Crews are closing in on the gaps, both long and short, in the cantilevered highway in the sky that curves through Tacoma's Nalley Valley.
One gap is over northbound and southbound Interstate 5. Next week, 6-foot-wide concrete segments weighing many tons will begin to creep out over the freeway.
The work will require 11 p.m.-to-4 a.m. closures of the northbound and southbound freeway exits to state Route 16 on Monday, Tuesday and Friday. The ramps will be closed on other nights as needed. Center Street will be closed under the state Route 16 viaduct from 9 p.m.-5 a.m. Monday through Friday.
A 650-ton crane will move onto I-5 each night to hoist a cantilevered segment into place before returning to the median in time for the morning commute.
As the gaps are closed, drivers will have a front-row seat. Each morning, they’ll see new lengths of roadway hanging over I-5 with nothing visible supporting them.
But not to worry.
The bridge design employs a so-called “balanced cantilever” technique to place lengths of roadway atop concrete columns.
To hold the hollow, 12-foot-high segments in place, hundreds of steel cables are threaded through the segments and then are pulled tight to compress each segment to the other.
One advantage of the design is that, by spanning a wide area without a forest of support columns underneath the roadway, the bridge can be built without lengthy closures of busy I-5.
When completed next June, the $10.4 million bridge will connect northbound I-5 to westbound state Route 16 and provide an exit to Sprague Avenue. It is part of the $184 million remake of westbound state Route 16 through Nalley Valley.
The balanced cantilevered design – suggested and designed by Atkinson Construction, which is rebuilding the viaduct – is a first for Washington, and state engineers appeared happy with the result and its progress.
Steve Roark, construction manager for the state Department of Transportation’s Olympic Region, led a recent tour to the top of the bridge.
A scaffold containing a staircase allowed access to one of the cantilevers that hung like an island of roadway off both sides of a column.
Down below, the one-lane roadway that for decades has taken traffic for northbound I-5 to westbound state Route 16 was choked with stop-and-go traffic.
The cantilevered roadway was both banked and curved. Each segment had been precast on the ground and engineered to marry up exactly with the one next to it. Cranes have been lifting them into place since June.
The segments, when finally connected, will be covered with concrete, Roark said. Side barriers of concrete will be poured in place.
Roark pointed out a 2-foot gap between two hanging segments. One side was about 5 inches lower than the other.
“When these things come together they don’t match up,” he said. “We would be stunned if they did.”
That’s because extending a balanced cantilevered roadway is like “putting out a measuring tape and attempting to hold it perfectly still,” he said.
To bring the two roadways together, steel beams are placed across the gap and a jack raises or lowers each side until they are even. Then concrete forms are installed and cement placed into the gap. More steel cables are threaded through the segments between two pillars, pulled tight and made fast to a steel plate called an anchorage.
“Then we are done,” said Troy Watts, the Transportation Department’s field engineer on the project.
He estimated the cantilever bridge will be connected sometime this winter.
“It’s really pretty neat,” Roark said.