Ultimate limit-state design of concrete structure

Ultimate Limit-State Design of Concrete Structure

Structural concrete members often show great deviation in structural performance from that predicted by the current code of practice. In certain cases the predications considerably underestimate the capabilities of a structure or member, while in others the predictions are unsafe as they overestimate the member's ability to perform in a prescribed manner. Clearly, a rational and unified design methodology is still lacking for structural concrete.

The adoption of the limit-state philosophy as the basis of currentcodes of practice for the design of concrete structures expressesthe conviction that this philosophy is capable of leading to saferand more economical design solutions. After all, designing astructural concrete member to its ultimate limit state requires theassessment of the load-carrying capacity of the member and thisprovides a clearer indication of the margin of safety againstcollapse. At the same time, the high internal stresses whichdevelop at the ultimate limit state result in a reduction of both thesize of the member cross-section and the amount of rein-forcement required to sustain internal actions. (Admittedly, thelatter economy and, of course, safety itself are dependent on theactual factor of safety adopted; nevertheless, the more accurateestimate of the true failure load provides an opportunity to reducethe uncertainties reflected in the factor of safety in comparisonwith, say, elastic design calculations.

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