Northgate Crossing Bridge Replacement
A four-span, 860-foot replacement bridge over the Northgate River, with 1,200 feet of approach roadway, a protected pedestrian crossing and scour protection for a 100-year event.
What made it hard.
The 1958 structure carried 34,000 vehicles a day and had been classified fracture-critical for more than a decade. Closing it was not an option: the nearest alternate crossing added eleven miles to emergency response times.
We also had to work through two fisheries windows and a river that ran six feet higher than the design flood during the first winter.
How we built it.
We built the new structure immediately downstream and shifted traffic onto it in two stages, keeping two lanes open in each direction throughout. Cofferdams for the two in-water piers were installed and removed inside a single work window, with turbidity monitoring downstream of every disturbance.
The superstructure combined cast-in-place approach spans with steel plate girders over the channel. Girders were erected during night closures, and deck pours were sequenced so the closure never lasted past 5:00 a.m.
Where it landed.
The bridge opened to traffic eleven days ahead of the contract date. There were no lost-time incidents and no exceedances of the in-water turbidity limit. Scour countermeasures and a 100-year design life give the corridor a structure that will not need major work for decades.
On site.
Their superintendent knew the bridge better than our own inspectors by the second month. When the river came up, they had already planned the work-around. That is the difference between a contractor and a partner.
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