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'Angel Wings' granted to new Sellwood Bridge

by: DAVID F. ASHTON - The steel beam which the crane is lifting will support an angel wing form, at the east-side bridge footing. Rebar frames in the foreground will become part of the new Sellwood Bridge footings structure. Much of the current work on the Sellwood Bridge rebuilding project continues on the west side of the Willamette River, as contractors there continue to build new retaining walls, the new Highway 43 traffic interchange, and the regional trail.

“Eastside commuters should be aware that a new traffic signal will be activated at the west end of the bridge on December 11,” said Multnomah County project spokesman Mike Pullen. “All traffic on Highway 43 and the current bridge will pass through the traffic signal until the new interchange opens in 2015.”

Touring the east-side construction site with THE BEE on December 4th, Pullen pointed out two new features under construction: The footing at Bent 6, and the eastside perched box caisson at Bent 5. As we’ve explained before, the “bents” are what hold up the bridge.

On the east bank of the river, crews were installing rebar that will eventually support the steel arch span where it touches down. This footing will look like a massive concrete box set into the riverbank. It will be 64 feet long, 42 feet wide, and 18 feet deep.

“Concrete for the footing will be poured in a 24-hour continuous operation near the first of January.” Pullen said. “The footing is so large that a system of water pipes will run through the concrete to cool it while it cures – something necessary, even with freezing weather – to keep it from cracking.

During our visit, just in front of the eastside footing, other workers were welding massive steel girders into place. “Those structures are temporary supports (referred to as “falsework” in the trade) for ‘angel wings’,” Pullen pointed out.

Angel wings, he described, are reinforced concrete “arms” that receive the steel arch spans in place, where they touch down at the east-side footing, until the concrete has fully cured. “This falsework will be later removed.”

Walking out to the end of the work bridge, near the center of the river, another construction crew is busily building a “perched box caisson” that will hang on Bent 5.

Instead of building a watertight retaining structure – called a caisson – on the river floor, and pumping out the water so workers can pour the footing for the concrete pier, Pullen said, engineers have specified a different construction technique.

Crews were welding together steel forms around a concrete “box”, hanging on the four piers that are the foundation shafts of Bent 5. In this box, workers will fasten rebar, and then fill it with concrete.

“The foundation shaft and the perched box caisson are designed to support it above the river bottom,” Pullen explained. “Once it is lowered into place it will remain permanently above the river bottom. This allows us to avoid impacting the river bottom outside of the ‘in water work period’, which is a big schedule advantage for the project.”

As construction continues, the bridge’s upper supporting structure will be built on the top box caisson.

As the new Sellwood Bridge rises out of the river, THE BEE will be there to capture the work taking place, as it happens.

View live construction from Multnomah County’s two live webcams online at: www.sellwoodbridge.org/?p=construction-camera.