February 01, 2003
A highway bridge in Oregon that uses duplex stainless steel is designed to last 120 years.
By Virginia Heffernan.
The state of Oregon in the northwestern United States is using S32205 stainless steel as an antidote to the corrosion problems that plague some of its older bridges along the Pacific coastline.
A US$12-million bridge now under construction over an estuary near Coos Bay will contain 363 tonnes of stainless steel rebar, believed to be the largest quantity of stainless steel used for any bridge in North America.
The Oregon marine environment is hard on bridges reinforced by carbon steel rebar. Wind blows salty, moist air under the deck and T-beams, where it condenses and causes corrosion. The resulting cracking along the contact between the rebar and the concrete is exacerbated by tensile cracking caused by heavy traffic loads. Eventually, rust forms and the concrete begins to crumble, weakening further the bond between the metal and the concrete. Structural failure has been known to occur in as little as 17 years.
Usually, designers recommend S31653 or S30400 stainless steel to prevent these problems. But Oregon has gone a step further. By choosing the S32205 alloy, which has much greater corrosion resistance, the state hopes to extend the maintenance-free life of the bridge to 120 years while providing the structure with enough strength to withstand potentially damaging seismic activity in the earthquake-prone area.
The S32205 was supplied by Carpenter Technology Corp. of Reading, Pa., U.S.A. It contains 22% chromium, 5.5% nickel and 3% molybdenum. The balance is made up of iron and smaller quantities of silicon, carbon, phosphorous, sulphur and nitrogen. The alloy has superior fatigue resistance because it exceeds the elongation requirement that allows bridges to withstand movement and stress under heavy truck traffic. Its yield strength is also considerably higher than the 410 megapascals typical of S31653 and S30400 stainless.
Even though the state stands to save a considerable amount on maintenance costs by using S32205, the choice of material did not add significantly to the capital cost of the project. The rebar represents only about 13% of the US$12 million budget.
"The designer, James Bollman, decided to use a stronger reinforcing bar than usual, to reduce the total weight, and therefore cost, of the stainless steel," says Frank Nelson in the bridge engineering section of Oregon's Department of Transportation. "At the 520-megapascals yield strength, the steel mill and the contractor agreed on using this specific alloy.
"In addition to the reinforcing, Bollman used S21800 for the massive hinge assemblies at the butt of each arch rib. These assemblies, which use S32205 stainless steel hinge pins, are exposed, and at high tide are submerged in the brackish water inlet."
Another, much larger (614-tonne) tonnage of uncoated carbon steel rebar will support areas of the substructure where corrosion is less of a concern.
The 235-metre-long bridge, a series of three spans of concrete deck arches, will carry an estimated 14,000 vehicles per day. The five-lane structure replaces a two-lane timbered bridge that has reached the end of its life and is being removed.
Nelson says a bridge near Newport is the next significant bridge in Oregon slated for stainless reinforcing.
Virginia Heffernan is a Toronto-based freelance writer.
Photos: ORIN RUSSIE/ODOT PHOTO/VIDEO