Retrofitting of Steel Moment-Resisting Frames under fire loading against progressive collapse

Generally, fire has been one of the most important threats which can lead to the collapse of structures. Due to the high temperature caused by fire, material strength decrease significantly and finally, could lead to failure of different members of a structure. When some parts of the structure are damaged, frames without sufficient resistance and ductility can progressively collapse. In this study, the retrofitting effect of a Steel Moment-Resisting Frame (MRFs) with X and inverted-V braces are investigated using dynamic explicit analysis for frames which had previously suffer progressive collapse under fire. Simultaneously, different bracing systems and fire conditions have been used. Furthermore, the effect of stiffness and strength of bracing systems were numerically analyzed. Studies show that the unbraced moment-resisting frame (MRF) lacks an effective mechanism for transferring loads from failed parts to the neighboring parts and the frame completely collapse due to the catenary actions under fire loading in central span. Systems braced with hat truss, due to their distance from the heated columns, have limited capacity to prevent the pull-in of columns in the heated floor. However, they can directly re-distribute vertical loads which are shed by buckled columns to the neighboring columns. But, their performance in central span fire scenario seems relatively better than edge span fire scenario. On the other hand, vertical bracing systems have a good effect and with restraining the columns, it can prevent them from pull-in. Also, they are able to effectively prevent the global collapse process. Furthermore, the stronger vertical bracing system can decrease the vertical movement of the heated columns and thus, enhance the re-distribution capacity of the frame. Accordingly, with combination of hat truss bracing system and vertical bracing system, new paths can be generated for redistribution of the forces which are shed by failed columns in the frame. At the end, comparing between X bracing systems and inverted-V bracing systems indicates that the frames with inverted-V bracing systems show more ductility.