By Robert L. Reid
A new reference guide highlights the various ways that uncoated weathering steel can help speed up bridge construction and provide financial and environmental benefits on transportation projects across the country. At the same time, the guide explains certain situations and scenarios that might prevent UWS from being used most effectively as well as cases when the material might not be the best choice at all.
Intended for bridge owners and designers, the Uncoated Weathering Steel Reference Guide was developed by the engineering firm Modjeski and Masters and sponsored by the American Institute of Steel Construction and the National Steel Bridge Alliance.
Available since the 1960s, UWS is a high-strength, low-alloy steel that “forms a patina during its initial exposure to the elements,” according to a Nov. 30 AISC press release. “The patina is essentially an oxide film of corrosion byproducts about the same thickness as a heavy coat of paint.”
As Jeff Carlson, P.E., the NSBA’s senior director of market development, explains in that same release, “Everyone is looking for materials with a proven track record of great corrosion protection while reducing maintenance costs and protecting the environment. But not everyone knows that uncoated weathering steel has done just that in a variety of climates across the country.”
To help promote that record, the guide explains that while the material cost of UWS is slightly more expensive than traditional, non-weathering steel, the product can produce initial cost savings of as much as 10% over painted structural systems. Moreover, in terms of life-cycle costs, the use of UWS can eliminate or reduce the need for maintenance painting during the service life of the structure, avoiding or decreasing the costs associated with materials, labor, equipment, and maintenance of traffic. Consequently, a UWS bridge can provide as much as a 30% savings in life-cycle cost compared with a painted steel alternative.
And by eliminating the need to apply paint and wait for that paint to dry, UWS can reduce the time needed to fabricate structural components, accelerating the construction schedule, the guide reports. Less painting also reduces the release of volatile organic compounds into the atmosphere and obviates the need to contain and dispose of the paint removed during repainting as well as the abrasive blast media used to prepare the bridge surfaces.
Although the guide states that UWS is generally appropriate to use “in the vast majority of locations,” there are certain factors, conditions, and situations that bridge owners and designers need to be aware of and take into consideration in deciding whether to use UWS or how to use it most effectively. These include locations with high atmospheric humidity, “extremely high” periods of wetness, high concentrations of chlorides, and “moisture traps,” such as when a bridge features a low vertical clearance over water or other factors that might prevent the bridge steel from drying. But many of these situations are of concern only in “extreme” cases, the guide notes.
To maximize the benefits of UWS, bridge designers should carefully consider the structure’s drainage system and work to eliminate or at least reduce the number of bridge joints, which can leak or fail. When joints cannot be avoided, the guide says, targeted painting of steel members — at, say, girder ends or where steel members are embedded in concrete — can help mitigate potential failures.
The 86-page guide can be downloaded for free from the AISC website.