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Clyde Arc failure: bespoke connection in spotlight

Investigations into the Clyde Arc failure on 15 January were this week focusing on the bespoke connection between the steel bowstring arch and one of its 35m longtension bars.

Failure of the connection caused a tension bar between the top of the 32m high arch and the bridge deck to fall onto the live carriageway (News last week).

Glasgow City Council has closed the bridge while investigations are being carried out.

The connection detail was a bespoke product made and designed by Macalloy.

At one end was a screw connector to fix to a Macalloy tie bar and at the other were two flattened lugs, which would sit either side of a fin welded on the main arch structure. A pin through the two lugs and the fin completed the connection.

The two lugs fractured across the pin hole. Photographs taken after the tie bar fell show a clean, straight fracture line.

Investigations into the cause of the failure are underway. No traffic was crossing the bridge at the time of failure and it was not thought to have been be particularly windy, even though heavy winds had been reported a few nights before. Air temperature was above freezing.

The bridge is currently in the second year of a three year maintenance contract and Nuttall, the contractor, has employed specialist materials consultant Sandberg to carry out investigations.

"All the elements have been collected and taken away for analysis by Sandberg," said Nuttall divisional director for Scotland, James Scobie "They are also visually checking the other elements."

Sandberg has removed the connection from the bridge and taken it to a laboratory for metallurgical and dimensional checks.

"


There are no hard and fast rules on what we're looking at," said Sandberg site engineer, Julian Hunt. "It's an information gathering exercise."

Halcrow's regional director, Donald Bell confirmed that a check was being carried out on the connection design in parallel with the visual inspection.

"We are looking at the appearance of the fork ends, the seating of the pins and to see if there's anything untoward," said Hunt.

The Clyde Arc has been recognised for construction excellence, winning three awards last year including the Saltire Award for
Civil Engineering and an award for quality in construction.

"The Clyde Arc is an iconic structure and a symbol of the regeneration of the Clyde waterfront in Glasgow," said ICE Glasgow & West of Scotland regional chairman, Gordon Pomphrey.

"However, while it is a modern design and was built using the most up-to-date methods, it is not so cutting edge that the security of the bridge should be in doubt."

The steel bowstring tied arch bridge spans 96m over the River Clyde. The £20.3M structure, carries four lanes of traffic and was opened in September 2006.

The Clyde Arc is designed to allow for the removal of one of the bridge supports at a time for repair and maintenance. There are no plans to replace the hanger until investigations are complete.

Glasgow City Council said that the testing was likely to take some time, so the bridge is could stay closed for several weeks.

» www.nce.co.uk

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