Drift and yield mechanism based seismic design and upgrading of steel moment frames.

Abstract

The behavior of an existing moment-resisting frame designed by conventional method was studied using nonlinear static and nonlinear dynamic finite element analyses. The results show that moment-resisting frames designed by conventional, elastic, method using equivalent static forces may undergo inelastic deformations in a rather uncontrolled manner resulting in uneven distribution of plastic hinges. Guided by the performance of this conventionally designed frame, a new design concept is proposed. The new design concept is based on plastic (limit) design theory and principle of energy conservation. The ultimate design base shear for plastic analysis is derived using the input energy from the design pseudo-velocity spectrum, a pre-selected yield mechanism, and a target drift. Parametric studies were carried out to verify the validity of the proposed design procedure. The results show that the proposed method can produce structures that meet a pre-selected performance objective in terms of both the maximum drift and the yield mechanism. The study was then extended to include seismic upgrading of existing steel moment frames. A possible scheme to modify the behavior of existing moment-resisting frames to have a ductile yield mechanism is proposed. This upgrading scheme uses rectangular openings in the girder webs reinforced with diagonal members as ductile fuse elements. A series of small-scale experiments were carried out to study the feasibility of the proposed upgrading system. A detailed design procedure for seismic upgrading of steel moment frames was presented. The results of nonlinear static and dynamic analyses show that it is possible to upgrade an existing moment frame using the special openings. Finally, a full-scale test of a one-story subassemblage was carried out to verify the proposed modification concept experimentally. The test results were very satisfactory. All inelastic behavior was confined to the designated elements of the web opening only. The results also confirm that the proposed upgrading system is very ductile.