Minimum weight design of structures based on buckling strength and plastic collapse

Abstract

A study on the plastic design method of plate girders whose webs buckled in shear is presented. Local buckling of flanges and shear buckling of webs set limitations to the plastic design of plate girders, and it is recommended that the dimensions of cross section shoule be so designed as to prevent various types of buckling from taking place up to the strain hardening range.The web hight, however, is regarded as the most important factor when we aim at the minimum weight, and girders are able to carry shear force by tension field action after buckling occurs in their webs. So it is necessary to introduce the concept of incomplete tension field into the plastic design method. The problem was studied by K. Basler, but the interaction curve he proposed seemed to be unsafe and the theoretical direction of tension field does not coincide with our experiments.The authors tried a different approach based on the lower bound theory and the assumption that the compressive principal stress after web's shear buckling maintains the value at buckling in the same direction. Conclusions are as follows.(1) There are two types of shear collapsing modes. One is that the web yields uniformly along the tension field and deforms in rhomic. The other is that the yielded band grows nearly in diagonal direction and the adjacent flanges are pulled into the web finally.(2) Interaction curve between bending moment and shearing force including the web buckling in shear is introduced. It is showed that the maximum carring moment is reduced by not only the shearing stress in web, but also the compressive stress in flanges caused by the tension field action.(3) The generalized method to decide the collapsing load of indeterminate girders using the linear programming method is showed.(4) Tests were carried out on 2 span continuous girders subjected to concentrated loads, and the tests results showed a good agreement with the theory.