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System Behavior Factors for Composite and Mixed Structural Systems

Roberto T. Leon, Virginia Polytechnic Institute and State University Jerome F. Hajjar, Northeastern University Tiziano Perea, Universidad Autónoma Metropolitana Mark D. Denavit, Stanley D. Lindsey and Associates, Ltd.

Sponsored by National Science Foundation and the American Institute of Steel Construction

This research aims to fill key gaps in the design provisions for frame systems that include steel-concrete composite columns. The experimental portion of the project, in which 18 full-scale slender concrete filled steel tube beam-columns were tested under a variety of load cases, is complete. The experimental data has been valuable in the assessment of behavior as well as to validate second-order inelastic analysis models capable of performing broad parametric studies. Among the studies performed with the models was an assessment of the seismic performance factors for special composite moment and braced frames. Following the FEMA P-695 procedure, numerous static pushover and dynamic response history (Figure 1a) analyses were performed. The results provide quantitative evidence in sup-port of the current factors used in design. Also, current stability design provisions for composite frame systems have been evaluated. The results indicate that while the current design provisions are safe and accurate for the majority of common cases, there exist cases where the current provisions result in unconservative error. One case of unconservative error is slender columns subjected to combined axial load and bending moment, where the error can be traced to the change in shape of the interaction diagram with slenderness (Figure 1b). Practical modifications to the current AISC design provisions, specifically changes in the direct analysis stiffness reductions and the effective flexural rigidity for the determination of the compressive strength have been proposed. Further design recommendations are in development including modifications to the ACI design provisions and effective flexural rigidity for drift determination.

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