| Steel takes center stage |
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| Steel takes center stage |
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148-ft. long steel beams will support the Hilgard bridge
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Steel usually plays a supporting role in highway bridge construction. Under the familiar surface of concrete lies a critical network of rebar that literally steels concrete to support heavy loads such as cars and trucks.
But structural steel has a starring role in the OTIA III bridge program, too: about 10 percent of the bridges being replaced will be made of it.
“Steel is appropriate in certain conditions, because it is lighter and because it can be bolted together,” said Steve Narkiewicz, project manager in the Bridge Delivery Unit. “For longer spans or when a bridge is curved, steel can be the best solution.”
The 148-foot-long girders for the Hilgard Interchange replacement bridge, on Interstate 84 near La Grande, were perfect candidates for this material. To minimize mobility constraints, the proposed bridge will span I-84 entirely, with no piers on the roadway, thanks to the strength of high-performance steel.
Oregon Iron Works built the girders using weathering steel, which includes alloys that create a dark-brown patina of rust on the surface of the steel when it’s exposed to the elements. Once the beams are covered, the patina protects them and they don’t need to be painted, which lowers the need for—and therefore the cost of—maintenance.
Another bridge that makes good use of steel beams, in this case, supplied by Fought & Company of Tigard, is a replacement bridge southwest of Springfield. The bridge sits above the Union Pacific Railroad tracks, part of the main freight and passenger transportation rail line between Oregon and California. There again, steel had an advantage because it could span more than 143 feet while maintaining vertical clearance over the tracks. A single span will increase the safety and stability of the structure by eliminating supports near the tracks that could be struck by a derailed train.
While precast concrete girders must be straight, steel girders can be curved horizontally by heating and cooling them repeatedly. On the bridge program, curved structures will be required to span several creeks, such as the bridge over Bear Creek or over the North Fork of the John Day River. The longer beams offer the added advantage of keeping piers out of the water and thereby providing more fish-friendly habitat. In the case of the Elk Creek Bridge, curved steel beams are also making the structure lighter, so it will be easier to slide into place when hydraulic skids turn it into a rapid replacement project in September.
The economics of steel vs. concrete are difficult to calculate consistently. In general, steel costs more. But because it is lighter, less concrete is required in the supporting columns and substructure. So steel can make a project more cost-effective by lowering construction or maintenance costs. Most often, the decision is based on the particular design required to do the best job of spanning a site.
As a natural resource, steel has a distinct advantage: It has no structural memory, so it can be recycled endlessly, and steel made from recycled material is just as strong as steel using virgin iron ore and coke. In 2006, two-thirds of the steel produced in the United States was made of recycled steel. When an old building or bridge comes down, it becomes something new.
“In Oregon, we’re fortunate to have some really good recyclers,” said Narkiewicz. “And building with steel helps Oregon’s economy by spreading bridge program work to yet another group of companies and workers.”
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