More than a month after the first piece of arch was set for the new Interstate 74 bridge, construction of the "basket-handle design" should move more rapidly, provided floodwaters do not interrupt.
The first segments of arch are the most complicated, and three of the four segments were set as of Tuesday.
"Hundreds of bolts must be installed in the arch segment in a complex, stressing procedure to properly attach the segment to the concrete pier," said Doug McDonald, district construction engineer for the Iowa Department of Transportation. "The bolts must be tightened, torqued, loosened, and tightened again.
"It’s a slow, painstaking process."
The arches are the new spans' signature, and they will rise 164 feet above the bridge deck.
Various city and DOT leaders announced in mid-March that bridge construction had fallen behind schedule. While the arches were to be completed this spring so the westbound span could be operational this fall, the new timeline has the westbound span opening by the middle of next year.
Several factors have delayed progress, including fall and spring flooding and a particularly harsh winter season that created dangerous working conditions on the platforms and barges in the river. Forecasts now call for a Sunday crest on the Mississippi River of just above 20 feet, which could create more problems in the work zone.
"The river velocity is sometimes too fast for some operations to be safe, which is slowing progress," said Sam Shea, transportation planner with IDOT. "We are anticipating more flooding, which could further slow the progress, and we plan to work through it as best we can."
When workers are cleared to start adding more arch pieces, Shea said, progress will be more swift.
Iron workers have erected two sets of towers — each 200 feet tall — that will help hold the arches in place as they are built from either side of the channel, ultimately meeting in the middle.
When workers get to the third arch segments, cable stays will be added to the towers to hold the arch pieces in place. Iron workers will enter the arches to secure hundreds of bolts to each arch connection.
Without the arches, concrete cannot be poured on the driving surfaces of the westbound span, which is making its way across the river.
See article and photos on qctimes.com
But the arches cannot be rushed. They require "extraordinary precision," DOT officials have said, because they must be placed at precisely the correct trajectory needed for the two sides to meet in the middle.