Study On Military Box Nose Girder Of Composite Materials And Shear Lag Effect

There are important theoretical and military application values to study the composite materials of box nose girder and its shear lag effect, which can bring high speed and great mobile capability to military bridge. Based on present research, thorough analyse of relative problems of composite materials box nose girder because of the insufficient published results and the significance of improving troop's mobile capability under military bridge, both theoretical method and numerical method are used to systematically research the structure design, bending under vertical load and shear lag effect of composite materials of box nose girder. The main contents and innovative work are as follows:(1) Based on published research, box nose girder of composite materials is primarily designed, the structural form and the size of section are confirmed, especially the basic principle of joint design to composite materials sandwich structure is mainly researched, which is adopted appropriate way to match different joint, and the length of nose girder under cantilever-push way is also optimized.(2) Basic hypothesis of preliminary beam theory and suitable equivalence of composite materials sandwich structure are applied to analyze bending normal stress of composite materials of box nose girder, whole-bending stiffness of box girder, shear stress and flexibility which takes account of shear deformation, providing a simplified theory to box nose girder of composite materials.(3) A FEM model of sandwich composite materials which is proved by experiment and theory is established. MSC.NASTRAN and primary beam theory are applied to compute bending flexibility and normal stress of composite materials of box nose girder simultaneously. The results show that basic theory can precisely analyze the flexibility of composite materials of box nose girder, but can't predict the actual normal stress, because of shear lag effect in existence.(4) The shear lag effect of composite materials of box nose girder is analyzed, and the definition of coefficient of shear lag effect is put forward to describe the extent of shear lag effect. The reason for shear lag effect of box girder is theoretically deduced. Numerical simulation is applied to analyze the coefficient of shear lag effect. Based on those important parameters, distributing principle for its shear lag effect is acquired. Finally, based on energy functional variation principle, the governing differential equations considering shear lag effect for composite materials of box nose girder are established, and the expression of normal stress is acquired.