Beam stiffness

Beams deflect when loaded and this must be limited to avoid damage and distress.

When a beam is loaded it will deflect. This deflection must be limited so that building occupants are comfortable and that building materials are not damaged. For instance, large deflections in a steel beam supporting a partition could cause unacceptable cracking in the plaster.

Beam deflections can be calculated and depend on the modulus of elasticity of the material, and the moment of inertia of the cross-section.

In general, the calculation of deflection is not straightforward. However, algebraic expressions are tabulated for many standard cases.

The deflection of a particular beam is inversely proportional to:

• modulus of elasticity
• moment of inertia

The modulus of elasticity is constant for all structural steels so the larger the moment of inertia the smaller the deflection.