The Effects of Yield Mechanism Selection on the Performance based Plastic Design of Steel Moment Frame

Abstract

This study concentrates on the effect of selecting a desirable yield mechanism that has a significant effect on seismic design and response of structures designed by Performance Based Plastic Design (PBPD) method. In PBPD method, the design base shear is obtained on the basis of energy-work balance equation implementing pre-selected target drift and yield mechanism. With considering the importance of selection of yield mechanism, a parametric study has been done considering the different types of yield mechanisms to design a model of steel moment frame. The results obtained by nonlinear time history analyses shown that the complete sway mechanism (strong columns-weak beams) is the most efficient in terms of required design base shear for a given target story drift. It also should be noted that it is often impossible to estimate the column moment demand during the event of sever ground motions because they undergo large moments not only from those delivered from beams but also from their own deformation. Therefore, assuming only the criteria of strong column-weak beam mechanism in PBPD method can’t prevent the yielding in columns. In this paper to improvement of PBPD method, some solutions are represented to precisely obtain the required moment of columns to prevent their yielding. Because the yielding of columns leads to form undesirable mechanisms before the structure reaches the pre-selected target drift. For example, a 10 story frame has been designed based on PBPD method using strong column-weak beam mechanism. The frame has been redesigned considering all represented equations of this study (modified PBPD frame). The results obtained by nonlinear static and dynamic analyses show that the modified PBPD frame perform according to expectations in terms of yield mechanism and target drift levels whereas the PBPD frame suffer large story drifts due to flexural yielding of the columns.