Arch completion photo story, part 2 of 3

From ODOT- Last time, we reviewed how the arches are initially formed. Now, we’ll describe how we went about lengthening them.



To minimize the impact on the Willamette River, the arch ribs will touch down only once in the water. Therefore, to span the wide and relatively shallow Willamette River, each arch must extend approximately 400 horizontal feet. The arches in the new bridge are fairly flat compared with those constructed during Roman times.



The arches will support the weight of the bridge by transferring the weight of the new bridge into shafts at either end of each arch. A “keystone” — a final insertion of concrete — ensures that the two sides of the arch transfer the force they bear outward, rather than collapsing in on each other.



To make way for the keystone pour, the crews left a 5 foot wide by 6 foot deep hole inside the crown of each arch. A crown reinforcement frame, seen below, creates a space to receive a steel-encased hydraulic ram capable of exerting the 965 tons of pressure necessary to jack the arch ribs apart.

Next, crews lowered a jacking box into the crown reinforcement frame at the center of each arch rib, as seen in the picture below. The team then activated the massive hydraulic ram, which pushed the arch ribs apart about two to three inches (a little goes a long way), lifting them off their false work and causing each rib to support its own weight. For the next step, crews poured concrete around the jacking box, casting it permanently into place with the job of supporting compression forces of 5.5 million pounds every second for the next 100 years.