Optimal design of frame structures with semi-rigid joints

Abstract

In this paper, a genetic algorithm is proposed for discrete minimal weight design of steel planar frames with semi-rigid beam-to-column connections. The frame elements are constructed from a predetermined range of section profiles. Conventionally, the analysis of frame structures is based on the assumption that all connections are either frictionless pinned or fully rigid. Recent limit state specifications permit the concept of semi-rigid connection of the individual frame members in the structural design. In a frame with semi-rigid joints the loading will create both a bending moment and a relative rotation between the connected members. The moment and relative rotation are related through a constitutive law which depends on the joint properties. The effect, at the global analysis stage, of having semi-rigid joints instead of rigid or pinned joints will be that not only the displacements but also the distribution of the internal forces in the structure must be modified. In this study, a simplified beam-to-co lumn connection is presented which was specified in EC3 Annex J. In order to capture the changes in the nodal force and moment distribution in terms of joint flexibility, the ANSYS finite element analysis is applied. The structural model is formulated as a combination of 3D quadratic beam elements and linear torsional springs. Present work deals with the effects of joint flexibility to the optimal design problem. The design variables - including joint properties - are discrete. Results are presented for sway frames under different load conditions.