REFINED PLASTIC HINGE ANALYSIS OF SPATIAL STEEL FRAMES

Method for computing the element restoring force, rather than the stiffness matrix, is the most important factor that determines the accuracy of a geometric nonlinear finite element problem. The formulation of a spatial beam-column element with shear deformation for the purpose of refined plastic hinge analysis of 3-D steel frames is presented. Second-order effects are captured through stability functions. Material nonlinearity is modeled using refined plastic hinge method, which can account for the gradual yielding of the section and the reduction of the element stiffness due to distributed plasticity resulting from residual stresses. The coupling terms between the axial, lateral and torsional displacements are included in element stiffness matrix. Using the User Programmable Features (UPFs) of ANSYS program, the element routines have been developed and successfully incorporated into ANSYS program. The results of numerical examples show that only one proposed element, in contrast to three or four BEAM189 elements of ANSYS program, is necessary to model a column of spatial frames with excellent accuracy. The analysis results also indicate that shear deformation of members must be considered in the analysis of multi-storey frame structures.