Tensile Membrane Action and the Performance of Composite Slabs in Fire

A common strategy for reducing passive fire protection on composite floor slabs is to specify protection only to primary and secondary beams on the column grid. While unprotected secondary steel beams lose much of their strength at high temperatures, large deflection of the central zone of the slab causes tensile membrane action (TMA), which enhances its load capacity compared with that of the small-deflection yield-line mechanism, and compensates for this loss of strength. Various simplified models of TMA, starting with the optimal small-deflection yield-line mechanism, have been developed as design methods for fire resistance, based mainly on research published in the 1960s. These methods have been published as design guides in the UK, the European Union and New Zealand, and have been implemented in public-domain design software. A critical appraisal of the original research, and the design methods developed from it, suggests that it is in need of re-development from the first principles of structural mechanics. Disparities exist in the logical approach, both in the treatment of enhancement of yield-line capacity with deflection, and in the limiting deflection corresponding to the "Integrity" limit of fire resistance. While the basic simplification of TMA is justifiable, the re-development needs to follow the sequence of behaviour as a composite slab with unprotected downstand beams is heated in fire. Initial studies of the yield-line behaviour of non-composite slabs and the enhancement of their capacity produced by TMA at finite deflections removed logical flaws from the existing treatments, and allowed the sequence of fracture of concrete and reinforcement to be tracked as deflections increase. However, since the compression zones of a concrete slab heat slowly in a fire and concrete tension cannot be considered in a design analysis, the effects of fire were not directly addressed. In this paper unprotected composite steel beams are included in a composite slab panel, and are heated as fire progresses. The initial yield-line mechanism differs from that of a non-composite slab, and varies with load intensity and beam temperature. This mechanism is fixed, but the slab deflects further as the beam temperatures rise. The formulation allows plasticity and fracture of the reinforcement, and the compressive strength of the concrete. Stresses around the yield lines can be monitored from negligible deflection to structural failure, and provides a rational way of predicting when integrity failure via a through-depth tensile crack will occur.