Post-tensioning the bridge

From ODOT- Driving by the Willamette River Bridge, you might have seen the contractor building the bridge’s falsework and pouring the concrete for the new bridge. Other activities that are equally critical to completion of the bridge can’t be easily seen.

One of the least seen and least understood activities is post-tensioning of beams and decks that make a bridge stronger than its concrete alone.

Post-tensioning allows the bridge to use longer spans with resulting in fewer support columns. Once post-tensioned, the bridge has the strength to meet the long-term demands of heavy traffic and to better resist an earthquake.

How do we post-tension the bridge? Crews place galvanized steel ducts and rebar in stemwall forms before placing the concrete. After the concrete cures, strands of steel cable (tendons) are anchored on one end of the deck and pulled through the ducts to the opposite side. Large hydraulic jacks attached to the free end stress the cables by pulling them to predetermined forces. Once tensioned, the cables are held in place with anchoring devices. The ducts are then filled with grout to prevent deterioration of the cable during the life of the bridge.

To understand the principles of post-tensioning, imagine a series of wooden blocks with holes drilled in them and a rubber band threaded through the holes. Fasten one end while you hold the other, letting the blocks sag. Now twist the rubber band, tightening the blocks together. When the rubber band is twisted and secured on both ends the blocks remain tight and strong.

On the new southbound Willamette River Bridge, one bridge section north and two bridge sections south of the river are post-tensioned. The deck supported by the two arches over the river is not a post-tensioned span. Instead, they are supported by the arches .