Upcrossing Rate Solution for Load Combinations

An exact expression for the mean upcrossing rate of a sum of two poisson square-wave process representations of stochastic loading functions is used to explore the accuracy of an earlier approximation (due to Wen) of the extreme value distribution of such processes. The approximation is found to be accurate over a broad parameter range despite the fact that it ignores the randomness of load durations and implicitly assumes very infrequent load events. The limits of the mean upcrossing rate as an approximation to the extreme value distribution are also studied.