Chances are, there won’t be overhead beams on the permanent span that replaces a collapsed section of the Interstate 5 Skagit River bridge.
State engineers are now focusing on a steel-girder bridge, in which the deck is supported from beneath, Harvey Coffman, bridge-preservation engineer for the state Department of Transportation, said Monday.
This sort of bridge can be installed faster, and with less traffic disruption, than a steel-truss bridge identical to the 1955-vintage span that crashed Thursday when a truck’s overheight load hit several overhead crossbeams.
Gov. Jay Inslee and DOT Secretary Lynn Peterson earlier announced they intend to get a reduced-speed, temporary roadbed installed by mid-June, followed by a permanent connection in September.
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The permanent span, Coffman said, would probably rest upon a half-dozen steel girders, each 160 feet long, that reach from the shore to the first in-water concrete pier.
Atkinson Construction has been hired to remove wreckage from the steel-truss span that fell Thursday, and build the temporary crossing.
The permanent span will be put out to competitive bids, said Coffman, part of a DOT design team working through the holiday weekend.
One technique might be the “roller” method, used recently for new I-405 overpasses in Bellevue.
Bridge parts would be assembled on the river’s north shore. The entire span then would be rolled south over the river on to temporary concrete abutments adjacent to the bridge. Finally, the new bridge parts would be moved sideways in alignment with the bridge. The final positioning would require a short-term freeway closure.
“It might be a two- to three-day operation,” Coffman said.
Coffman said steel girders are preferred because they’re lighter than concrete girders, and therefore put less strain on the existing columns, built in 1955.
There shouldn’t be any structural problems having a girder-supported deck on the same crossing as three original truss-supported spans. Each span is independently fastened to the concrete river piers.
Mike Lindblom: 206-515-5631 or firstname.lastname@example.org.
On Twitter @mikelindblom