Structural design

Abstract

Traditional structural steel designs were based on the idea that if the calculated stress in any part of the structure did not exceed an allowable stress, it would safely support the load it was designed to carry. In this allowable stress or working stress, as it is sometimes called, tension and compression was set as a fraction of the yield stress or tensile strength. For pressure vessels and some structures, this allowable or design stress was once set at a quarter of the ultimate tensile strength, but later this was changed to two-thirds of the yield stress. This approach is called the elastic method of design because under the design load nowhere in the structure is the nominal stress intended to exceed the yield stress; but clearly there will be stress concentrations at bolt and rivet holes and other openings where the stress may be up to yield stress. Although being a simple method of design, the elastic method produces rather inefficient steel frame structures in terms of the weight of steel used to support a load. By making the end joints rigid, the maximum moment is reduced and moments are introduced at the ends; a smaller beam section can be used and more effectively since more of its length is working nearer its design strength. Beyond this, rigid joints offer a further opportunity in steel that was to be exploited by the plastic method of design.