Direct displacement-based seismic design of semi-rigid steel frames

Semi-rigid steel frames are often considered a cost-effective alternative to conventional fully rigid steel frames, because they use the economical partially restrained (PR) connections instead of the costly moment connections while performing satisfactorily under seismic loads. So far, semi-rigid steel frames have typically been designed using the traditional force-based method that is meant for the design of fully rigid steel frames. With this method, the sizing of various structural components is firstly carried out per code strength requirements and then revised iteratively to satisfy the stiffness requirements on both individual structural components and the overall frame. As a result, a designer often encounters considerable difficulty in achieving a structural solution that meets the target displacement and attains a desired yield mechanism. To address this issue, we propose a facile displacement-based method for the seismic design of semi-rigid steel frame structures by directly and explicitly relating the target displacement and deformation to the appropriate sizing of various structural components in terms of their strength, stiffness, and ductility. The new method is based on a simplified analytical semi-rigid frame model assuming that the structural plastic deformation is mainly concentrated within the PR connections while beams and columns largely remain elastic. A step-by-step procedure for implementing the new method is described and illustrated with example semi-rigid steel frame designs. The usefulness of the new method is demonstrated by comparing the target displacement and deformation values specified as a design objective with the actual values predicted by a nonlinear static pushover analysis.