J acques Berthellemy

This communication will discuss the importance of the correct evaluation of the fatigue category of resistance for bridges made of weathering steel.

Clause 7.1(5) of the French National Annex to EC3 1-9 has put bridges designers under the obligation to take into account stress concentration factors since 2011.
 However, the temporary version of a french guidance document published in 2015 about weathering steel considered that there was no specificity of these steels regarding fatigue.

This document was rewritten in 2023 to take into account all the clauses provided for in Eurocode 3 part 1-9 concerning weathering steels. In addition, the monitoring and maintenance actions needed by these bridges have been specified because they are actually much more demanding than for painted steels. As an example, various deposits, in particular bird droppings, must be removed as quickly as possible when they occur on horizontal surfaces. This applies to I-beam bridges, whereas box girder bridges do not have horizontal surfaces wide enough to allow deposits to form or birds to land.

Bird droppings adhere to weathering steel, whereas paints are designed to be water-repellent. This case is worse than the effect of sea water: the likely occurrence is the formation of corrosion pits. The effects of corrosion pits on the category of detail is studied with a finite element model.

Even in the absence of bird droppings, studies by Ludwig Kunz have shown that craters form on the surface of weathering steel. The effects of these corrosion craters on the category of detail is again studied with the effective notch stress method. The thickness transition zone of the I-beam bridge is particularly sensitive to the presence of corrosion craters. Model shows that the presence of other craters in the vicinity of the crater placed at the hot spot does not improve the category of detail by a smoothing effect.

A confocal microscope can be used to analyse in detail the corrosion effects on the surface over an affected area in order to asses an existing bridges. A chart is provided to deduce the category of detail based on the depth of the observed craters. Prudent road owners can also use this data in France to design new structures using both the Clause 7.1(5) of the French National Annex to EC3 1-9 and the work of L. Kunz which is referenced in the Cerema-UGE 2023 guidance document about weathering steel.

Session