Properties of structural steel

Structural steel is iron with a small, controlled amount of carbon (typically under a quarter of a percent for weldable grades) plus manganese and traces of other elements. That chemistry gives the combination civil engineers want: high strength, ductility, toughness and weldability. Steel is the same in tension and compression, which concrete is not, so it suits members that reverse stress, like truss diagonals and crane girders.

A tension test shows the key properties. Stress rises linearly with strain up to the yield stress fy; the slope is Young's modulus, about 200 GPa for all common grades. Mild steel then shows a yield plateau where strain grows with no extra stress, followed by strain hardening up to the ultimate tensile strength fu, and finally necking and fracture at an elongation of around 20 percent. Indian grades are named by yield strength, for example E250 and E350. Higher grades give more strength but the same stiffness, so deflection and buckling do not improve with strength.

Other properties matter in specific situations. Toughness, measured by the Charpy impact test, controls brittle fracture in cold climates and thick welded plates. Fatigue strength governs bridges and crane runways. Steel loses strength above about 400 °C, so fire protection is needed. Corrosion requires painting, galvanising or weathering steel. Density is about 7 850 kg/m³, Poisson's ratio 0.3 and the coefficient of thermal expansion about 12 × 10⁻⁶ per °C.