Tensile membrane action of reinforced concrete slabs at ambient and elevated temperatures.

The aim of this research was to extend the knowledge of the performance of slabs at ambient and elevated temperatures, to look at their failure mechanisms and the influence of temperature on their performance. This involved undertaking a programme of experimental work in which a series of slabs were tested in a custom made loading rig at ambient and elevated temperatures. An in depth investigation of the BRE Simplified Design Method was undertaken.
This thesis presents the results from 15 small-scale tests conducted on horizontally unrestrained slabs at ambient temperature which were subjected to large vertical displacements. All the tested slabs showed a load-carrying capacity far greater than the design capacity using the well-established yield-line theory. The results of the tests have been compared to the Simple Design Method which incorporates membrane action of composite floor slabs into the estimation of their load capacity in fire and shows that the design method compares well with experimental results and is generally conservative. The numerical results at ambient temperature showed that tensile strength of concrete is significant at low deflections and not a significant factor at high displacements as the strength and type of wire used is important as it affects the load capacity of the slab.
This thesis also presents the results from a series of tests conducted on horizontally unrestrained slabs at elevated temperatures. The purpose of these tests was to investigate the influence of thermal curvature on the failure mechanisms of rectangular slabs, since this is not explicitly allowed for in the simplified design method. Observations from the high temperature tests have shown that the mechanism of failure differs from that assumed in the Simplified Design Method. The observed slab behaviour at high temperatures is that high double-curvature deflection is created quickly, and this leads to frill-depth cracking across the short span of the slab, but the association with a yield-line mechanism is much less obvious. The results from the numerical study showed that up to temperatures of 200°C, the influences of concrete tension, load ratio, type of wire on the displacements is negligible, and that thermal gradient dominates the slab behaviour.
The studies have proven that the proposed design method is not always conservative and is only based on one pre-determined collapse mode. At high temperatures, the experimental and numerical work has shown that the slab behaviour is dominated by different mechanisms. The design method is based a final failure mechanism of the slab which was observed in the tests during the latter stages of the heated tests which questions the applicability of the method. The Vulcan predictions were accurate at elevated temperatures even though it does not consider bond effects.