Generalized modified modal superposition procedure for seismic design of rocking and pivoting steel spine systems

Abstract

To mitigate structural damage and associated losses due to strong earthquakes, rocking and pivoting steel spine systems have been proposed to control story drifts and localize inelastic deformations in ductile energy dissipating components. One of the challenges of designing these systems is proportioning the spine members to remain essentially elastic under earthquakes. Previous studies have shown that modified modal analysis can efficiently and accurately capture the forces developed in rocking braced frames. One of these approaches, the modified modal superposition (MMS) method, consists of superimposing the first inelastic modal force(s) with the elastic higher modal forces. The methodology, previously developed for single rocking braced frame, is extended for coupled and stacked rocking braced frames as well as strongback systems, equipped with various hysteretic and viscous nonlinear dampers. Nonlinear dynamic analyses run with a suite of 50 hazard-consistent ground motions for seven archetype rocking and pivoting systems demonstrate the accuracy of the capacity design procedure for predicting member forces in braced frame steel spines as well as global forces: story shear and overturning moments. Using the MMS methodology, peak story drifts and floor accelerations are reliably estimated. Comparison of the alternative spine configurations demonstrate the effects of various energy dissipation properties, global hysteresis behavior and earthquake frequency content on the nonlinear dynamic response of these systems.