Swell Bridge: Two Years Later, $14M Drawbridge Still Sticks

By: Alan Pollock

Topics: Bridges

Contractors work on the Mitchell River drawbridge.  FILE PHOTO

CHATHAM Contractors are hoping that the most recent adjustments to the Mitchell River drawbridge will allow it to function properly, at least through this summer season. The two-year-old bridge has been prone to swelling during humid weather, causing the center span both to stick shut and to not close completely after being opened.

Last week, crews from bridge contractor SPS New England, Inc., spent three days shaving timber pieces, grinding metal elements and generally adjusting the span.

“Much of it, if not all of it, is related essentially to the wooden superstructure,” Chatham Coastal Resources Director Ted Keon said.

In a compromise with preservationists who wanted a replacement bridge to be made entirely out of wood, engineers designed a span with a steel-and-concrete substructure that supports wooden deck panels. The wear surface, the top planks that vehicles pass over, rests on top of the deck panels. The problem is with those panels, Keon said.

“On the [movable] span itself, the panels are oriented such that the width of the bridge has expanded slightly,” Keon said. But the fixed portions have expanded longitudinally, causing the movable span to bind during periods of high humidity.

“The designers have anticipated some of this, but they’re actually expanded beyond what they had anticipated,” he said.

Several times since the $14 million bridge was opened in June 2016, the contractor has returned to shave some of the wooden joints.

“Some of the swelling has now put metal on metal,” Keon said. Crews have trimmed metal pieces where they can, making changes that do not affect the structural integrity of the bridge. “Most of this stuff is non-critical,” he said.

Still, the bridge has been difficult to open on several occasions, “and then when it does open, often it does not seat itself in the proper, fully closed position,” he said. If the bridge doesn’t lower fully, locking pins do not enter the sockets designed to receive them, and that prevents the safety gates from automatically opening. “It’s a cascade of other issues if the bridge does not get into its fully closed position,” Keon said. Occasionally, bridge operators have had to drive their vehicle on top of the span “to close it that last little bit,” he said.

Despite the problems, the town has officially taken ownership of the drawbridge, but Keon said MassHighway has acknowledged the design problems and is committed to fixing them.

“The state has taken responsibility to continue to address this,” he said.

The contractor expects that the bridge will function properly through the remainder of the summer, but crews may return in the fall to make additional adjustments.

The bridge was entirely funded by the Massachusetts Department of Transportation through the federal Accelerated Bridge Program. It took more than six years to design and build the span, thanks largely to contentious negotiations over a design that would be both functional and historically sensitive.