The Oregon Department of Transportation (ODOT), using highly alloyed stainless steel reinforcing bar in its concrete structures, is building a bridge in North Bend, Ore., that is expected to provide maintenance-free service for 120 years--nearly double the service life of the bridge it is replacing.
Frank Nelson, bridge preservation managing engineer for ODOT, figures the taxpayers are getting a huge bargain. When finished by the end of 2003, the bridge will cost approximately $12 million. The stainless steel rebar, utilized in the most critical structural elements, accounts for only 13% of the total bridge cost.
For that small increase, he observed, ODOT will save the cost of bridge replacement in 50 years. That is a sum likely to be $25 million, or at least twice the cost of bridge construction today. As an alternative, the money saved could be used to build another bridge. Meanwhile, the new structure will require little more than routine examination.
The new bridge, carrying U.S. 101 over the Haynes Inlet Slough near the coastal town of Coos Bay, is using what is believed to be more stainless steel rebar than any bridge in North America - nearly 400 tons. Yet this is not an ordinary stainless steel structure because it had to meet some very challenging requirements for corrosion resistance, strength and site seismicity.
Along the Oregon coast, the marine environment is very hostile to bridges. Salt-laden air and fog from the Pacific Ocean condense under the deck and T-beams of this bridge. Wind blows the chloride-containing moisture underneath the structures, initiating corrosion. Rain washes the chlorides off the road surface, but flushes away nothing below.
ODOT considered stainless-clad bar and epoxy coating of carbon steel rebar, but decided neither possessed sufficient durability nor long-term resistance to chloride-induced corrosion. The concrete in which the rebar is embedded will eventually become contaminated with corrosive chlorides.
Extraordinary strength was required of the stainless to facilitate design of the new bridge and to deal with the potentially devastating seismic activity in this area. ODOT specified that the stainless alloy needed must have a minimum yield strength of 75 ksi. That strength level is new to bridge building and substantially higher than the 60 ksi minimum yield strength required of the Type 316LN stainless that was used for rebar in ODOT's Brush Creek and Smith River bridges replaced a few years ago. In addition, the alloy also had to provide high ductility (25% elongation) so it could be effectively fabricated.
In view of the area's geological history, Bridge Designer James Bollman had a study done to determine design seismicity and collapse criteria. Ground surface accelerations were intended to forecast a 1,000-year probability seismic event. Ground surface acceleration was calibrated at 1.05 g maximum, and peak bedrock acceleration at 0.54 g. The bridge, consequently, has been designed to remain serviceable with only a 10% probability over its lifetime of the site seismicity exceeding the design seismicity.
It was clear that ODOT, its goal set on extending bridge life, wanted this to be its strongest bridge yet. With a higher strength stainless alloy than any it had used to date, Bollman also expected to enjoy an economic advantage of less stainless rebar weight than would have been required using 60 ksi alloy.
For more on the story, read the May issue of ROADS&BRIDGES.