Rolled sections
Most structural sections used by architects and engineers are formed by hot-rolling in a range of standard sizes.
White hot slabs of cast steel are sent into the rolling mill and are passed through sets of rollers which gradually change the profile into the familiar 'I' and 'H' sections. These are produced in a wide range of section sizes. These are known as universal beams and columns, identified by serial size and mass per unit length. Each serial size corresponds to a different set of roller sizes. Changes in mass per unit length are achieved by increasing the thickness of the flanges during rolling.
As in any manufacturing process, there are very small variations in the geometry of hot-rolled sections which may need to be considered in design.
Although modern rolling mills can achieve a relatively high degree of accuracy and consistency it is essential to have certain rolling tolerances. It is more critical for hot-rolled materials. This is because of physical wear during rolling and inevitable temperature variations.
Although these tolerances generally have very little effect when the products are used in normal fabrication for structural applications, they should be considered when detailing structures which need a close fit to more precise components such as cladding.
Coordination with the steel fabricator at the design stage should avoid the possibility of problems due to the specified tolerances, enabling manufacturers to take them into account.
Adjustment can often be provided, for instance by using slotted connections. Co-ordination with the steel fabricator and other parties at the design stage should avoid possible problems. The tolerances given to a cladding manufacturer for example must take into account this kind of inaccuracy.
Smaller steel sections can be manufactured by cold forming.
In steel construction there are two main families of structural members. One is the familiar group of hot-rolled shapes/sections including members built up of plates. The other group is one with which most designers are less familiar but which is of growing importance. These are composed of sections cold-formed from steel sheet, strip, plates or flat bars in roll-forming machines or by press brake or bending brake operations. Only the smaller structural sections such as those used in lattice trusses, cladding rails and cladding panels are produced by cold forming.
