Chapter 6: High Strength Steel Structures

摘要

The efficiency of steel structural members and connections can in many cases be enhanced by using steels with high values of yield stress and/or tensile strength. Good economy can beachieved by utilizing the superior strength-to-weight ratio of such steels, particularly inrelatively heavy construction. In light gauge construction, high strength steels offer ease ofhandling and quick construction.The mechanical properties, notably strength and ductility, of steel depend primarily on itsmicrostructure, that is the arrangement and chemical composition of the microscopic crystalsof which the steel is composed. The microstructure depends mainly on: 1. chemical composition, 2. thermal history and 3. work-hardening history, and can be changed greatly by changing any of these influencing factors. With respect to chemical composition, the mechanical properties are most significantly influenced by the carbon content. The strength increases with increasing carbon content but the weldability decreases. If the steel is to be weldable, the permitted range of carbon content is relatively limited and consequently, the most common processes for enhancing the mechanical properties of steel are by changing the thermal history and/or the work-hardening history. In relatively heavy construction, say plate thickness of 5 mm or larger, the most common process for enhancing strength is by quenching and tempering. This is a thermal process in which the steel is first cooled rapidly (quenched) to achieve high strength and subsequently heated (tempered) to partially anneal the steel and gain ductility and weldability. The most common grades of structural quenched and tempered steels have yield stress values in the range from 420 MPa to 690 MPa. The process for producing quenched and tempered steels and the design of such steels are described further in Section 6.2. Cold-reduced light-gauge steels fall in the category of work-hardened steels. These steels have typical thicknesses less than 3 mm and yield stress values in the range from 300 MPa to 550 MPa depending mainly on thickness. They gain increased strength by the plastic deformations occurring during the thickness-reducing rolling process. The production of these steels and the design of such steels are briefly described in Section 6.3.