Complex and comprehensive method for reliability calculation of structures under fire exposure

Numerous recent studies have pointed out that in case of extreme effects (such as fire or seismic effects) the reliability level of structures is lower than in case of conventional effects. Proper estimation of reliability in fire design situation is complicated since there is no comprehensive methodology defined for it; and the commonly used methods apply great simplifications. In former studies the reliability calculations are carried out mainly for simple, separated elements, while simple structural behaviour is assumed. Further research work is needed to refine rules and define targeted safety levels required by fire codes. The current study presents methodology for calculating the reliability of structures subjected to fire; its main novelties are as follows: a) the reliability calculation is not limited to a single, isolated element but the whole structure as a complex system is considered; b) the methodology is capable of taking any type of fire curve into account; c) in the reliability analysis nonlinear global analysis of the whole structure is involved; d) the structural reliability is assessed on time basis. To justify the applicability of the proposed methodology, reliability analysis for a tapered steel frame protected by intumescent coating is presented as an illustrative numerical example. Probability of its failure is calculated by using First Order Reliability Method; the computed failure probabilities are verified by using Monte Carlo Simulation. FORM approximation underestimates the failure probability, the observed error is within −1% to −34%. Based on the results, it has been found that for low and moderate consequence classes the calculated reliability indices are in better agreement with the recommendations of ISO 2394 standard and Joint Committee on Structural Safety than with the values recommended in EN 1990:2002 standard.