Commentary on Safety Factors in Plastic Design

Published specifications, design tables and texts have made plastic design of steel structures a standard practice for many structural engineers. Utilizing the ductility of steel in the stress range between yielding and strain hardening, the analysis of a continuous structure can be simplified to a problem of statics by assuming that, at ultimate load, the structure is composed of a system of simple beams spanning between plastic hinges, when such hinges have formed simultaneously or progressively in an ideally proportioned structure. Such a system is called a collapse mechanism. The ultimate load, or collapse load, is obtained by multiplying the service load by a load factor equal to the desired safety factor.