UC San Diego

In recent years the tenets of Performance-Based Seismic Engineering (PBSE) have been introduced for design of earthquake resistant structures. Thus, it is necessary that a design methodology be capable of producing a system that can achieve a performance target. Research has identified limitations in conventional force-based design practices in meeting the needs of PBSE. In response, a significant movement has been made towards displacement- based design in an attempt to bypass these limitations. This research proposes a Direct Displacement-Based Design (DDBD) methodology for design of new seismic resistant steel moment frames. Two crucial issues in earlier DDBD methods that need resolution are (1) assumption of frame yield displacements and (2) determination of system equivalent damping. To resolve the first, a procedure using beam mechanics is proposed to construct a yield displacement profile. The procedure illustrates that yield displacement is essentially a function of beam geometry, suggesting that displacement ductility demand can be controlled via design. Secondly, the total energy dissipated by the frame from beam yielding is commonly estimated by evaluating the base shear - roof displacement hysteresis. From which an estimate of equivalent damping is computed by applying a damping function developed for a single yield mechanism. This is limiting in that ductility contributions from each mechanism or the effects of higher mode contributions are not considered. A more rational procedure is proposed where floor ductility contributions are accounted for and an equivalent modal damping computed. In so doing, a better estimate of equivalent damping for design can be made