below 200 kN, there wereno cracks in EHT or GHT. The first cracks formed in the bottomof the lower slab at loads of 291.7 kN and 234.4 kN in EHT andGHT, respectively. In the case of FHT, the first cracks occurred inthe bottom of the lower slab at 145.2 kN, but the cracks at theother locations such as the upper slab and the top of the lower slaboccurred at an applied load greater than those of EHT and FHT.

5.2. Prestressing efficiency

Each specimen had the same prestressing force of approxi-mately 432 kN (60% of the ultimate tensile strength of the pre-stressing tendon) applied using hydraulic jacks. In order to applythe same prestressing force to each specimen, the prestressing

K.-H. Jung et al. / Engineering Structures 32 (2010) 3866–3878 3877

Table 8Cracking mechanism of EHT, FHT, and GHT.

Load (kN) FHT GHT EHT

200

400

800

(a) HC girder. (b) HT girder.

Fig. 17. 3D modeling of FHC and FHT.

Fig. 18. Measured strains of the compressive and tensile rebars in FHC and FHT.

force was directly measured using load cells and the elongationlengths of the tendons were checked. Also, the specimen longitu-dinal strain variations weremeasured throughout the prestressingprocedure to determine the prestressing efficiency for each spec-imen. Fig. 21 presents the strain variations in the reinforcementsin the longitudinal directionwhen the same prestressing force wasapplied to the lower slab of thehybrid girderswith steel trusswebs.The longitudinal strains of EHT and GHT were higher than that ofFHT, which indicated that EHT and GHT had higher prestressingefficiencies than FHT, since the flange plates of both EHT and GHTdid not resist axial prestressing forces. Finally, from the fact thatthe first cracking of FHT occurred at a lower applied load, it can bededuced that FHT had a lower prestressing efficiency than those ofEHT and GHT.

6. Conclusions

This paper has focused on verifying the structural safety ofhybrid girders with steel webs and evaluating the behaviors ofhybrid girders according to web type and connection system.

(1) The structural safeties of the flexural capacities of two typesof hybrid girders are governed by the connection systems aswell as the web structures. From the test results, the stiffness,ultimate strength, and behavior of each type of hybrid girder

Fig. 19. Measured principal strains of the corrugated steel webs in FHC and PHC.

Fig. 20. Measured strains of both the maximum compressive and tensile trussmembers in FHT, GHT, and EHT.

can be very different in the linear elastic region as well as inthe nonlinear region depending on the connection system.

(2) The structural safeties of the shear capacities for the two typesof hybrid girders were guaranteed, since the failure modesof all specimens presented the typical flexural failure moderegardless of web type. The maximum strain of the steelwebs remained in the linear elastic region until all specimensreached the ultimate state.

3878 K.-H. Jung et al. / Engineering Structures 32 (2010) 3866–3878

Fig. 21. Prestressing efficiencies of FHT, GHT, and EHT.

(3) Both FHC andPHChave a sufficient number of shear connectorsto allow for perfect composite behavior; however, the ultimatestrength of FHC is approximately 14% higher than that of PHC.The study results can be attributed to FHC having continuouslyarranged studs in the longitudinal direction as opposed toPHC, which has discontinuously arranged perfobonds. Thisindicates that the arrangement pattern and the number ofshear connectors are important parameters that affect theflexural behaviors of hybrid girders.

(4) EHT is an embedded-type connection system with no localmoment at the connection joint since there is no eccentricitybetween the centroid of the concrete slab and the cross pointsof the two truss axes. FHT and GHT have shear studs and a steelplate as force transfer elements and always produce a localmoment at the connection joint since eccentricity between thecentroid of a composite slab and the cross points of two trussaxes always occurs.

(5) The EHT used in this study can be a useful connection systemfor hybrid truss girders since it provides sufficient structuralsafety without requiring any shear connectors or weldingprocedures during construction.

(6) The stiffnesses of hybrid girders with corrugated steel webs(FHC) are higher than those of hybrid girders with steel trusswebs (FHT) due to FHC having a typical regular cross section,while FHT has an open irregular cross section. This result isrelated to the shear capacities of hybrid girders.

(7) The longitudinal flange plates of hybrid girderswith steel webscan play a vital role in resisting the prestressing forces, therebyenhancing the flexural capacity but decreasing the crackingload in the lower slab.

Acknowledgements

This work was supported by a grant (05 construction core C14)from the Construction Core Technology Program of the Ministryof Construction & Transportation of the Korean Government. Theauthors wish to express their gratitude for the financial support.The opinions, findings, and conclusions of the paper are solely fromthe authors anddonot necessarily reflect the views of the sponsors.

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