Efficient collapse analysis techniques for framed structures

Abstract

An efficient computational procedure for collapse analysis of framed structures is described. The procedure makes use of co-rotating nonlinear beam–column elements, connected at nodes, that may develop plastic hinges. The formulation incorporates a series of novel features that greatly enhance the efficiency of the computations. The secant and tangent stiffness of the beam–column element are obtained from a six-component equilibrium format in explicit form as a nonlinear function of the normal force, and initial imperfections and shear flexibility are incorporated into the explicit format. The yield functions used in the plastic hinges are extended to the full generalized stress space in a form that optimizes the convergence rate of the iterative return algorithm, and the effective length of the plastic hinge is assessed from the moment distribution in the element. The equilibrium format of the beam–column element enables a simple explicit modification of the theoretical elasto-plastic tangent stiffness to obtain the corresponding algorithmic stiffness that accounts for the finite stress increments of the numerical algorithm. These features have been implemented into a finite element collapse analysis program RONJA, and two realistic offshore analysis examples are included to illustrate the performance of the computational procedure.