This paper presents the experimental and numerical investigation on resistance of two-storey steel sub-frames subjected to a middle column removal scenario. Two different types of 1/2 scaled steel sub-frames with seismic configuration were fabricated and tested, they are: (1) welded connections with gross beam section (GBS) and (2) welded connections with reduced beam section (RBS, typically used for energy dissipation in earthquake design). Based on the test results, finite element models using explicit software LS-DYNA which can accurately replicate the response of the frames were created and validated. Using the validated model, the parametric studies were also performed to investigate the effect of different parameters on the progressive collapse resistance of the frame. Base on the experimental and numerical studies, the failure mechanism and collapse resistance capacities of these two types of frame systems are first time investigated. The test results indicated that the welded connections with RBS exhibits better performance due to the guaranteed formation of plastic hinges at the location of reduced section, as well as the avoidance of welding heat effect and brittle weld fracture at beam to column connections. Thus, RBS exhibits larger deformation capacity which is favorable in mitigating the possible progressive collapse.
External installation of steel braces is one of the effective approaches to increase the lateral load resistance of the steel moment-resisting frames. However, the effects of existence of steel braces on the robustness of steel moment-resisting frames to resist progressive collapse is still not clear as little study has been carried out. To fill this gap, in this paper, six multi-story steel moment-resisting subframes (three bare frames and three braced frames) were fabricated and tested. Test results indicated that the specimen with reduced beam section in the connection zone performed best among three types of connections, due to the guaranteed formation of plastic hinges at the location of reduced section and avoiding brittle fracture of weld at the connection. Experimental results proved that steel braces could increase the load resisting capacity by 45.1% and 83.9% of the frame with weld connection and end plate connection, respectively. As the gusset plate restricted the rotation of the plastic hinges in the second story of the braced frames with V-shaped bracing, which decreased its deformation capacity and degraded its catenary action capacity. Actually, the ultimate load of the braced frames with Vshaped bracing is only 87.5% of that of the counterpart without any braces. As the compressive braces were severe buckled before the displacement reached 0.4% of the beam span, it has little effects on yield load but increases the initial stiffness of the bare frames. Thus, majority of the benefits of the bracing system were attributed into the tensile braces. Moreover, the analytical results evaluated the differences in load resistance and development of load resisting mechanisms in different stories.Furthermore, the contribution of compressive and tensile braces was de-composed individually by analytical analysis.
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