Steel is a ductile material by nature, which also has a stable behavior under load investments and has a convenient strength-to-weight ratio.

Steel can accept significant deformations beyond the elastic limit without failing, so it has the capacity to allow inelastic deformations that may be required. It can be used to construct statically indeterminate structures that meet seismic design requirements. It is therefore very convenient for buildings located in areas of high seismicity. However, the intrinsic ductility of steel is not necessarily preserved in the finished structure, so care must be taken during design and construction to avoid the loss of this property.

It is this property, intrinsic characteristic of structural steel, that does not exhibit any other construction material in a completely clear way, and that makes possible the application of plastic analysis to the design of structures.

Not all steels meet the ductility requirements that are required in areas of high seismicity; specifically, a behavior like the one shown in figure 2 is required in which the graphical effort of forming a specimen during the stress test is schematically represented.

The fact that structural steel is ductile does not imply that the structure made of this material is also ductile; on the contrary, to achieve this, a series of precautions, sometimes not easy to achieve, must be taken into account, which are what make a steel structure considered suitable for operation in seismic areas.

Ductility depends primarily on the chemical composition of steel, the metallographic structure and the shape, size and distribution of nonmetallic inclusions and segregations. This property has acquired a fundamental importance in the current criteria of seismic design of structures.