A mechanical failure of a sheave in the south tower of the 76-year-old, 2,804-ft-long Sarah Mildred Long Bridge on Aug. 21 has led to a permanent closure—10 weeks before construction of its replacement would have closed it anyway.

The steel-truss lift bridge carries 14,000 daily vehicles on the U.S. Route 1 Bypass over the Piscataqua River between Maine and New Hampshire. After two days of investigation, engineers from the New Hampshire Dept. of Transportation, MaineDOT, Hardesty & Hanover and contractor Cianbro determined the bridge would not be safe for operation without repairs that would cost $1 million and take at least six weeks.

By Aug. 22, crews lifted the bridge into enough of an “up” position to allow ships to pass. Kaven Philbrook, Cianbro senior project manager for the $158.5-million replacement project, says that while demolition of the old bridge will start immediately, it will not impact new bridge construction, more than 50% complete.

“The failure of a thrust block had been caused by the trunnion moving to the north,” says a DOT spokesperson. “The trunnion is not supposed to move relative to the sheave, causing damaging forces to the thrust block.”

The alignment of the new bridge, designed by a joint venture of Figg and Hardesty & Hanover, is designed to allow larger ships to access local port facilities, according to MaineDOT. A larger, 56-ft vertical clearance in “resting” position will mean 68% fewer bridge openings.

Closure of the old bridge was originally scheduled for Nov. 1. The new bridge is scheduled to open to traffic in September 2017, with project completion slated for June 2018.