This paper presents an approach to derive probability models for use in structural
reliability studies. Two main points are made. First, that it is possible to
translate engineering and physics knowledge into a requirement on the form of a
probability model. And second, that making assumptions about a probability model
for structural failure implies either explicit or hidden assumptions about material
and structural properties. The work is foundational in nature, but is developed with
explicit examples taken from planar and general stress problems, the von Mises
failure criterion, and a modified Weibull distribution.