The Clyde Arc, a labour of love and four years hard work for the Nuttall and Halcrow design team opened in 2006 to awards and applause. It fast became a symbol of Glasgow, with its stunning shape integrated within local company insignia. The structure itself formed the backdrop for the BBC Glasgow evening news.
But when the project was back in the news in January, it was for all the wrong reasons. At 11.30pm on Monday 14 January, a connection failed causing a 35m long Macalloy bar to fall onto the carriageway below. The bar was one of 14 tension bars which suspended the deck from the bridge’s bowstring arch. It was connected to the steel arch with a bespoke connection. Fortunately the bridge was free from traffic at the time of failure (NCE 24 January). It was closed immediately.
A second crack in another connection was found 10 days later, prompting a decision to replace all the existing connections. And so it is back to site for the construction team. Contractor Nuttall and steel supplier Watson Steel have worked tirelessly to secure the bridge and install the temporary works necessary to allow the bars to be taken down one by one and their connections replaced. After continually working for the last three months, the temporary works are now finally in place. The team has just had its first weekend off in months.
When the hanger came down in January, the team had to act fast to secure the bridge. The first job was to replace the failed hanger; the bridge is designed to function with one hanger missing but, with the performance of the other hangers called into question, it was imperative
that the failed hanger be replaced as soon as possible.
But a new bar had to be made in Sweden and needed between 16 and 20 weeks to procure. Therefore a temporary measure had to be employed in the interim. Twinned M90 Macalloy bars were used as they were readily available. They were connected to the arch and the
steel deck by bespoke Watson Steel connections.
"Watson installed the replacement hanger in the dark," recalls Nuttall contracts manager Dougie Grant. "They were lifting a 30m long bar with a lifting beam [used to stop the bending of the bar during installation], trying to twist it to get it where they wanted it to sit and levering it in from a basket on a crane."
They started looking at the simplest solutions: straddling the arch with steel support units or saddles and then sending strand jacks straight down to pick up the ends of the transverse beams. But this would have involved taking the cables straight down and drilling straight through the deck, a solution, not favoured by the client, Glasgow City Council.
The replacement hanger was installed by the end of January and discussions on how to support the deck while all the hanger connections were replaced was something that happened in parallel.
"We could have propped the deck from underneath," explains Grant. "But the piles that were used in the original temporary works had been burnt off under water and it would have been difficult trying to locate them."
That option was swiftly discounted. Another option was to support the deck’s transverse beams – which connect to the hangers – at each end using piles. But this too was ruled out.
"We would have needed a 300t crawler crane," says Grant. "At that time of year it’s very windy and there would have been considerable risk. Also it takes four to six weeks to procure piles and due to the tapering nature of the outrigger beams we would have needed all kinds of packing."
Having ruled out supporting the deck from underneath, the team decided to, as Grant puts it, "use the arch to support the bridge deck as it was originally intended to do."
The temporary works designer suggested taking the strand jacks down to temporary outriggers in a similar way to the original bars. After looking at several variations the optimum solution was decided on. This involved installing five saddles along the top of the arch. Temporary outriggers were welded to the ends of transverse beams which pick up the ends of the strand jacks. The outriggers protruded enough to eliminate the need for the strands to pass through the deck.For a detailed diagram click here
Halcrow had to check the impact of the temporary works on the permanent works, including what equipment would be used during the repair works. A forklift and a cherry picker were decided on. The design of the saddles involved complex loading and geometric modelling. "The geometry was key," says Grant. "Each saddle was different and had to exactly pick up the angle from the top of the arch. Also each of the outriggers was bespoke."
When the saddles arrived on site, Watson had to use ultrasound to detect 50mm diaphragm plates embedded in the arch. The saddles were then positioned on these higher strength locations. Once the outriggers and saddles were in place, the strand jacks could be installed.
"We had to lift the saddles up and bolt them on," says Grant.
"Then we dragged the cables across and fixed them. The cables were very heavy, with one of the strand jacking components weighing two and a half tonnes."
The strand jack stressing operation took place in three stages; 15%, 45% and then finally 100% of the load was put into the strand jacks. At 100% load the strand jacks are carrying the load of the deck. Also the strands had to be stressed in a specific order to ensure that the arch was loaded evenly and did not deform.
All the strands were fully stressed by Friday 11 April, allowing work to begin on replacing the tension bar connections the following Monday.
The connection components being replaced are milled steel instead of cast steel; a decision taken by the connector suppliers. The new milled steel connectors are made from 250mm thick plate.
Nuttall and Watson Steel need one and a half days to take down the existing hanger, take the old connections off, screw the new connections on and replace the hanger. This means that 21 days will be needed to change all the hangers. Once they are all replaced and stressed, the load is transferred back to the hangers and the temporary works can be removed. During May and June, peripheral works such as repairing the parapet and road surface damaged in the initial fall and repainting will be completed in time for the grand reopening of the bridge this June, restoring the Clyde Arc to its former glory.