Research On Shear Lag In Box Girders And The Influence On Behavior Of Bridges

The box girders have been widely used in the bridge engineering because their beautiful external shapes, good behaviors and structural functions, but they show the shear lag or negative shear lag effect when symmetry bending loads act longitudinally .If the effect of shear lag is neglected ,high risks up to collapse of bridges may take place . As a classic subject, shear lag effect has been studied by scholars all over the world past several decades, some conclusions have been achieved. The bridge codes and specifications in many countries have taken shear lag effect into account, but many problems with shear lag stay continuously. The bridge engineering disasters in the middle last century made the scholars pay more attention to shear lag effect again and farther research restarted.The box girder mechanical characters and the concept of shear lag are introduced in this dissertation, the theories and methods for solution to shear lag effect are discussed; Their suitability and constraint of use are discussed too. 3—bar simulation method governing equations with different boundary conditions are derived according to assumptions considering the compact-bending effect, and the solutions to simple bearing and cantilever girders under uniform and concentrated load are derived. The uniform formulas for simple bearing and cantilever girders under bending loads and compact-bending loads are constructed , the solution to normal stress of 3—bar simulation method for box girders is extrapolated to multi-bar simulation and governing equations are derived . General potential enery equations are constructed according to the principle of minimum potential energy based on the 3-degree distribution of longitudinal displacement along transverse direction and a set of governing differential equations with different boundary conditions are derived according to the energy equation considering the compact-bending effect, and the solutions to simple bearing and cantilever girders under uniform and concentrated load are derived. The uniform formulas for stress and deflection of simple bearing and cantilever girders under bending loads and compact-bending loads are constructed. Shear lag effect of box girders are studied with bar simulation,energy variation and FEM , verifications of these methods show that are believable . Negative shear lag effect is studied and the factors leading to negative shear lag are analyzed too .Some girders of bridges are studied and compared in the end according to the bridge specifications and the methods discussed in the dissertation . Some suggestions to the shear lag effect are given to the design of the box girders.