Reliability for load and non-load induced strength deterioration - Derivation of partial safety factors

Computation of structural reliability under non-stationary conditions is known for a number of important loading processes and deteriorating resistances. While there is a one to one correspondence between theoretical results and practical design rules via partial safety factors - at least ideally - no such relationship has been established for time-variant problems. Making use of the outcrossing approach together with FORM/SORM methods it can, however, be shown that time variant failure probabilities can be factored into five different terms representing the coincidence probability, the frequency of load changes, a local failure probability in the most likely failure point, a SORM correction factor and a term which accounts for the time behaviour of the structural state function around the critical point. This enables to define partial safety factors in the point in time distributions as opposed to factors for normal extreme value design where the partial safety factors for loads are related to the extreme value distributions. The concept is illustrated at three examples.