Conventional steel bridges that take advantage of the latest advances in automated fabrication and construction techniques are able to provide economic solutions to the demands of safety, rapid construction, attractive appearance, shallow construction depth, minimal maintenance, and flexibility in future use. Weathering Steel bridges have all these qualities, plus the following further benefits.
Periodic inspection and cleaning should be the only maintenance required to ensure the bridge continues to perform satisfactorily. Hence, weathering Steel bridges are ideal where access for future maintenance is difficult or dangerous, and where traffic disruption needs to be minimised, such as over major roads or railways.
Although weathering steel is slightly more expensive than ordinary structural steel, savings from elimination of the paint system offsets the additional material cost. Hence, the initial cost of a weathering steel bridge is very similar to that of a conventional painted steel alternative. This was illustrated in a study on eight bridges in the UK .
However, weathering steel bridges have the added benefit of much lower whole life costs. The minimal future maintenance requirements of weathering steel bridges greatly reduce both the direct costs of the maintenance operations, and the indirect costs of traffic delays during maintenance.
With little maintenance, the risks associated with future maintenance are clearly minimised. The health and safety issues relating to initial painting are also avoided. Such issues are particularly relevant to the fabrication and maintenance of steel box girders, for which weathering steel is becoming increasingly specified in order to minimise internal access requirements
Exposure to high concentrations of chloride ions, originating from seawater spray, salt fogs or coastal airborne salts, is detrimental. The hygroscopic nature of salt adversely affects the ‘patina’ as it maintains a continuously damp environment on the metal surface. In general, weathering steel should not be used for bridges within 2km of coastal waters, unless it can be established that airborne chloride levels do not exceed the salinity classification of S2 (i.e. cl < 300mg/m2/day) to BS EN ISO 9223.
The guidance that weathering steel should not generally be used within 2km of coastal water comes from research by BISRA (British Iron & Steel Research Association) in the mid 1980s. They measured airborne chloride levels at various distances from the coast at a number of locations around the UK, and found a dramatic reduction in airborne chloride levels at a consistent distance of approximately 2km from the coast. An exercise for CEGB (in relation to transmission towers) showed similar results.
However, it should be noted that the airborne chloride level (and hence the suitability of weathering steel) depends on the microclimate at the bridge site (i.e. the local topography and prevailing wind direction etc.) so this figure of 2km should not be considered as a fixed limit; it is merely guidance based on the available data.