Seismic Hybrid Experimental Testing of Six-Story Special Concentrically Braced Steel Frame with Stiff Rocking Core

A rehabilitated 1/3 scale six-story concentrically braced frame (6SCBF) 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 maximum considered earthquake (MCE) ground motion followed by a near fault (NF) earthquake ground motion were selected to examine the effectiveness of the SRC during extreme and near-fault seismic events. The specimen frame was designed and detailed according to the AISC Steel Construction Manual and Seismic Provisions. The SRC was attached to the specimen frame at the first and second floor elevations using pinned-pinned links designed to axially transfer the interaction forces between the two frames. 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 modern SCBF designs under MCE loading.

The physical substructure consisted of the bottom three stories of the 6SCBF specimen and two-story SRC. All tributary gravity framing, including mass and gravity loads, was modeled as a single column in the Open System for Earthquake Engineering (OpenSees, 2012). The top three stories of the six-story specimen frame were also modeled analytically using OpenSees. Braces were modeled to simulate buckling behavior, gusset plate folding along yield lines, and fracture due to low-cycle fatigue. The analytical substructure 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).