Fox, David

Stratford, Tim

Usmani, Asif

English

Fire induced geometrical changes produce large deflections in floor slabs and even if there is significant loss of strength, the change of shape of the slab allows stability to be maintained by the slab acting as a tensile membrane. This of course depends upon the amount and arrangement of reinforcement and the end restraint conditions of the slab. If the slab is continuous over the supports and the continuity of reinforcement over the support can also be assumed then the "anchoring" of the tensile membrane action is not in doubt. There is however a mechanism by which even simply supported slabs can produce a degree of tensile membrane action. In this case although there are no lateral (or in-plane) reactions available at the supports, there is a self-equilibrating mechanism that occurs in the form of a "compressive ring" between the central and perimeter regions of the slab which provides restraint to the membrane tensions (rather like the rigid ring surrounding a trampoline). Some authors have used this to develop design methods for non-continuous simply supported slabs in fire; however there is a great deficit of knowledge regarding the quantification ofthis effect. One of the key parameters that may govern the magnitude of "membrane enhancement" available for simply supported slabs is the ratio of slab thickness to its "overlap length" from the edge of the support. This paper will present initial conclusions obtained from this study and discuss the implications for design

Understanding the Compressive Ring in Reinforced Concrete Slabs in Fire

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