Validation of the Seismic Performance of Composite RCS Frames: Full-Scale Testing, Analytical Modeling, and Seismic Design

Composite RCS moment frames integrate reinforced concrete columns with structural steel beams, providing several advantages over conventional steel or concrete moment resisting frames. Past studies have shown these systems to be efficient in both design and construction stages while able to maintain sufficient strength and ductility necessary in seismic applications. Despite this past research, use of this hybrid structural system in the United States has been limited to non- or low-seismic zones, which is largely due to the reluctance of the engineering community to accept this “new” system as well as a lack of comprehensive seismic design criteria in building codes and specifications. In addition, past studies have acknowledged that there is a fundamental need to test full structural systems, both analytically and experimentally, in order to (1) substantiate the knowledge that has been accumulated up to this point and (2) act as a proof of concept for the composite RCS frames.

The primary goal of this research program is to fill this knowledge gap and facilitate the greater acceptance and use of composite RCS systems as a viable alternative to conventional lateral resisting systems. This research synthesizes and interprets some of the latest provisions and past studies on RCS systems and applies the accumulated knowledge to (1) develop and validate improved seismic design provisions for RCS frames, (2) assess and demonstrate the seismic performance of RCS structural systems through full-scale frame testing and analytical simulations of prototype building systems, and (3) develop and validate modeling guidelines for nonlinear analysis and performance simulation. The cornerstone of this study is the planning, design, and testing of a fullscale 3-story composite RCS moment frame. Using the pseudo-dynamic loading technique, this specimen is subjected to a series of earthquake motions ranging in hazards from frequent to extremely rare events. In addition to providing insight into the seismic performance and design of composite RCS frames, the frame and supporting subassembly tests also provide a rich data set for the validation of nonlinear analysis and damage models. This also helps address one of the broader aims of this investigation, which is to provide support in the development of performance based earthquake engineering.

Designed to evaluate the minimum limits of current building code requirements, the fullscale test frame exhibited excellent seismic performance up through the maximum considered earthquake intensity level. The damage patterns after each pseudo-dynamic earthquake event were representative of the performance expected in well-detailed moment resisting frames designed by current building codes, with no instances of brittle failure (e.g. fracture). Nonlinear beam-column fiber elements and two-dimensional joint elements were independently calibrated to multiple subassembly tests and modeling recommendations are proposed. These models are used to simulate the test frame conditions and loading protocol. The analytical results correlate well with the experimental response within interstory drifts up to 3%, beyond which, the physical damage in the frame exceeds what the analytical models used in this study are expected to accurately capture.

Using the recommendations presented herein, trial designs of three case study buildings (3, 6, and 20-stories) are generated, analytically modeled, and subjected to a suite of earthquake ground motions at a range of hazard levels. The response of these case study buildings are probabilistically evaluated considering several key engineering demand parameters. These results, coupled with the response of the test frame, validate the seismic behavior of composite RCS frames and also help assess and improve several key design issues that are applicable to this system, as well as others.