That stretch includes more bridges than just the Lewis River crossings, but Greenwell said “work done on bridges other than the North Fork and the East Fork was very minimal” — in other words, the Lewis River crossings are the squeakiest wheels of the bunch, and they’re getting squeakier.
The spans face unique challenges due to their disparate designs and ages. The East Fork’s northbound span was built in 1939, and the North Fork’s southbound span followed the next year. They carried both directions of U.S. Highway 99 until the northbound North Fork bridge was built in 1968 and the southbound East Fork bridge in 1969 as part of the construction of I-5.
The two older bridges both have weight limits due to their outdated designs. Trucks pulling loads in excess of 20,000 pounds per axle have to detour and use the more modern East CC Street bridge in Woodland, according to WSDOT bridge management engineer DeWayne Wilson.
The northbound North Fork bridge is 30 years younger than its southbound counterpart and doesn’t have the same structural weight limits, but it’s currently subject to a different restriction. Signs posted along I-5 northbound warn truck drivers to move into the center lane as they approach the bridge.
“Typically, trucks travel in that far right lane, and they’ve been traveling in that lane for 50-plus years,” Wilson said.
The extra weight accelerates the deterioration of the bridge deck, and WSDOT has had to perform increasingly frequent emergency maintenance on the right-hand side, pulling crews away from other scheduled work, according to Greenwell. The lane restriction is intended to reduce the need for those repairs until a full deck rehabilitation project gets underway.
The bridges each carried about 42,000 vehicles per day as of 2019, according to Wilson and Greenwell, about 16 percent of which was truck traffic.
The only bridge out of the quartet that isn’t currently subject to any restrictions is the southbound East Fork span, the youngest of the four. It will need some upkeep of its own in another few years, Wilson said, but it’s in fair condition and is better at handling heavy loads.
The first project on WSDOT’s Lewis River docket is a rehabilitation of the decks on both North Fork bridges, which will end the need for the truck lane restriction on the northbound span. The project is scheduled for 2022 and includes a similar rehabilitation of the northbound Toutle River Bridge deck.
The project won’t replace the decks in their entirety. Instead, crews will fix up the worn-out spots and then add overlays — an extra layer of road surface to protect the decks from wear-and-tear. Overlays can extend the life of a deck and only cost about a third as much as a full replacement, Wilson said, provided that they’re installed before the deck has degraded too much.
“If we wait too long, let it go out to 10 percent (degradation), then we’re looking at having to replace the bridge deck,” he said.
The southbound span already has an overlay because its deck is 30 years older, but Wilson said WSDOT wants to replace it with a polymer layer to reduce the overall weight of the bridge. A follow-up project in 2023 will focus on strengthening the truss, with the goal of eliminating the weight restrictions on the southbound span.
The deck rehabilitations will cost about $19.3 million in total, plus another $10.3 million for the southbound span’s structural tune-up.
The biggest project on the list is a $75.4 million full-scale replacement of the northbound East Fork bridge, which will begin next year and wrap up in 2023.
The bridge’s problems are similar to those of the southbound North Fork span — it’s more than 80 years old and has weight limits stemming from its outdated design — but the cost of repairing it doesn’t pencil out, Wilson said, because its particular truss design has been shown to be prone to steel cracks in the long term.
“If (repair) costs are about 50 percent of a replacement cost, then we lean toward replacing the bridge,” he said.
WSDOT will build a temporary adjacent detour bridge to carry northbound traffic while the original bridge is torn down and the replacement is built in the same alignment.
Like many states, Washington has a backlog of bridge repair projects competing for limited funding. WSDOT is funded to tackle about 40 percent of what will need to be done in the next 10 years, Wilson said. The Lewis River bridges rank high on the triage list because they carry I-5, but there’s no shortage of equally pressing repairs.
WSDOT generally plans for 100-year bridge lifecycles, Wilson said. The typical model would involve a first round of major rehabilitation after about 40 to 50 years including an overlay, which would buy another 25 to 40 years.
“You can kind of rehabilitate a bridge deck twice, then the deck overall will need to be replaced,” he said.
The challenge is that a significant number of the state’s bridges are hitting that 50-year mark all at once, in part because many of them were built in same time period as part of the freeway system — meaning they’re all due for the first round of major repairs.
“They all come relatively around the same time,” said Devin Reck, WSDOT assistant regional administrator of development and delivery.
The agency began focusing on deck rehabilitation in the 1970s, Wilson said, and has completed about 600 rehabilitation projects since then, compared with only about 15 full deck replacements — but another 600 decks are due for rehabilitation in the next 10 years alone.