Abstract
A simple proposal for ultimate seismic demand evaluation of multi-degree-of-freedom moment resisting steel frames is developed from limit analysis and first order reliability method (FORM). First, a procedure is proposed for 'vibration mode-failure mode' integrated analysis, wherein the restoring force characteristics are represented by a global convex yield polyhedron model, instead of a set of member hysteresis based models usually adopted in the inelastic structural analysis. Then, FORM is extended to seismic design in the form of 'random pushover analysis', to choose an appropriate number of failure modes to be considered in the vibration mode-failure mode analysis. Random pushover shows a positive correlation with the deterministic analytical procedures and can be further used to determine the lateral load patterns for pushover by the nonlinear static procedures. A six-storey two-bay frame is taken for case study and is analysed by the proposed methods for three ground motions and an impulse. By comparing the results with the member-hysteresis based analytical procedures, this study indicates that the proposed simple methods can serve as a good tool for seismic demand evaluation for practical purposes.
