Weathering steel bridges should be detailed to ensure that all parts of the steelwork can dry out. Details should be such as to avoid moisture and debris retention, and to ensure adequate ventilation. The following good practice should be observed.
Grind flush weld details that may cause water traps
Provide 50mm radius cope holes in the corners of web stiffeners where they are attached to the bottom flange
Where stiffeners are not attached to the bottom flange, they should be curtailed (see GN 2.05)
Avoid closely spaced girders to aid ventilation
Avoid overlaps, pockets and crevices, which can attract moisture by capillary action
On half-through railway bridges, avoid contact with continuously wet ballast by the use of concrete encasement up to top of ballast level, and use ‘weather flats’ along the web at the top of the encasement
Ensure that web plates of box girders extend 20mm below the bottom flange, so water running down the web drips off and does not run along the underside of the flange.
For the common arrangement of concrete slab on top of steel girders, consideration should be given to providing wide deck cantilevers, with well-formed drip details, to shelter the girders from wind blown rain. In addition, any outlet pipes from the slab above the girders should be non-metallic, and of sufficient length that the salt laden water will not spray onto steelwork surfaces.
Run-off from the steelwork during the initial years, as the ‘patina’ develops, will contain corrosion products, which can stain substructures and paving slabs. The risk is highest during the early months, when the corrosion rate is highest. As the corrosion rate slows, the risk of rust staining also reduces, as there are fewer corrosion products in the run-off.
This potential problem can be avoided by providing drip details (details that cause water to drip off the steelwork, rather than run along its surface) on the bottom flanges of girders, and ensuring that bearing shelves have generous falls to internal substructure drainage systems. The key is to manage the drainage such that run-off does not flow directly over concrete surfaces.
Although rust staining should not occur on a well detailed weathering steel bridge, it is worth noting that concrete, stone and unglazed brick are difficult to clean. Hence, it is recommended that vulnerable substructures be sealed with washable organic coatings to facilitate cleaning with commercial products, should rust staining occur.
Concern has been expressed in the past about the possibility of corrosion products in the run-off contaminating surrounding vegetation, plants and watercourses. However, this is unlikely to occur due to the nature of the corrosion products. Weathering steel is predominantly iron which under wet or damp conditions dissolves to form ferrous ions which are then oxidised to form a series of ferrous and ferric oxyhydroxides. These compounds are of a transitory nature and evolve into more stable compounds e.g. hydrated ferric oxide. The small amounts of alloying elements in weathering steel contribute to the formation of protective compounds e.g. copper can form insoluble copper hydroxysulphates, which are precipitated in the rust layers. Nickel also forms similar compounds.
Other elements such as chromium and phosphorous have been found in the rust layer closest to the substrate and therefore are not freely available to contaminate the run-off; the run-off does not have an adverse effect on surrounding vegetation, plants or watercourses etc