Progressive Collapse Mechanisms of Steel Frames Exposed to Fire

Abstract

OpenSees is an open-source object-oriented software framework developed at UC Berekeley. The OpenSees framework has been recently extended to deal with structural behavior under fire conditions. This paper summaries the key work done for this extension and focuses on the application of the developed OpenSees to study the fire-induced progressive collapse mechanisms of steel structures. The implicit dynamic analysis method (Newmark method) is applied and the influences of the load ratios, beam sizes and fire scenarios on the collapse behavior of frames are investigated. Single-compartment fire scenarios in the central bay and edge bay are considered, respectively. A total of four collapse mechanisms of steel frames are proposed by varying the three influencing factors. Most of the collapse of steel frames is triggered by the buckling of the heated columns. The thermal expansion of heated beams at early heating stage and their catenary action at high temperature have great influences on the collapse mechanisms. The most common collapse mode of steel frames are in the form of lateral drift of frames above the heated floor together with downward collapse of frames along the heated bay. As the load increases, the collapse behavior of structures is dominated by a downward collapse of the whole frame with little sign of the upper frame drift. The collapse modes of steel frames with strong and weak beams are column failure mechanism and beam failure mechanism, respectively. The former mechanism is due to the buckling of the columns below the heated floor represented by a global collapse of the frame and the latter is initiated by the premature development of plastic hinges at the ends of beams denoted by an obvious lateral drift of the heated floor. Generally, the edge bay fire is more prone to induce the collapse of structures than the central bay fire. It is found that the most dangerous situation is the frame subjected to high load ratios exposed to a central bay fire where its progressive collapse may occur as early as 250°C.