CHATHAM — If this summer’s humidity has you down, you’re not alone.
Last weekend, the span of the Mitchell River drawbridge was kept in the lowered position because of continued problems related to swelling of bridge timbers during humid weather. Crews made the latest round of stopgap repairs Monday, and are hopeful that the $14 million bridge will remain operational until more extensive repairs can be made this fall.
The bridge is of unique design, with a mostly wooden superstructure on top and steel and concrete below. The wooden components, specifically the timbers that are just below the wooden driving surface, experience swelling during times of high humidity. The expansion has caused the center span both to stick shut and to not close completely after being opened.
Coastal Resources Director Ted Keon said crews appear to have removed enough wood to solve the problem of longitudinal swelling, which elongates the center span slightly, causing it to contact the fixed portions of the bridge. But certain timbers that run laterally continue to swell, causing the fixed portion of the bridge to crowd the movable span, he said.
“This last, very humid, wet weather just made a couple of the areas start to impact the operation of the bridge again,” he said.
Despite a closure for repairs in late June when humidity caused the bridge to stick, the drawbridge has been mostly operational this summer. But crews will need to make more extensive modifications to the bridge panels this fall, so the swelling problems do not recur next summer. To that end, users should expect the drawbridge to be taken out of service for more than a few days sometime this fall.
“The engineers are working on a longer-term fix,” Keon said.
While small boats can pass under the span when it is closed, taller vessels require the bridge to be raised in order for them to pass up the Mitchell River to Mill Pond and Little Mill Pond. Many of the boaters who need the drawbridge are customers of First Light Boatworks, but owner Woody Metzger said the bridge problems have not affected his business.
“When you have a complicated piece of equipment that’s brand new, and it’s sort of outbound of typical engineering, there’s bound to be what we call in our business some ‘shakedown,’” he said. A specialist in wooden boats, Metzger said he’s optimistic that the problem will be fixed.
“This bridge will continue to get better and better with its post-construction refinements,” Metzger said. “That wood will find its home at some point.”
“Sooner or later,” Keon said. “It’ll only swell so far.”
While the town has officially taken ownership of the drawbridge, which was opened in June 2016, MassHighway has acknowledged the design problems and is committed to fixing them, officials say. The wooden superstructure was a compromise with preservationists who wanted a replacement bridge to be made entirely out of wood, rather than steel and concrete, which are more stable.
“It was a lot to ask, to build it out of wood, in some respects,” Metzger said.
Email Alan Pollock at alan @capecodchronicle.com