developing composite action in existing non-composite steel girder ...

connectors. All specimens were an 11.6-m long simply supported beam, with a concentratedload applied at midspan. For all specimens, the steel beam was a W30x99 section of ASTMA922 steel. The reinforced concrete slab was 2130-mm wide and 178-mm thick, with a specifiedconcrete compressive strength of 20.7 MPa. Details are shown in Fig. 3.Figure 4 shows the computed load-carrying capacity of the test specimens with respect to theshear connection ratio from simple plastic analysis. These beam strength values werecalculated using the minimum specified concrete strength ( f 'c= 20.7 MPa) and the minimumspecified yield stress of the steel ( Fy= 345 MPa). The contribution of the longitudinalreinforcing bars was neglected in the strength calculation. The minimum specified tensilestrength of ASTM 193 B7 threaded rod for the DBLNB and HASSAA connectors and ASTMA325 high-strength bolt for the HTFGB connectors is 862 MPa and 827 MPa, respectively.Based on these calculations, the non-composite beam has a capacity of 609 kN. As shown inFig. 4, adding shear connectors significantly increases the computed load capacity of this beam,even with low values of shear connection ratio. Based on this analysis, it was decided to designthe four partially composite beam specimens with a 30-percent shear connection ratio, resultingin a predicted increase of 48 percent in load-carrying capacity compared to the non-compositebeam. To achieve a 30-percent shear connection ratio requires 16 shear connectors for eachshear span (a total of 32 shear connectors for a beam). The shear connectors were 22 mm (7/8in.) in diameter, and their static strength was predicted using Eq. 3.The specimen designations used below begin with the connection method, followed by theshear connection ratio in percentage. “BS” stands for Beam Static test.1782134305 #5 & #4 @152#4@305Load (kips)12001000800901 kN1048 kNW30x99Unit: mm600609 kN 30%0 0.2 0.4 0.6 0.8 1Shear connection ratio (N/N f )Figure 3 – Details of specimen cross sectionFigure 4 – Predicted load capacity of testspecimens versus shear connectionMaterial propertiesMechanical properties of structural components used in the test specimens are shown in Table1. Concrete for the specimens was delivered by ready-mix truck. The specified compressivestrength was 20.7 MPa with 19-mm river aggregate. Five Star ® Highway Patch, a fast-settinghigh-strength grout, was used to fill the hole in the concrete slab after installation of the DBLNBand HTFGB connectors. Hilti HY 150 adhesive was used for the HASAA connectors.The steel beams used for the full-scale beam tests were of ASTM A992, which has a minimumspecified yield stress of 345 MPa. Steel coupons were taken from both the beam web and theflange. The Grade 60, #4 longitudinal reinforcing bars used in the specimens were also tested intension to evaluate mechanical properties. The ultimate tensile strength of the ASTM A193 B7

threaded rod for the DBLNB and the HASAA connectors and of the ASTM A325 high-strengthbolt for the HTFGB connector was determined from tension tests.Table 1: Measured material properties ( MPa)SpecimenFlangeBeam*WebConcrete**Connector**Grout**Reinforcingbar*NON-00BS 392.6 (533.4) 419.8 (542.2) 43.12 - - 424.9 (713.8)DBLNB-30BS 392.6 (533.4) 419.8 (542.2) 25.38 1014 52.21 424.9 (713.8)HTFGB-30BS 379.1 (520.6) 408.8 (536.7) 28.02 1024 62.91 435.0 (707.1)HASAA-30BS 392.6 (533.4) 419.8 (542.2) 24.90 1014 - 397.2 (683.6)HASAA-30BS1 379.1 (520.6) 408.8 (536.7) 22.21 1014 - 435.0 (707.1)* Yield strength, () ultimate strength, ** ultimate strengthInstallation of shear connectorsThe shear connectors in the laboratory specimens were installed using the proceduresdescribed below, which were developed to be realistic for field installation. The DBLNB andHTFGB connectors require access from both the top and the bottom of the slab. The HASAAconnection method, however, requires access only from the bottom of the slab.(a) Coring concrete slab (b) drilling beam flange (c) Drilling and coring from the bottomFigure 5 – Installation of shear connectorsDouble-Nut-Bolt (DBLNB): Installation of the DBLNB connectors requires access from both thetop and the bottom of the slab. First, a 60-mm diameter hole was drilled through the top of theconcrete slab using a concrete coring machine as shown in Fig 5(a). Second, a 24-mm diameterhole was drilled through the steel beam flange from the top side of the slab using a portablemagnetic drill. A hollow round bar was placed inside of the cored hole in the concrete to guidethe steel drill bit and to keep the inside surface of the concrete clean of cutting oil. Next, a 190-mm long ASTM A193 B7 threaded rod was placed from the top to provide a 130-mmembedment length, and the connector was tightened to a pretension of 170 kN using an impactwrench. Finally, the hole was filled with grout.High-Tension Friction-Grip Bolt (HTFGB): Installation of the HTFGB connectors requires moresteps than the DBLNB connectors. First, a 70-mm deep and 50-mm diameter hole was drilledinto the concrete from the top using a rotary hammer drill. Next, a 25-mm diameter hole,concentric with the 50-mm diameter hole was drilled through the concrete slab from the top

The Hognestad (1951) stress-strain relationship was used to model the concrete stress-straincurve for compression. A smeared cracking model was used to model concrete behavior intension (ABAQUS 2007). In this model, cracking is assumed to occur when the stress reachesa failure surface. The concrete model does not track individual macro cracks. Instead, thepresence of cracks affects the stress and material stiffness of the corresponding integrationpoints (ABAQUS 2007). To model stress-strain response of the steel beams and reinforcingbars, an elastic-perfectly plastic stress-strain relationship was used. Strain hardening was notincluded in the material model. The load-slip relationship proposed by Ollgaard et al. (1971) wasadopted for shear connector behavior. The equation by Ollgaard et al. (1971) was developed forconventional welded shear studs, not for post-installed shear studs. However, it was decided touse this equation due to limited static test data for individual post-installed shear connectors.Load (kN)120010008006004002000max: 1029 kNmax: 1142 kNmax: 1065 kNmax: 726 kNNON-00BSDBLNB-30BSHTFGB-30BSHASAA-30BS0 50 100 150 200 250 300Deflection (mm)Figure 7 – Load-deflection relations Figure 8 – Deflected specimen after typical testTEST RESULTSStructural behavior of test specimensDuring each test, in addition to electronically recording data, the specimens were visuallyexamined for cracks in the concrete slab, yielding and buckling in the steel beam and failure ofthe post-installed shear connectors. Figure 7 shows the measured load versus midspandeflection response for non-composite Specimen NON-00BS, as well as for the partiallycomposite Specimens DBLNB-30BS, HTFGB-30BS, and HASAA-30BS. These partiallycomposite beam specimens were provided with post-installed shear connectors that wereuniformly distributed along the span at a spacing of 725 mm. All partially composite beamspecimens showed significantly higher stiffness and strength than the otherwise identical noncompositebeam even with a low shear connection ratio of 30 percent.For Specimens DBLNB-30BS and HASAA-30BS, a sudden strength drop was observed ataround 120-mm deflection due to the nearly simultaneous failure of multiple shear connectors.Specimen HTFGB-30BS showed better deformation capacity than the other two partiallycomposite beams. It is believed that the oversized hole in the concrete associated with theHTFGB connectors gave them a large slip capacity, resulting in a large deformation capacity forSpecimen HTFGB-30BS. It is, however, noteworthy that 120-mm deflection before shearconnector failure for Specimens DBLNB-30BS and HASAA-30BS is significant and thespecimens resisted significant load even after shear connector failure. Figure 8 shows thedeflected test specimen at the end of the test for Specimen NON-00BS.

End Slip (mm)50403020100NON-00BSDBLNB-30BSHTFGB-30BSHASAA-30BS0 50 100 150 200 250 300Deflection (mm)Neutral axis height (mm)7006005004003002001000NON-00BSDBLNB-30BSPlastic neutral axis forpartially composite beamNeutral axis fornon-composite beam0 50 100 150 200 250 300Deflection (mm)Figure 9 – Load-End slip relationships Figure 10 – Neutral axis locations of test specimensEnd slip and neutral axis locationsAll of the test specimens showed an increase in slip at the interface between the concrete slaband the steel beam as the deflection increased. Figure 9 shows the interface slip at the ends ofpartially composite beams with uniformly distributed shear connectors along with the noncompositebeam specimen. The partially composite beams showed much less slip before anyshear connector failure. Behaviors of Specimens DBLNB-30BS and HASAA-30BS were similarto that of Specimen NON-00BS after the multiple shear connector failures.Specimens DBLNB-30BS and HASAA-30BS showed beam end slip values, at the point ofconnector failure and sudden strength loss, of 5.8 mm and 6.9 mm, respectively. For SpecimenHTFGB-30BS, the first shear connector failure occurred at an end slip of 11.4 mm, much largerthan for the other two partially composite beam test specimens.Composite action in the retrofitted partially composite beams can be further evaluated bylocating the neutral axis during the tests. Figure 10 shows the measured neutral axis location atmid-span of the girder throughout the tests. Neutral axis locations were obtained byinterpolating strain data read from the strain gages on the beam section. For Specimen NON-00BS, the neutral axis was located near mid-height of the steel section at most load levels, asexpected. At very low load levels at the start of the test, the neutral axis was located higher upin the cross-section, suggesting some degree of initial composite action, probably due to bondand/or friction between the steel and concrete. As indicated in Fig. 10, however, this compositeaction occurred only at very low load levels. Once the load exceeded about 10% of the girder’sfull capacity, the girder subsequently behaved in an almost purely non-composite manner. ForSpecimen DBLNB-30BS, the neutral axis stayed above mid-height of the steel section at all loadlevels. All of the partially composite beam specimens showed almost full composite action in theearly stages of loading, likely due to friction at the steel-concrete interface. However, theneutral axis moved down as the load increased, indicating partial composite interaction betweenthe steel beam and the concrete slab.DEVELOPING INCREASED DEFORMATION CAPACITYAnalytical approachThe non-composite Specimen NON-00BS exhibited very high deformation capacity, asindicated in Fig. 7. Specimens DBLNB-30BS and HASAA-30BS showed less deformationcapacity due to the limited slip capacity of the high-strength connectors and also due to the lowshear connection ratio. Specimen HTFGB-30BS showed significantly higher deformation

capacity in its overall load-deformation response due to the higher slip capacity of thisconnector. It is believed that high slip capacity of the HTFGB connectors enabled the shearconnectors at the steel-concrete interface to redistribute interface shear among the connectors.However, the HTFGB connectors are more difficult and time-consuming to install than theHASAA and DBLNB connectors. Because of the easier installation characteristics of the HASAAand DBLNB connectors, an approach was developed to increase the deformation capacity ofbeams retrofitted with these connectors.Oehlers and Sved (1995) indicate that concentrating shear connectors near zero momentregions, resulting in the increase of Ashin Eq. 2, can reduce slip at the steel-concrete interfacewhen the beam reaches its full capacity. This suggests that simply supported beams with shearconnectors concentrated near the supports can likely show higher deformation capacity thanbeams with uniformly distributed shear connectors along the span.Finite element analysis was used to evaluate the effect of moving shear connectors near thezero moment regions. First, the full-scale beam test specimens were modeled with ABAQUS.Figure 11 shows load-deflection relations for Specimen HASAA-30BS from the test along withthe finite element analysis results. As shown in the analysis result of Specimen HASAA-30BS,the model used in this research could not simulate the behavior after shear connector failure.However, the model is considered useful for predicting the behavior before shear connectorfailure and the failure of the shear connectors. Figure 12 shows the longitudinal stressdistribution of Specimens HASAA-30BS from the FE analysis. As shown in the figure, neutralaxis of the beam moved toward the beam top flange due to composite action between the twostructural components.Figure 11 also shows the analysis results of a partially composite beam with shear connectorsconcentrated near the supports. In the FE model, shear connectors were moved near thesupports and were located at a 300-mm spacing. The total number of shear connectors was notchanged in the model. The analysis model with concentrated shear connectors was same withthe analysis model of Specimen HASAA-30BS except the shear connector locations. ForSpecimen HASAA-30BS, the connectors were uniformly distributed along the length of thebeam, at a spacing of 725 mm. Compared to Specimen HASAA-30BS, the deformation capacityof the partially composite beam with shear connectors relocated near the supports wasincreased significantly. The increase of its deformation capacity can be attributed to thedecrease in slip at the steel-concrete interface as shown in Fig. 13. The partially compositespecimen with concentrated shear connectors near the support shows much less slip than thespecimen with uniformly distributed shear connectors in ABAQUS.Load (kN)120010008006004002000HASAA-30BS (Test)HASAA-30BS (ABAQUS)Specimen with 300mmconnector spacing (ABAQUS)0 50 100 150 200 250 300Deflection (mm)Figure 11 – Test vs. FE Analysis resultsFigure 12 – Longitudinal stressdistribution of a composite beam

End Slip (mm)302520151050HASAA-30BS (ABAQUS)HASAA-30BS1 (ABAQUS)HASAA-30BS (Test)0 50 100 150 200Deflection (mm)Load (kN)120010008006004002000max: 1042 kNmax: 1065 kNmax: 726 kNNON-00BSHASAA-30BS1HASAA-30BS0 50 100 150 200 250 300Deflection (mm)Figure 13 – Load-End slip relationships Figure 14 – Load-deflection curves for test specimensTest Results for Partially Composite Beam with Concentrated Shear ConnectorsSpecimen HASAA-30BS1 was tested to verify the FE analysis results and evaluate the globalbehavior of partially composite beams with post-installed shear connectors concentrated nearzero moment regions. Specimen HASAA-30BS1 had the same number of shear connectors asSpecimen HASAA-30BS. For Specimen HASAA-30SB1, the shear connectors were movedtowards the ends of the beam, and were spaced at 300 mm.Figure 14 compares test results for Specimens HASAA-30BS1 and HASAA-30BS. Thedeformation capacity of Specimen HASAA-30BS1 (concentrated connectors) is much greaterthan that of Specimen HASAA-30BS (uniform connectors), and is comparable to that ofSpecimen NON-00BS. Specimen HASAA-30BS1 showed first shear connector failure at a 170-mm deflection, after which the applied load increased slightly before another shear connectorfailure at 210 mm. The maximum load was 1043 kN at 197-mm deflection. It appears thatconcentrating shear connectors near the supports not only decreases slip at the steel-concreteinterface, but also helps redistribute loads among shear connectors.Test results from Specimen HASAA-30BS1 indicate that the stiffness and strength of noncompositebeams can be improved significantly by using relatively small number of postinstalledshear connectors without sacrificing deformation capacity.SUMMARYThe study reported herein was the final phase of a research project to develop methods forstrengthening existing non-composite bridge girders using post-installed shear connectors.Previous phases of this study identified possible post-installed shear connectors and evaluatestatic and fatigue performance of those shear connectors. Based on this earlier works, threetypes of post-installed shear connectors were selected for full-scale beam tests.The number of post-installed shear connectors needed to strengthen an existing bridge girder isdetermined based on the concept of partial composite design. Partial composite design is notnormally used for new composite bridge girders, because fatigue typically controls the requirednumber of shear connectors. Because of the superior fatigue characteristics of the post-installedshear connectors tested in this study, however, fatigue is not likely to control the requirednumber of shear connectors, and partial composite design is therefore possible.With partial composite design, 50 to 70 percent of the shear connectors normally needed for fullcomposite design can be eliminated, while still achieving a 40- to 50-percent increase in loadcarryingcapacity in positive-moment regions of a girder. Based on the test data from singleshear connector tests, overall performance of girders retrofitted with post-installed shear

connectors was evaluated with a series of large-scale beam tests, supplemented with finiteelement analysis. The results of this study suggest that strengthening existing non-compositebridge girders using post-installed shear connectors may be an effective and economicalalternative to replacement of existing bridges.ACKNOWLEDGEMENTSThe authors gratefully acknowledge the financial support provided for this study by the TexasDepartment of Transportation. The authors extend a special thanks to Jon Kilgore and ClaraCarbajal of the Texas Department of Transportation for their support, assistance and advicethroughout the entire course of this project. The authors also gratefully acknowledge thecontributions of Brad Schaap, Brent Hungerford, and Hulya Kayir in the earlier phases of thisresearch study. The experiments described herein were conducted at the Phil M. FergusonStructural Engineering Laboratory at the University of Texas at Austin.REFERENCESAASHTO (2007). “LRFD Bridge Design Specifications Interim Customary U.S. Units, 4thEdition.” American Association of State Highway and Transportation Officials, Washington, DC.ABAQUS (2007). “ABAQUS Analysis User’s Manual (Ver. 6.7).” ABAQUS, Inc, Pawtucket, RI.ACI (2005). “Building Code Requirements for Structural Concrete (ACI 318-05) andCommentary (ACI 318R-05).” American Concrete Institute, Farmington Hills, Michigan, 2005.Hognestad, E (1951). “A Study of Combined Bending and Axial Load in Reinforced ConcreteMembers.” University of Illinois Engineering Experimental Station, Bulletin Series No. 399.Hungerford, B.E. (2004). “Methods to Develop Composite Action in Non-Composite Bridge FloorSystems: Part II.” MS Thesis, Department of Civil, Architectural and Environmental Engineering,The University of Texas at Austin.Kayir, H. (2006). “Methods to Develop Composite Action in Non-Composite Bridge FloorSystems: Fatigue Behavior of Post-Installed Shear Connectors.” MS Thesis, Department ofCivil, Architectural and Environmental Engineering, The University of Texas at Austin.Kwon, G. (2008). “Strengthening Existing Steel Girder Bridges by the use of Post-InstalledShear Connectors.” PhD Dissertation, Department of Civil, Architectural and EnvironmentalEngineering, The University of Texas at Austin (expected completion in August 2008).Lehman, H.G., Lew, H.S., and Toprac, A.A. (1965). “Fatigue Strength of 3/4 in. Studs inLightweight Concrete.” Center for Highway Research, The University of Texas at Austin.Oehlers, D.J., and Sved, G. (1995) “Composite Beams with Limited-Slip-Capacity ShearConnectors.” Journal of Structural Engineering, 121(6), 932-938.Ollgaard, J.G., Slutter, R.G., and Fisher, J.W. (1971). “Shear Strength of Stud ShearConnectors in Lightweight and Normal-Weight Concrete.” AISC Engineering Journal, 8, 55-64.Schaap, B. A. (2004). “Methods to Develop Composite Action in Non-Composite Bridge FloorSystems: Part I.” MS Thesis, Department of Civil, Architectural and Environmental Engineering,The University of Texas at Austin.Slutter, R.G., and Fisher, J.W. (1966). “Fatigue Strength of Shear Connectors.” HighwayResearch Record 147, 65-88.Viest, I.M. et al. (1997). “Composite Construction: Design for Buildings.” McGraw--Hill, NewYork, NY.