Abstract
During previous fire events such as the World Trade Centre Towers (WTC) 1, 2 & 7 in New York (2001), the Windsor Tower in Madrid (2005), and the Plasco building in Iran (2017), flames were observed to travel horizontally across the floor plate and vertically to different floors. Such fires are not considered as part of the traditional prescriptive structural design for fire. Recently, the Travelling Fires Methodology (TFM) has been developed to account for such horizontally travelling nature of fires. A dozen of studies have investigated the structural response of steel, concrete, and composite structures to a single-floor travelling fire. 5 out of 6 of the vertically travelling fire studies have been limited to the structures with a long span composite truss system as in the WTC Towers. The aim of this work is to investigate the response of a substantially different structural system, i.e. a generic multi-storey steel frame, subjected to travelling fires in multiple floors, and varying the number of fire floors, including horizontal and vertical fire spread. A two-dimensional 10-storey 5-bay steel frame is modelled in the finite element software LS-DYNA. The number of multiple fire floors is varied between 1 and 10, and for each of these scenarios, 5 different fire types are investigated. They include four travelling fire scenarios and the standard fire. In total, 51 fire simulations are considered. The development of deflections, axial forces, bending moments and frame utilization are analysed. Results show that the largest stresses develop in the fire floors adjacent to cool floors, and their behaviour is independent of the number of fire floors. Results indicate that both the fire type and the number of fire floors have a significant effect on the failure time (i.e. exceeded element load carrying capacity) and the type of collapse mechanism. In the cases with a low number of fire floors (1–3) failure is dominated by the loss of material strength, while in the cases with larger number of fire floors (5–10) failure is dominated by thermal expansion. Collapse is mainly initiated by the pull-in of external columns (1–3-floor fires; 1–9-floor fires for 2.5% TFM) or swaying of the frame to the side of fire origin (5–10-floor fires). This study has assessed a different structural form compared to previous literature under an extensive range of multiple floor travelling fire scenarios. We find that although vertically travelling fires result in larger beam axial forces and initial deflections, simultaneous travelling fires result in shorter failure times and represent a more onerous scenario for the steel frame investigated.
Highlights
The understanding of the fundamental mechanics of a whole building behaviour in fire has significantly increased in the last decades, especially following the Broadgate fire in London in 1990 [1,2], which took place in a 14-storey steel framed building under construction
The development of the beam mid-span displacements, axial forces, bending moments and column lateral displacements for the 5 bays in Floor #5 for a 25% travelling fire scenario is shown in Fig. 4, where comparison is made between a single floor and 5-floor fire scenarios
The results show that under multiple floor fires the structural response is significantly different compared to a single floor fire
Summary
The understanding of the fundamental mechanics of a whole building behaviour in fire has significantly increased in the last decades, especially following the Broadgate fire in London in 1990 [1,2], which took place in a 14-storey steel framed building under construction. Even though the majority of the steelwork was unprotected and active fire protection methods were not functional, the building showed robust behaviour and did not collapse. Following this accident, full-scale tests of various multi-storey buildings were carried out in Cardington between 1994 and 1999 [3]. The Broadgate fire and Cardington tests showed that steel framed buildings as a whole performed better in fire than indicated by the prescriptive design of individual members. The collapse of the buildings during these accidents showed that for buildings with non-conventional structural layout (unlike in the Broadgate fire and Cardington tests) the prescriptive guidance assuming single elements can be non-conservative [5].
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