Effects of Steel Braces on Robustness of Steel Frames against Progressive Collapse

External installation of steel braces is an effective approach to increase the lateral load resistance of 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 because little study has been carried out. To fill this gap, in this paper, six multistory 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. 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 V-shaped bracing is only 87.5% of that of the counterpart without any braces. Because the compressive braces were severely buckled before the displacement reached 0.4% of the beam span, it had little effect on yield load but increased the initial stiffness of the bare frames. Thus, a majority of the benefits of the bracing system were attributed to 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 decomposed individually by analytical analysis.