SORM correction of FORM results for the FBC load combination problem

The old stochastic load combination model of Ferry Borges and Castanheta and the corresponding extreme random load effect value is considered. The evaluation of the distribution function of the extreme value by use of a particular first order reliability method was first described in a celebrated paper by Rackwitz and Fiessler.

The method has become known as the Rackwitz-Fiessler algorithm. To make the paper selfcontained, the original RF algorithm is described herein allowing to introduce a modest but quite effective accuracy improving calculation following after termination of the RF calculations. The calculation gives a limit state curvature correction factor on the probability approximation obtained by the RF algorithm.

This correction factor is based on Breitung’s celebrated asymptotic formula. Example calculations with comparisons with exact results show an impressing accuracy improvement.