The present paper discusses the ability of a developed optimization technique to obtain parameters that can be used in the simulation of a finite element model of 3D portal frame structure where a set of parameters that gives acceptable response of the structure is needed. In order to find the values of these parameters, an optimization technique based on a
modified genetic algorithm (GA) was developed and linked to a finite element package. In the formulation of the optimization problem, the objective function is the differences between the computed structural response and that measured experimentally where the design variables describe the rotational stiffnesses of various connections of the bare frame, the steel sheet cladding thickness, the orthotropic properties of the cladding and the stiffness of the fasteners.