Role of yield-to-tensile strength ratio in the design of steel structures

Avoidance of structural collapse during extreme events like earthquakes relies on the available ductility of the structure and its members and components. Over the years, the yield stress-totensile strength ratio (Y/T), which is a measure of the ductility of steel, has increased from 0.5 to 0.85 or more. This paper discusses; (a) Yield stress-tensile strength- Y/T ratio relations, (b) relevance of Y/T ratio in structural design and (c) treatment of Y/T ratio in the North American steel design codes. The focus of this paper is limited to ductility aspects of civil engineering structural designs, such as design of buildings and bridges. Statistical analysis of mechanical properties of steels shows that the Y/T ratio increases with increasing yield stress, and with increasing tensile strengths of steel. However, Y/T ratio is significant only in steel structural systems and members that are expected to withstand strain-hardening range stresses and strains, such as ductile elements of earthquake resistant structural systems, stress concentration situations like holes in tension members and beams, and structures designed based on plastic analysis. This paper discusses the required geometric and material conditions to prevent premature failures of steel members and structures and to ensure adequate ductility. (AU)