| Steel–concrete composite girders are widely employed in bridge structures,building structures,and other long-span structures.The cross-sections in bridge engineering are mostly non-compact sections with lager web height–thickness,which introduces web stability problems.For a composite girder,in addition to the steel flanges,the concrete slab also provides restraint to the web.Therefore,the assumption in existing codes that the web is simply supported on four sides will underestimate the elastic shear-buckling load of composite girder webs.Furthermore,when calculating the shear capacity of composite girders,most design codes neglect the contribution of concrete slab and assume that the acting shear is borne by only the steel web.This conservative design method inevitably results in unnecessary wastage of materials.Therefore,funded by the National Natural Science Foundation of China(51878099),the web stability,shear capacity,and moment-shear interaction of composite girders in negative moment regions are investigated in this paper.The main research contents and conclusions are as follows:A total number of 12 specimens,including 11 simply-supported composite girders and one bare steel girder were investigated experimentally.The results show that the web buckling load of the reference composite girder was 40.0% higher than that of the pure steel girder,indicating that the concrete slab provides a strong restraint to the web,which greatly increases its buckling load.With the increase of the web height–thickness ratio and web aspect ratio,the web buckling load decreased obviously,while the web buckling load only increased by 1% when the concrete slab thickness increased by 20%,indicating that the web height–thickness ratio and web aspect ratio are important factors affecting the web buckling load,while increasing the slab thickness has little effect.The ultimate load of the composite girders increased by 10–35% compared with that of the bare steel girder,indicating that the concrete slab has significant contribution on the capacity of composite girders.With the increase of the web height–thickness ratio and web aspect ratio,the ultimate loads of composite girders were apparently reduced,and when the concrete slab thickness increased by 20%,the ultimate loads increased by 7–14%.This indicates that the web height–thickness ratio,web aspect ratio and slab thickness all significantly influences the capacity of composite girdersA total number of 16 specimens,including 13 simply-supported girders(11 simplysupported composite girders and one bare steel girder)and three continuous girders with two equal spans were investigated experimentally.The results show that the ultimate load of the composite girders were 10–20% higher than that of the bare steel girder,and 8–16%higher than that calculated with AASHTO and the EC-4 codes,which further indicates that neglecting the shearcontribution of the concrete slab yields a conservative result in existing codes.When the reinforcement ratio was doubled,the slab thickness increased from 115 mm to 180 mm,and the web thickness increased from 6 mm to 8 mm,the shear capacities of composite girders increased by 4.4–8.9%,9.1–11.4%,and 25.2–49.1%,respectively.It is indicated that increasing the longitudinal reinforcement ratio,slab thickness,and web thickness all will increase the shear capacity of composite girders,and the increase is much more of the slab thickness and web thickness.Adopting PBL connectors has little influence on the shear capacity of the composite girder,but improves its ductility.Considering the actual boundary conditions,the effects of flanges and concrete slab restraint on the web shear buckling were investigated using the finite element analysis(FEA).The result shows that the top flange and the concrete slab together provide a fixed boundary condition at one edge for the web.The boundary conditions of the composite girder web are simply supported on two opposite edges,fixed on one edge,and restrained by the bottom flange on one edge.The effect of restraint provided by flanges on the web is depended on the ratio of flange torsional stiffness to web bending stiffness.When the ratio decreases,the web boundary tends to be plates with three simply supported edges and one fixed edge,and when the ratio increases,its boundary tends to be plates with two simply-supported opposite edges and two fixed opposite edges.Based on the finite element(FE)model,the shear distribution of the cross-section was investigated.The FEA reveales that the shear distribution of the section can be divided into four stages,namely elastic,slab cracking,steady after slab cracking,and web apparent buckling stages,during the entire loading procedure.In steady stage,the shear proportion of the steel beam reaches its maximum,whereas that of the concrete reaches its minimum,and the shear ratio of the concrete slab is about 6–21%.Furthermore,because the stress states of the web and the slab vary at different positions,some or all these stages may be experienced.Based on the FE model,parametric analyses were conducted on the factors affecting the shear capacity in negative moment regions of composite girders.The parameter analysis results show that within the scope of practical engineering,with the increase of the reinforcement ratio,thickness,and width of the concrete slab,the shear capacity of the composite girders increased by 14–55% compared with the bare steel girder;when the web height–thickness ratio was increased from 90 to 150,the shear capacity of the composite girder was 1.14–1.36 times that of the bare steel girder.This indicates that the shear contribution of the concrete slab cannot be neglected,and the higher the degree of non compaction,the higher shear contribution of the slab.With the increase of the reinforcement ratio,thickness,and width of the slab,the shear capacity increases linearly;with the increase of the web height–thickness ratio,the shear capacity decreases nonlinearly,in which the web height–thickness ratio has the greatest influence,followed by the slab thickness and reinforcement ratio,and the influence of the slab width is the least.Based on the FE model,parametric analyses were conducted on the factors affecting the moment–shear interactionr in negative moment regions of composite girders.The result shows that changing reinforcement ratio and concrete slab thickness have little influence on the moment–shear interaction law,and when the section shear force exceeds80% of the shear capacity,the composite girders begin to experience obvious momentshear interaction.The FE models with web height–thickness ratios of 90,120 and 150,begin to experience obvious moment-shear interaction when the shear force exceeded82%,73% and 64% of the shear capacity,respectively.It is indicated that the higher the web height–thickness ratio,the earlier the composite girder appear obvious moment–shear interaction.Therefore,for composite girders with web height–thickness,which are commonly used in bridge engineering,the moment–shear in negative moment regions can not be neglected.Based on the parameter analysis results,considering the influence of actual boundary conditions,a formula for calculating the web shear-buckling coefficient of composite girders in negative moment regions was proposed.According to the web tension field model and the proposed calculation formula for web shear-buckling,the shear capacity calculation formula of the steel beam was modified.Considering the influence of the width,thickness,reinforcement ratio and tensile strength of the concrete slab,a fitting formula of the shear capacity for the concrete slab was proposed using the experimental data.By superimposing the capacity of the steel beam and the slab,a shear capacity calculation formula for composite girders was proposed,and its accuracy was verified using a large number of FE results.Based on the experimental and numerical results,a formula for calculating the moment-shear interaction of composite girders in negative moment regions was proposed through regression analysis. |
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