The structural reliability is evolved thoroughly so far and is currently categorized under three different levels (Level 1, Level 2 and Level 3) that depend mainly on the degree of sophistication of the analysis and the available input information. Level 3, which sometimes is referred to as the fully probabilistic approach, is the most demanding in terms of the required input information. However, even if the input information is available, the analytical or numerical evaluation of the resulting integrals for estimating the probabilities of structural failure is extremely difficult.