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Realistic Performance Requirements for Steel in Structures

Steel offers significant advantages for construction: it has high strength and stiffness and ample deformation and stress redistribution capacities for many applications, it does not crack or otherwise fracture under normal conditions, and is available in many grades and geometric forms. On the other hand, structures will be subjected to high deformation demands due to various conditions during fabrication, construction and service. A dynamically loaded structure may experience fatigue or fracture; seismic events create major deformation demands on structural members and connections; and fabrication methods such as welding require very large local deformability of the steel under certain conditions. However, the chemical composition and metallurgical structure of steel are very complex, and the models that are used by codes to reflect the mechanical response bear little resemblance to what it will experience under actual conditions. For example, steel is anisotropic, as a result of production operations and other plastic deformation effects. Although the anisotropy normally is of no consequence, it will affect the response of the steel in many loading and deformation demand situations. For another, the behavior of steel is a function of deformation history, to the effect that it may respond as a high strength, low ductility material, given the prior occurrence of large displacements. The paper addresses the properties of a range of structural steels, how these are incorporated into design standards and how the standards define deformation characteristics and demands. Several examples from practice illustrate the primary behavioral characteristics. However, most of today's design requirements are strength-oriented, with focus on element load-carrying and load-transfer capacities. With the current move towards performance-based design standards and especially the demands imposed by seismic and other extreme load conditions, it is clear that deformation considerations need to be better recognized and incorporated into the structural design criteria.