MECOSTA TOWNSHIP, Mich. (WOOD) — The twin span bridge that carries US-131 traffic over the Muskegon River south of Big Rapids provides a nice view and a gateway to northern Michigan.
But inside the concrete box that supports the bridge are signs of a common problem for this style of bridge.
“Whenever you get a crack, that’s an avenue for water and salt to get in there and start corroding our steel,” explained Corey Rogers, an engineer with the Michigan Department of Transportation Bridge Build Service.
To combat that problem, MDOT would have to monitor the bridge and send in crews on a regular basis to patch the cracks, hoping to fend off a total rebuild and the $30 million to $50 million price tag that would go with it.
Instead, MDOT is trying a $6 million fix that engineers say could last up to a century.
“We call this harp post tensioning, ’cause it kinds of replicates the strings of a harp,” Rogers said, pointing out large conduits that surround strengthened steel cable inside the northbound bridge span.
The series of tightly strung cables are run through holes cut into the span and set in mortar.
“Every one of these tendons is stressed to 500,000 pounds,” Rogers said.
The process essentially tightens up the bridge.
“So all of these are like huge rubber bands that squeeze the bridge together and keep it from cracking,” Rogers said. “Now that this is in place, we should see no additional cracking. No more crack injection.”
Though the work is done on the inside of the bridge, the mortar that sets the cable has to cure, so the bridge has to be shut down to keep vibration to a minimum.
But the closure is not as long as more traditional repairs and upgrades, saving drivers the cost and hassle of lengthy detour.
The northbound lanes of US-131 over the river have already reopened. The southbound lanes are set to reopen Friday morning.