Proposal for seismic design coefficients for rocking steel braced frame

Abstract

Rocking steel braced frames are capable of reducing seismic damage through directing damage in energy dissipation elements. This paper quantifies seismic design factors for the controlled rocking self-centering braced frame including response modification, over-strength, and ductility parameters through probabilistic safety assessment methodology. For this purpose, twelve self-centering braced frames differ from the number of stories, plan location, and site class is designed. A nonlinear model is developed for the rocking braced steel frame in Opensees software to simulate the degrading and collapse of the frame and its components (post-tensioning strands and yielding replaceable damper). Over-strength and ductility factors of self-centering systems are determined using nonlinear static analysis. The incremental dynamic analysis is conducted to obtain collapse limit state fragility curves of self-centering frames. Considering total uncertainty and effects of spectral shape, the fragility curves are modified. Through modified fragility curves, proposed response modification factor is verified by comparing the adjusted collapse margin ratio with its acceptance criteria. Finally, the effects of modeling and seismic parameters on the collapse probability of the system are examined. Results indicate that controlled rocking systems are satisfied acceptance criteria and the design of the system with the proposed coefficients provide sufficient safety margin against collapse.

Keywords

• rocking braced steel frame
• seismic design parameters
• collapse assessment
• collapse margin ratio
• fragility curve