Collapse analysis of regular and irregular tall steel moment frames under fire loading

Abstract

This study is carried out to evaluate the progressive collapse of steel buildings under fire event. To this end, a 15-story steel structure with moment-resisting system and composite floors is considered. The effects of various parameters such as beam section size, gravity load ratio, vertical irregularity of resisting system, and location of fire compartments on collapse modes are investigated numerically. Different temperature-time curves are defined across the composite floors according to the Eurocode 4. It is found that local collapse of the frames at the ground floor fire is triggered by the buckling of the interior heated columns at approximately 540°C. The redistributed loads by floors delay the global collapse at least 45 min. Increasing gravity loads accelerates the global collapse of the frames significantly. The heated columns of the middle floor buckle at higher temperature compared to the ground floor heated columns and no global collapse occur due to this scenario. In general, the potential of collapse of the regular and irregular frame due to fire in the edge bay is higher compared to the fire in the middle bay. It is also found that the local and global collapse of regular frames occur earlier than irregular frames. KEYWORDS collapse mode, column buckling, concrete slab, progressive collapse, tall steel frame, vertical irregularity 1 | INTRODUCTION The fire-induced collapse of real tall steel buildings such as the World Trade Centre (WTC) Towers in New York (2001), Windsor Tower in Madrid (2005), and the Plasco building in Tehran (2017) provided important information about the stability and progressive collapse of high-rise buildings during fire events. The progressive collapse of structures is defined as "the spread of an initial local failure from element to element, resulting eventually in the collapse of an entire structure or a disproportionately large part of it". [1] The level of damage to the buildings due to fire can be extensive and is dependent on various factors such as material properties, intensity and duration of fire, and also external factors such as wind conditions, firefighting operation, and building height. While steel materials have been widely used in many tall structures, temperature sensitivity of steel material is a major disadvantage, since the mechanical properties of steel deteriorate significantly at high temperatures. According to Eurocode 3, [2] the yield stress of steel remains unchanged up to 400°C. But at 700°C and 900°C, it reduces to 23% and 6% of ambient temperature yield stress, respectively. Also the yield stress and modulus of elasticity of steel material reach almost zero at 1,200°C.Fire and heat in buildings cause changes in the geometry (thermal expansion) and material properties (reduction of stiffness and strength) of structural members. In the frame structures in early stages of fire, the thermal expansion effects are crucial, and then, if further increase in temperature occurs, the reduction of stiffness and strength of materials will be more import...

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