Steel construction - Fatigue verification according to Eurocodes

Crack initiation necessarily starts from a defect. In the case of a smooth-appearing detail, the crack may originate from microscopic defects at the grain scale, or from edge defects due to flame cutting, drilling, etc., or it may originate from the surface.

In the case of structural details with welds, it's more likely to be a defect in the weld bead that initiates a crack (solid or gaseous inclusion, retassure, crack, etc.). These defects, combined with the notches caused by the more or less abrupt change in geometry due to the weld bead, are more significant than those in parts without welds.

Fatigue verification, which is used to prevent cracking, can be decisive in the case of bridges, runways, very slender structures exposed to wind, or hydraulic structures. Fatigue testing is not required for ordinary buildings subject to actions such as wind or snow, which are considered to be quasi-static.

To carry out a fatigue check, reference is generally made to standard NF EN 1993-1-9, "Design of steel structures – Part 1-9: Fatigue". However, other Eurocodes provide additional information on fatigue testing for structures that may be susceptible to fatigue, such as bridges, towers, masts, chimneys or runways.

The calculation of crack propagation is dealt with, but without much precision, in NF EN 1993-1-10, or rather in its reference document.

Having to refer to different parts of Eurocode 1 for fatigue loads, and to different parts of Eurocode 3 for the verification itself, makes understanding the verification process rather complex. That's why it's a good idea to bring together all the information on this subject in a single document.

This is the purpose of this article, which summarizes the principles of fatigue testing of structures susceptible to this phenomenon.