The safety of structures is described and estimated by the probability of failure of the structures. The optimum structure is defined as the structure consisting of members possessing approximately equal values of the probability of failure. A computer program which can calculate, iteratively, the probability of failure of the individual members as well as of the whole structure for the spaced frame structure is generated herein. The weakest link model is assumed to be available both for statically determinate and statically indeterminate structures. The dead load, seismic load and the strength of material are considered to be log‐normally distributed random variables. By changing the cross‐sectional area of the members, the probability of failure of individual members can be kept to a certain pre‐decided value, for example, 10–5. An example of the design of a space frame is presented. All the wide‐flange cross sections are selected from the AISC design manual.