Study On Short-and Long-term Shear Lag Effect In RC And PC Box Girders

In recent decades,excessive mid-span deflections and cracking,which can seriously affect the structural safety and serviceability,have been commonly observed in numerous concrete box girder bridges around the world after a few years of service.Based on the National Natural Science Foundation of China(51178416),the main objective of this study is to explore the methods for the short-and long-term analyses of shear lag effect in concrete box girders.The details of this study and achievements are mainly summarized as follows:(1)The longitudinal displacement function was introduced into the displacement field of box girders for eliminating the calculation error caused by supposing that the neutral axis coincides with the centroidal axis in the conventional shear-lag analysis.Then an analytical method considering variation of neutral axis and shear deformation was proposed.The values of an example obtained by the proposed method were compared with the measured values and those by the finite element method and the conventional method.The results show that the shear lag effect in box girders can be accurately calculated by the proposed method.(2)The proposed method in(1)was used for quantify the effects of the ratio of width to span,the ratio of width of cantilever slab to half-width of top slab,the ratio of width of bottom slab to width of top slab,the ratio of height to width and the ratio of flange thickness to web thickness on the stress difference ratios of flanges in box girders.The results show that the ratio of width to span,the ratio of width of cantilever slab to half-width of top slab and the ratio of width of bottom slab to width of top slab can significantly affect the ratios of stress difference at the intersection of the top flange and the webs in the cross-sections under concentrated load and at the interior support of box girders,and that the ratio of height to width and the ratio of flange thickness to web thickness can only remarkably affect the ratios of stress difference at the intersection of the top flange and the webs in the cross-sections under concentrated load and at the interior support of continuous box girders.(3)An analytical method for the long-term analysis of RC box girders considering shear lag,creep and shrinkage was presented based on the virtual theorem and the aging-adjusted effective modulus method.The effectiveness and accuracy of the presented method were verified by the finite element method and the elementary beam theory.In addition,the effects of creep and shrinkage on the long-term behavior of RC box girders were analyzed in detail.The results show that creep and shrinkage are the main factors which affect the long-time deflections and concrete stresses of box girders,and that creep and shrinkage also have considerable influence on the support reactions and the distribution of internal forces.(4)A method for shear lag analysis of box girders under prestressing effect considering shear deformation was developed on the basis of the prestress equivalent load method and the Timoshenko beam theory.The presented method can consider the shear lag effect caused by axial forces.The accuracy of the presented method was validated by comparing the values calculated by the presented method with the test values and those obtained by the finite element method.The shear lag analysis of simply-supported and two-span continuous box girders with typical tendon profiles was performed.The results show that shear deformation has a significant effect on the deflection calculation of box girders under prestressing effect,and that the equivalent axial forces of prestressing tendon can result in the obvious shear lag effect at the anchorage location.(5)By taking the time-dependent material properties of concrete into account,the finite element models of a typical PC continuous box girder bridge,which can simulate the cantilever construction,were established by beam and solid elements,respectively.The effects of non-uniform shrinkage and creep were compared with those of uniform shrinkage and creep on the time-varying deflection,tendon stresses and concrete stresses considering shear lag.It was demonstrated that neglect of shear lag can result in the significant underestimation of deflection at mid-span of long-span PC box girder bridges.Moreover,the results show that compared with uniform shrinkage and creep,non-uniform shrinkage and creep have smaller impact on the long-time deflection at mid-span,and have larger effect on the tendon stresses and concrete stresses.