Following full-scale fire tests on a steel-framed building, together with o
bservations from real fires, it has been shown that membrane action, at lar
ge displacements, of composite floors comprising steel deck, concrete, and
anti-crack mesh, is extremely beneficial to the survival of the building. I
t was therefore decided to review previous research conducted on unrestrain
ed concrete slabs, under large displacements, at normal temperatures. It wa
s found that the assumptions used to develop previous theoretical predictio
ns for the load-carrying capacity, for a given vertical displacement, are o
nly valid for square slabs and do not conform to test observations for rect
angular slabs. A new theoretical approach is therefore presented which is v
alid for both square and rectangular slabs and conforms to the mode of beha
viour observed in tests. The design method is shown to give excellent corre
lation with published test data. A prediction for ultimate collapse of the
slab due to fracture of the reinforcement is also presented, which limits t
he allowable mechanical strain in the reinforcement. Comparison with availa
ble test data shows that this prediction is always conservative. (C) 2001 E
lsevier Science Ltd. All rights reserved.