Analysis of Hollowcore Concrete Floor Slabs under Fire

Andrew H. Buchanan

Professor, University of Canterbury, Department of Civil Engineering, Christchurch, New Zealand

Peter J. Moss

Associate Professor, University of Canterbury, Department of Civil Engineering, Christchurch, New Zealand

Rajesh Dhakal

Senior Lecturer, University of Canterbury, Department of Civil Engineering, Christchurch, New Zealand

Jeremy Chang

PhD candidate, University of Canterbury, Department of Civil Engineering, Christchurch, New Zealand

Precast prestressed hollowcore concrete floor systems have become very common in New Zealand and in many other countries. Hollowcore concrete floor systems consist of several hollowcore concrete slabs with or without a layer of reinforced concrete topping. The benefits of using hollowcore concrete floor system are the low onsite labour cost, low self-weight, consistent quality, and economical use of concrete. The structural behaviour of a hollowcore concrete floor system under fire is complicated, and precise computer models for simulating the structural behaviour of hollowcore concrete slabs under fires have been developed to improve the understanding of this behaviour. There are many existing studies investigating this behaviour with different approaches. However, very detailed finite element analyses for modelling the structural fire behaviour of hollowcore concrete slabs are too time-consuming to apply in the day-to-day design process. At the other end of the spectrum, simplistic approaches using a simple code rules are insufficient to capture the thermal expansion across the units or the effects of the support conditions. This paper aims to propose a simple yet sufficiently accurate method for designers to model the structural fire behaviour of hollowcore concrete slabs, and then based on the simulation results to provide some recommendations on the fire design of hollowcore concrete floor systems.

It is widely recognised that the behaviour of hollowcore concrete slabs under fire is more complicated than that of solid slabs. The cavities at the centre of the slabs cause discontinuity of the thermal transfer, and the thermal gradient needs to be addressed correctly to accurately model the temperature induced mechanical strains in the webs. The support conditions also have significant influence on the structural behaviour of floors, this is especially so in hollowcore concrete floor system, and the effect of the support conditions should be considered in design. The presence of prestressing stress has been proven to also considerably influence the predicted overall structural performance as the hollowcore concrete units have no reinforcing and the resistance to tensile stresses comes from the prestressing tendons. Therefore, the fire design of the hollowcore concrete floor system needs to be able to accommodate different support conditions in different buildings, and the designers must recognise the fact that prestressed structural members demonstrate different behaviour to ordinary members.

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