Theory of Plastic Mechanism Control of Seismic-Resistant MR-Frames with Set-Backs

Abstract

Seismic codes do not provide specific hierarchy criteria for irregular Moment Resisting Frames (MRFs). In fact, the provisions for irregular frames are exactly the same adopted for regular MRFs with the only exception of a 20% reduction of the design value of the q-factor, accounting for a presumed worsening of the frame energy dissipation capacity. It is well known that seismic code provisions, based on hierarchy criteria, are often unsuitable to prevent partial mechanisms and to assure the development of a global collapse mechanism. In particular, irregular structures are prone to develop partial or soft storey mechanisms in case of significant set-backs. As already showed in previous work [1-10] a rational design procedure based on plastic mechanism theory can be adopted for different design typologies leading to excellent results in the field of mechanism control. This procedure is herein briefly presented and applied to different study cases. It is based on the application of the kinematic theorem of plastic collapse extended to the concept of mechanism equilibrium curve in order to consider second order effects. Sending the reader back to the original work on the theory of plastic mechanism control [1] for an exhaustive presentation of the theory, referred to regular MR-Frames, the work herein presented focuses on the issues to be faced to apply such theory to the particular case of steel frames with setbacks. In order to verify the results obtained by the application of the proposed design procedure, non linear static analyses [11] have been carried out for all the structures considered and a comparison with the results coming from the application of Eurocode 8 design procedures [12] is also performed. © Montuori et al.