Post-blast fire resistance of low-rise buildings through membrane action of composite floor slabs

Sever fires in buildings can lead to local failures, instability, partial or total collapse of the structure. In majority of the times, fire is a secondary event, after blast or impact, while the building has experienced some damage. Examples of widely known events include the 1968 Ronan Point collapse in the UK, the 1995 Oklahoma City bombing, the World Trade Center Collapse in New York in 2001, the 2014 collapse of a building at New York’s Harlem neighborhood due to a gas explosion, and the recent 2015 collapse of a building at New York’s East Village also due to a gas explosion.
The initial shock to the building can be conservatively modeled by removing an intermediate vertical supporting element (i.e. loss of load-carrying capacity in a critical element), leading to an increased span for composite floor slabs. In a lowrise building, if there is enough reinforcement throughout the slab and enough continuity and restraint, despite large deflections that will develop, the slab is capable of carrying the loads by membrane action. Fundamentally, the floor system behaves as an inverted dome structure with radial tensile forces and a compressive hoop stresses. This holds true at ambient temperature, yet a similar resisting mechanism forms during fire. Previous research and experimental work shows that fire performance of composite floor slabs can be used to reduce the fire protection requirement of the steel elements, i.e. the designer should take advantage of reserve capacity in the composite floor slab membrane action.
The utilization of membrane action in the design of composite floor slabs has been used, to some extent, for mitigating collapse from single events (blast or fire only). Given that, often the initial blast is followed by a secondary fire event, this work investigates the system-level performance of low-rise damaged buildings subject to post-blast fires. The hypothesis is that, when incorporated in the design, low-rise buildings can withstand the post-blast fires through membrane action of composite floor slabs. Application of this concept, within a performance-based framework, can be used to avoid progressive collapse, or at the minimum increase fire resistance to allow for safe evacuation. This work investigates the design requirements for beam sizes, fire protection, concrete reinforcement and cover thickness to develop membrane action for a pre-defined fire resistance rating under cascading post-blast fires.