ELASTIC-PLASTIC TANGENT STIFFNESS MATRIX OF TUBULAR MEMBERS

Plastic hinge models based on the concept of flow rule and a yield surface represented by generalized stresses are often used for elastic-plastic analysis of steel space frames. However, since the plastic deformations in plastic hinge are obtained directly by flow rule in those models, it is impossible to introduce distinctly the generalized strain hardening modulus which relates the infinitesimal generalized stress increment to the infinitesimal generalized strain increment as the strain hardening modulus in the finite element analysis of a continuum. This is a main factor that reduces the accuracy of the analitical results. A procedure to form an elastic-plastic tangent stiffness matrix of tubular members is presented in this paper. The generalized strain hardening modulus can be used directly to estimate the behavior of plastic hinge in the procedure and the effect of finite deformations of a frame are taken into account also. Numerical examples are made for two kind of tubular beam-columns and the results are compared with available theoretical and experimental results to verify the proposed method.