School buses can continue to roll safely across Maclay Bridge.

A message from the Montana Department of Transportation on Friday told Missoula County that fully loaded buses can cross the aging one-lane bridge over the Bitterroot River on a regular basis without damaging it.

The question arose after the bridge’s weight limit was downgraded from 14 tons to 11 tons last year. Overweight fire trucks from Missoula Rural Fire got the go-ahead last week to resume use of the bridge if they followed certain conditions, including slowing down to no more than 5 mph.

Missoula County had banned the Type I fire trucks more than a year ago until results of an MDT analysis were known. But the county requested the analysis on just the fire trucks and “kind of forgot about the buses,” county engineer Erik Dickson said last week.

Bob Mitchell of Beach Transportation said four school buses cross Maclay Bridge each school day – three carrying Target Range Elementary School students and one servicing Big Sky High. The bridge is also used frequently to transport students on field trips to Blue Mountain and Maclay Flats.

In February, Dickson submitted the bus dimensions supplied by Beach Transportation to the state transportation department to receive load rating factors. He said the buses that cross the bridge weigh 19,200 pounds empty and are “closer to 22,000 pounds” – or 11 tons – when loaded with 40 elementary-age students.

The county gave Beach Transportation the go-ahead to continue using Maclay Bridge while the state’s analysis was pending. Based on a discussion with Amanda Jackson, a civil engineering specialist for the state’s bridge bureau, Dickson didn’t anticipate a ruling from the state until after April 2, when the department’s submittal of bridge data to the Federal Highway Administration is due.

But Jackson responded early, via e-mail Friday.

“I have finished rating the school bus on the Maclay bridge,” she wrote to Dickson. “I have determined that a school bus with the weight and axle spacing you provided … plus the weight of a full load of schoolchildren that is following the posted speed limit of 15 mph can cross the bridge safely on a regular basis without damaging it.”

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