**Behaviour of steel beams**

Laterally stable steel beams can fail only by (a) Flexure (b) Shear or (c) Bearing, assuming the local buckling of slender components does not occur.

These three conditions are the criteria for limit state design of steel beams.

Steel beams would also become unserviceable due to excessive deflection and this is classified as a limit state of serviceability.

The factored design moment, M at any section, in a beam due to external actions shall satisfy

**M≤ M**_{d}

Where **M**** _{d}**= design bending strength of the section

**Design strength in bending (Flexure)**

The behaviour of members subjected to bending demonstrated

• When the beam is adequately supported against lateral buckling, the beam failure occurs by yielding of the material at the point of maximum moment. The beam is thus capable of reaching its plastic moment capacity under the applied loads. Thus the design strength is governed by yield stress and the beam is classified as laterally supported beam.

• Beams have much greater strength and stiffness while bending about the major axis. Unless they are braced against lateral deflection and twisting, they are vulnerable to failure by lateral torsional buckling prior to the attainment of their full in plane plastic moment capacity. Such beams are classified as laterally supported beam.