Performance Assessment of Hybrid Steel Frames under Near-field Seismic Excitations

Abstract

Assessment of the seismic performance of steel structures is topical research for a variety of earthquakes. Moment-resisting rigid steel frames are generally designed and used for high seismic zones. However, the famous Northridge in 1994 and Kobe in 1995 earthquakes exposed the shortcomings of moment frames. During these events, the beam-column connections of rigid moment frames were severely damaged. Since then, an alternative of welded connections called bolted or semi-rigid connections is considered for designing or retrofitting of steel moment frames. From the last few years, the semi-rigid connections combined with rigid connections, termed as hybrid or dual frames have become widely popular for earthquake designers and engineers due to enhanced ductility and reduced cost of construction. Earlier research work was focused on far-field (FF) earthquakes, whereas, the near-field (NF) earthquakes are crucial for civil engineering structures. In this paper, an extensive numerical study is carried on rigid and hybrid steel moment frames for NF and FF earthquakes. For this purpose, a ten-story rigid steel frame is analyzed and designed for seismic requirements as per Indian Standards. The rigid connections of frames are replaced by semi-rigid connections with a moderate degree of semi-rigidity. The six different patterns (locations of semi-rigid connections) of hybrid frames are considered here to compare the seismic performance of hybrid frames under near field earthquakes. The nonlinear response-history analyses are executed using the SAP2000 software at two scaled PGAs defined as low (i.e., 0.2g) and high (0.5g) level.  An ensemble of three time-histories, each for both types of earthquakes are considered for the numerical simulation. Response quantities of interest, namely, the maximum value of base shear, story displacement, inter-story drift ratio, the total number of plastic hinges, and the square root of the sum of squares of maximum plastic hinge rotations are considered for the decision of a most appropriate pattern for hybrid frames. The results of the numerical study indicate that the response behavior for the NF earthquakes is distinctly different both in nature and magnitude. It is observed that the seismic demands under the NF earthquakes are substantially more as compared to those obtained for FF earthquakes for all cases of hybrid frames. Further, it is also found that the pattern and number of semi-rigid connections in the hybrid frame have a vital role in seismic performance.