Design strengths
The design strength of structural steel is assumed to be equal to the yield stress.
The structural properties of different materials are conveniently expressed in terms of their design strength. This does not necessarily correspond directly to failure of the material, but rather to some point at which the structural performance becomes unacceptable. In the case of structural steel this is defined as the yield stress of the material. The quality of structural steel is carefully controlled with a high degree of quality assurance, and its design strength, py, is expressed as a guaranteed minimum yield stress. Values of py corresponding to different grades of steel are given in BS 5950.
For composite construction the design strength of concrete is taken as its characteristic strength divided by a safety factor.
Where steel and concrete are used compositely the strength of both steel and concrete must be considered. Compared with steel, concrete is much more variable in terms of its structural properties. The specification of concrete is in terms of a characteristic strength and particular samples of concrete may exhibit strengths below, as well as above, those specified. In this case it is therefore necessary to apply a further factor of safety to the material properties. A factor of about 1.5 is typically applied to the characteristic strength to obtain the corresponding design strength.
