Composite moment frames consisting of steel beams and reinforced concrete columns (so called RCS moment frames) are one of several types of hybrid systems gaining acceptance as cost-effective alternatives to traditional steel or reinforced concrete frames for seismic design. This research presents an extensive analytical study whose focus is on the seismic behavior of composite frames with the objectives to (1) develop analytical models and techniques for the nonlinear inelastic time history analysis of RCS moment frames under various seismic hazards, (2) propose damage (i.e., performance) indices to assess seismic performance of such frames, (3) develop and correlate stability limit states to performance levels, and (4) investigate response dependency on ground motion parameters so as to reduce the uncertainty in estimating median response.Our approach toward establishing a performance-based design basis for composite RCS frames involves both evaluation of seismic damage indices with test data on member and connection response and comparative behavioral studies between RCS and conventional structural steel moment frames. Trial designs of six- and twelve-story RCS and steel framed buildings are developed to exercise the latest seismic design criteria and standards in the United States. Nonlinear static and time-history analyses are run under two sets of earthquake records (general versus near-fault records with forward directivity) that were selected and scaled to different hazard levels. Peak and cumulative damage indices are then developed, calculated and compared with structural acceptance criteria established using data from tests of structural components. A new methodology is proposed to quantify system stability limit states by integrating the destabilizing effects represented by such local damage indices through modified second-order inelastic stability analyses avoiding the need for questionable ad-hoc averaging techniques to relate local to global performance. Correlation between ground motion intensity measures and structural damage is investigated, and statistical measures of global response are reported.Supported by test data on structural components, the analyses demonstrate excellent seismic performance of composite framed structures when evaluated both on their own merits and in comparison with steel frames. The study does, however, suggest areas for improving current design criteria, in particular, the minimum strength and stiffness requirements for proportioning beams and columns to resist seismic loads. By improving understanding of the seismic response of composite RCS frames this research should lead to their broader utilization for seismic regions and will contribute towards the development of more transparent and reliable performance-based design methodologies.
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