Origins of warping
When steel structures are hot-dip galvanised, the steel is submerged in a zinc melt at a temperature of approximately 450°C. During this process, the steel heats up and the inherent stresses that are present in virtually all steel structures may be released and result in warping.
In general, inherent stresses do not result in permanent warping because the limit of elasticity of steel is adequate to cope with temporary warping. However, when the inherent stresses are so large that they cannot be absorbed, warping will be permanent.
Possible warping depends upon, among other things:
- the levels of the inherent stresses;
- the distribution and the direction of the stresses in the steel structure;
- the stiffness of the steel structure;
- the composition and the thickness of the material.
The warping of steel structures during galvanisation is caused, in most cases, by inherent stresses resulting from welded joints and the associated point stresses. It is therefore essential to keep the stresses in a steel structure as low as possible and to avoid point stresses.
The warping of steel structures during galvanisation can be prevented to a large extent by taking structural measures. Weld sequencing can also be used to keep inherent stresses as low as possible. As a result, the steel structure can be more flexible in the zinc bath and absorb inherent stresses completely without warping
Important measures for preventing the warping of steel structures during galvanisation:
- use profiles with symmetrical cross-sections;
- use symmetrical welding sequences;
- avoid large differences in welds.
With structures made from sheet steel, it is important to take into account the fact that the sheet steel can expand during galvanisation. Sheet material warps easily and this can result in denting and waving. To prevent this type of warping, sheet steel should be stiffened before galvanisation.
Some structures are larger than the available zinc baths and so it is not always possible to galvanise a structure in a single dip. In cases like this, the structure will have to be galvanised in two stages. This is known as 'double dipping'.
Double dipping can result in stresses in the steel structure and so the builder and galvanisation plant should consult one another so that this factor can be taken into account during construction. It often emerges during consultations of this kind that structures can be split up, and this circumvents the need for double dipping.
The double dipping of long rolled profiles that are used as columns and frames is not generally a problem. There is some warping during the galvanisation process but the profiles almost always return to their original form when they cool down.