Deflections of concrete floor slabs exposed to standardised fires and some implications for design

Fourteen simply supported precast reinforced concrete floor slabs of 4.5m span and 900mm width were exposed to two standardised heating regimes used in fire resistance furnace tests. The tests were designed to show the effect of varying the slab thickness, type of concrete, imposed load, nature of fire exposure and soffit protection on the mid-span flexural deflection and axial movements of the slab ends. Measured deflections showed that during the 90 minute design period of fire resistance thermal bowing was dominant and the effect of the 1.5 kN/m2 design live load was small. The NPD hydrocarbon fire exposure caused a doubling of the flexural deflections achieved using the standard BS 476: Part 8 (now Part 20) fire exposure in the first 20 minutes of exposure. Some implications for the design of buildings are discussed by reference to design examples

Gordon M E Cooke BSc, PhD, CEng, MIMechE, MICE, FIFireE
Visiting Professor, Department of Civil Engineering, City University, London