Estimating sizes

Simplified procedures can be used to estimate the required size of beam section.

In many cases it is not necessary to perform detailed calculations to determine beam sizes, and simpler methods can be adopted. These include rules of thumb which enable a simple estimate of approximate section sizes, and safe load tables which provide a rigorous alternative to design calculations for standard cases.

Rules of thumb provide an estimate of the required depth of different types of beam in relation to span.

A typical structural frame may include both primary (main) and secondary beams. Roof beams tend to be more lightly loaded than floor beams, and the required section is therefore generally somewhat smaller. Some guidance on maximum beam spans and the required provide depth (as a proportion of the beam span) is given in the table below.

Safe load tables provide more rigorous guidance on required sizes.

The bending strength of a beam is related primarily to the material strength and the section modulus, which itself depends on the size and shape of the cross-section. It is therefore possible to calculate the maximum bending strength for any cross-section, assuming that there is possibility of lateral-torsional buckling. Such tables enable the section size required for a given bending moment to be read directly. In using such tables, it is assumed that shear forces are relatively small (as is often the case in practical design conditions), and some guidance is given in relation to deflection control.