Back in June, a routine inspection of the Patton Bridge over the Green River revealed a serious structural problem with the 58-year-old span. Now four months later, the bridge is fixed, travel across the river is back to normal, and King County engineers have valuable new experience that will be put to good use in evaluating and reinforcing other aging bridges.
Brackets were installed at the four corners of the span.
At 430 feet in length, the Patton Bridge on Southeast Green Valley Road east of Auburn is one of the longer bridges in the county’s 185-bridge inventory. It was built in 1950 using what was then state-of-the-art design and construction, with two massive concrete approaches supporting a single steel span across the river. Once the steel span was in place, its supports were encased in concrete.
Like all King County Road Services Division bridges, every two years the Patton Bridge undergoes an intense inspection, which is in addition to regular maintenance and monitoring. A special Under Bridge Inspection Truck (UBIT) is used for bridges that are not accessible from the ground or a ladder. The UBIT is stationed on the deck of the bridge, and has an arm that lifts engineers in a bucket up and over the railings to inspect all the areas that are normally hidden from sight.
Jim Markus, the managing engineer for the Road Division’s bridge unit, was in the UBIT bucket that day in June when he noticed that one corner of the span appeared to be dipping or deflecting toward the river. Some of the concrete encasing the area was beginning to crumble, and inside Markus could see a big problem.
“When the bridge was built, they connected that steel span to the concrete approaches, and then reinforced with welded rebar. Today, the general rule is that you don’t weld rebar because it creates a brittle point for load-bearing structures,” said Project Manager Ben Blegen, who was in the bucket that day with Markus.
“That rebar was totally covered in concrete, and it wasn’t in the original design plans. It looked like the decision to weld the supports was made in the field during construction. Without the UBIT inspection, we would have never seen the problem,” said Blegen.
The lane of traffic closest to the trouble spot was immediately closed, and the bridge unit began working on a solution. At first, the engineers thought they might have to put pilings in the river and jack up the entire bridge – a costly fix that would not be environmentally friendly. Plus, it would require a total closure of the bridge, which would create a 22-mile detour for most vehicles, including local school buses.
“As a temporary fix, we had bracketed that one corner with steel supports, and as we researched solutions we realized we could make a permanent fix by bracketing all four corners,” said Blegen.
Hydraulic jacks were used to slightly lift the bridge and insert new support brackets. Four 600-lb. plates were bolted to the bridge and an 800-lb. beam was welded to each plate. This was easier said than done, since everything was located under the bridge on the scaffolding where cranes can’t reach. And, there are high-voltage transmission lines crossing above the top of the bridge.
“The project turned out to be simple in concept, but complex in implementation,” said Blegen. “It was the first time we used this approach for a bridge in King County, but it probably won’t be the last.”
The bracket solution increased the load-bearing capability of the Patton Bridge, and also saved a lot of time and money. Originally, the engineers thought the project would cost $750,000 to $1 million to complete. Instead, the repairs totaled about $200,000, and at least one lane of the bridge was open almost at all times.
“This really reinforces our inspection schedule and practices,” said Blegen. “You can’t always trust plans that were drawn up decades ago, and each bridge weathers the years and the elements differently.”
