Structural steel is the heart of modern engineering. The European designation 'S' (Structural) is followed by a number indicating the minimum yield strength in MPa, defining the exact point at which the material passes from the elastic to the plastic phase.
The most widely used grades in construction are S235, S275 and S355. The safety of steel structures (especially in seismic zones) is based not only on absolute strength but on ductility: the ability to deform in the plastic range, absorbing energy before reaching failure. Steels of these grades ensure a wide yield plateau and a significant percentage elongation before collapse.
To form the joints of a framed structure, the members must be readily weldable. As strength increases (e.g. moving from S235 to S355), the carbon content generally rises, which makes the material harder but more brittle in the heat-affected zone (HAZ) of the weld. For this reason, codes impose strict limits on the carbon equivalent (CEV), ensuring safe welds without the need for excessive preheating.
Bare steel loses about 50% of its load-bearing capacity at 500°C. To meet REI/R requirements, protection by intumescent paints, calcium silicate boards or fire-resistant renders is mandatory.
Standards
European and international references applicable.
Physical properties
Usage environment
Suitable for all civil and industrial structural exposure environments. Requires active protection against corrosion (galvanizing, painting, intumescents) and fire: bare steel loses about 50% of its load capacity at 500 degrees C.