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Glasgow's Squinty Bridge could be shut for weeks after one of its struts crashed on to its carriageway.
Engineers last night admitted they had no idea how long it would take to find out why the 115ft support broke off the Clyde crossing's landmark arc.
They said they would not let cars and pedestrians back on the bridge until they had exhausted every possibility.
The Squinty Bridge - which is formally known as the Clyde Arc - only opened to the public 15 months ago at a cost of £20m. It was built, despite much controversy, to link new developments at Pacific Quay, such as the BBC headquarters, on the river's south bank with the city's centre and west end.
Officials at Glasgow City Council yesterday said they were confident the bridge would hold up: the strut, although described as the central support of the bridge, is one of 14 that secure the arc to the roadway.
Robert Booth, the council's head of land services, said: "We don't believe the integrity of the bridge is affected. Clyde Arc is designed to allow for the removal of one of the bridge supports at a time for repair and maintenance without affecting its operation.
"However, our number one priority is public safety and, until we are completely satisfied the bridge is safe to use, it will remain closed."
The strut broke off just before midnight on Monday, hitting the deck of the bridge and crushing part of its balustrade.
"It was a scary sound, like a bomb going off," said John Colvin, night manager of the City Inn, a hotel at the northern end of the crossing. "We felt the vibrations of it. We looked out and the whole thing was shaking."
Council and police officials yesterday asked motorists to use alternative crossings. Walkers were told to use the nearby pedestrian-only Bell Bridge.
The structure - Glasgow's first new road crossing of the Clyde for 30 years - was designed by Halcrow and built by construction firm Edmund Nuttall Ltd, which is also still responsible for its maintenance. Nuttall, not the council, will have to foot the bill for any repairs or investigation.
Mr Booth said: "The bridge is still under guarantee by the main contractor, who will report back to the council once they have established the cause following their investigations and independent testing of the broken part."
Nicholas McCormick, a manager at Nuttall, said he did not know how long the investigations would take.
He said: "It all depends on what our investigation identifies in terms of whether there's additional work required or whether it's just replacing something that has failed.
"There will be a need to assess the structure in a lot of detail not just the section that has failed.
"It's impossible for us to give a realistic time-frame on reopening the bridge."
Mr McCormick declined to speculate about what might have caused the strut to break lose from the arc, a single span that engineers called a "steel bowstring asymmetric tied arch".
Could it have broken in the recent high winds? "We wouldn't discount anything at the moment. All these considerations will be looked at. That is certainly one of the possible reasons that will be considered, but one of a number."
The Squinty Bridge's struts have two-pronged forks at each end, which are attached to lugs on the bridge's arc.
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