Dominant mechanisms in stochastic plastic frames

In structural system reliability analysis it is usually assumed that the elastic properties of the members are deterministic while the loading parameters and/or member resistances are assumed to be stochastic in nature. Theoretically, all potential modes of failure must be considered in an estimation of system failure probability. In reality, however, the probability of occurrence of many of these failure modes is so low, compared with the structural system failure probability, that they make only an insignificant contribution, and hence can be neglected. Thus it is usually sufficient to consider only the so-called dominant failure modes in a system reliability calculation. For rigid frames composed of ideal plastic or elastic-plastic material the dominant failure modes of interest in the reliability assessment are drawn from the set of all plastic collapse mechanisms. Several empirical procedures have been previously developed to enumerate these dominant mechanisms with varying degree of success. Although in principle it is possible to enumerate all dominant modes relative to some criteria, in practice no method has yet proved to be applicable to large frames, and the validity of the identified mechanisms is often uncertain. This paper proposes a modification of the existing truncated enumeration method (TEM) applied to plastic frames. An example is demonstrated, and the results compared to other methods recently developed. The results show that almost all the dominant mechanisms may be successfully found. The proposed procedure can be systematically programmed and may be applied to estimate reliability of large structural systems.