Seismic Hybrid Experimental Testing of Sub-Standard Three-Story Concentrically Braced Steel Frame with Stiff Rocking Core

A rehabilitated 1/3 scale sub-standard three-story concentrically braced frame (3NCBF) was subjected to two back-to-back ground motions performed quasi-statically using hybrid simulation to investigate the effectiveness of rehabilitation using a stiff rocking core (SRC). A design basis earthquake (DBE) ground motion followed by a maximum considered earthquake (MCE) ground motion were selected to examine the effectiveness of the SRC under multiple hazard levels. The specimen frame was designed using forces obtained from a 1959 SEAOC equivalent lateral force procedure (ELFP), and several connection details were purposely under designed based on a survey of older steel construction. The SRC was attached to the specimen frame at each floor elevation using fixed-pinned links designed to dissipate energy through yielding in flexure. The SRC allows lateral forces to redistribute along the height of the frame as strength and stiffness degrade due to damage in the braces. This prevents the formation of a soft-story collapse mechanism commonly observed in older steel construction under seismic loading.

All tributary gravity framing, including mass and gravity loads, was modeled as a single column in the Open System for Earthquake Engineering (OpenSees, 2012). This model was subjected to ground motions selected from the SAC ground motion bins for the LA site. Target displacement values were imposed by actuators attached to the physical specimen at each floor elevation, and measured forces in the actuators were communicated back to the analytical model for calculation of the next target displacement. The physical substructure and laboratory hardware were integrated with the analytical substructure through the Open Source Framework for Experimental Setup and Control (OpenFresco, 2008).