Plastic Frame Analysis under Fire Conditions

Abstract

This paper presents the use of plastic analysis methods for the calculation of critical temperature distribution for structures at collapse under fire conditions. The methods are based on the well-known plastic hinge concept using both elastoplastic and upper-bound approaches. The simple concept of plastic load factor for collapse analysis is extended to elevated temperature analysis which leads to the development of the unit load factor method for critical temperature factor calculation. The critical temperature factor is used to indicate the factor of safety for a structure under certain fire scenarios. In this approach, linearly varying distributed loads can be included in the analysis. The upper-bound approach is based on the conventional collapse mechanism calculation method, modified to include the temperature-dependent material properties. It is found that the critical collapse mechanism may become noncritical when temperature changes. A demarcation temperature multiplier is proposed to monitor such a phenomenon. Worked examples for both the elastoplastic and upper-bound approaches are presented.