Define structural steel; resistance after yielding

Structural steel

Structural steel is rolled steel (iron + 0.15-0.25% carbon and alloying elements such as Mn) of grades covered by IS 2062 that is used as the load-carrying material in buildings, bridges and towers.

Factor of safety

It is the ratio of the failure (or yield) stress of the material to the permissible (working) stress, $FOS = f_y/\sigma_{perm}$. In working stress design a single factor (about 1.67 on yield, i.e. $\sigma_{perm}=0.6f_y$) covers all uncertainties in loads and material.

Partial safety factor

In limit state design separate factors are used: $\gamma_f$ multiplies characteristic loads (e.g. 1.5 for DL and LL) and $\gamma_m$ divides characteristic material strength (e.g. $\gamma_{m0}=1.10$ for yielding, $\gamma_{m1}=1.25$ for ultimate strength, 1.25 for bolts, 1.25/1.5 for shop/field welds). Each factor reflects the uncertainty of one particular quantity.

Resistance after local yielding

Mild steel is ductile, with a long yield plateau (strain about 10-15 times the yield strain) and then strain-hardening up to $f_u$.

• At stress concentrations (holes, notches, welds) the local stress may reach $f_y$ and the fibre yields, but it keeps carrying the stress $f_y$ while the strain increases.
• The extra load is transferred to the adjoining elastic fibres, so stresses redistribute and the stress concentration is relieved.
• Residual stresses and bending stresses at connections also redistribute in this way, and a section can go on to form a plastic hinge and resist moment up to $M_p$.
• Hence local yielding does not lead to failure; the structure gives large deformation as warning before collapse.