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Milled steel replaces cast components on Clyde Arc

Design and fabrication of hangar connection details used on Glasgow's troubled Clyde Arc structure remained in the spotlight this week as the investigations into the cause of the last January's failure were completed.

No official statement on the precise causes of failure has been released but NCE understands that the report on investigations by Sandberg Consultants is now complete and has been issued to all parties involved in the bridge construction.

The first of the Macalloy bar connectors – similar to the 35m long bar that failed in January – was replaced by contractor Nuttall last week.
A repair programme has been agreed by client Glasgow City Council and will result in all of the 28 original cast connectors replaced under warranty with milled steel components.

"I can't say anything about the cause of the failure at this point, but we are confident work is progressing well," said a Glasgow City Council spokesman.

Repairs to the steel bowstring tied arch are expected to finish as early as June, allowing the bridge to reopen to traffic after six months of closure.

The spokesman added: "We remain very optimistic that the bridge will be available and safe for use within the timescale predicted."

Nuttall was also unable to comment on the Sandberg report's findings but it is understood to have used the report to ensure that its ongoing repair work was targeting the right area. The report is expected to play a significant part in determining who will eventually pick up the bill for repairs.

A spokesman for hanger supplier Macalloy said: "We can't comment on what's in the report." He added that the firm was continuing to support the design team's ongoing repair works.

He said that there were no fears for any other projects using similar hangers. "The Clyde Arc should be kept in isolation from other Macalloy projects."

The Clyde Arc was closed to traffic in January when the connection of one of the 35m long Macalloy bars failed, causing it to fall onto the carriageway . Discovery of a crack in a second connection led to the decision to replace all of the connectors and extra precautions being taken when working on the bridge.

The line of failure for both incidents appeared to be across the hole on the connection lugs. Each connection had two flattened lugs which sit either side of a fin welded onto the main arch structure. A pin through the two lugs and the fin connected the two, and it was across the hole for the pin that the lugs fractured.

Five temporary saddles have been installed and sit on top of the arch. Strandjacks are then looped between the saddles and outriggers attached to the deck. They are stressed to take the load of the deck, destressing the Macalloy bars.

This then enables Nuttall and Watson Steel, who are carrying out the repairs, to remove one bar at a time, to replace the screw on connectors and reconnect the bar. The hanger that fell in the initial failure was damaged beyond repair. The production time for a new bar is between 16 and 20 weeks and the new bar is due on site in May.

The temporary works are expected to be removed in May after which repairs will be made to the damaged parapet and paintwork.

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