Pros and cons of weathering steel

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Pros and cons of weathering steel

Pros and cons of weathering steel

Interior box girders 0. However, they may be included in the section properties for global analysis of the structure. However, the defects or imperfections inherent in welded details lead to a greater reduction in fatigue life than the reduction for pits in weathering steel and hence fatigue need not be of any more concern than Pros and cons of weathering steel ordinary structural steel.

Typical bolted connection Dalscone Viaduct, Dumfries Weathering grade preloaded boltsnuts and washers, with chemical compositions complying with ASTM F [9]Grade A, Type 3, or equivalent, should be specified for all bolted joints.

Pros and cons of weathering steel

The slip factor may be taken as that for ordinary structural steel, and research [10] has shown that the effect of rusting caused by weathering of the faying surfaces between their preparation and assembly is not normally significant. Bolted connections inevitably introduce overlapping plates, and the potential problem of crevices.

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However, careful design of the joint by adopting the following guidance will avoid the subsequent corrosion risks associated with crevices. Ensure good fit-up using flexible cover plates The bolt spacing adjacent to an edge should not exceed the lesser of 14t and mm The edge distance from bolt centreline should not exceed the lesser of 8t and mm Where t is the thickness of the thinner plate If this guidance cannot be met, the edges of the joint should be protected by a suitable sealant.

Hence, it is important that the bridge be detailed to ensure such cycles can occur. Advice on good structural detailing for weathering steel bridges is widely available [3] [6]and there is also a Steel Bridge Group Guidance Note on the subject GN 1. The key issues are summarised here.

Hence, the selection and detailing of joints requires careful consideration. Ideally, expansion joints should be avoided by the use of continuous and integral construction. However, if joints are unavoidable they should be located away from the ends of the beams, and a positive non-metallic drainage system should be provided to convey any leakage away from the steelwork.

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It may also be prudent to paint the ends of the beams directly beneath such deck joints if this will not detract from overall appearance of the bridge.

The colour should be selected to match the expected final colour of the mature steel. 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.

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.

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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.

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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.

The small amounts of alloying elements in weathering steel contribute to the formation of protective compounds e. 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.

Hence, if the seal cannot be guaranteed then there is a risk that moisture will get in, creating a potential for accelerated corrosion due to ponding. Consequently, many designers assume that some ingress of the external environment will occur and detail the steelwork to allow free drainage.

The ability of the structure to breathe and allow a flow of air to dry out wet or damp surfaces is desirable. It is equally important that the internal diaphragms and stiffeners etc.

The openings should be sufficiently large enough not to become blocked, but not allow a large intake of the external atmosphere. It is suggested that drainage tubes of a non-corroding material be used to ensure that any water containing corrosion products is diverted away from the external surfaces, especially those of absorbent material such as concrete that could easily be stained.

Grilles or meshes are also recommended to prevent birds and rodents from entering internal chambers of the structure. Examples include the edges of the top flange and the profile of a beam that is cast into the end wall of an integral bridge.

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Disadvantages. Using weathering steel in construction presents several challenges.

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American Shotcrete Association Weathering Steel - Disadvantages Disadvantages Using weathering steel in construction presents several challenges. Ensuring that weld-points weather at the same rate as the other materials may require special welding techniques or material.
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