The Replacement Forth Crossing will strengthen the route across the Firth of Forth when complete in 2016. Jessica Rowson reports on the symbiosis between the old and the new.
Celebrating its 45th birthday this year, the Forth Road Bridge spanning the Firth of Forth just north of Edinburgh is already starting to feel its age. In 2004, the main cables were found to have corroded and lost about 8% of their strength. Left to its own devices, the strength of the suspension bridge would continue to deteriorate, which would lead to weight restrictions being imposed on the bridge by 2014.
The bridge is a key route between Edinburgh to the south of the Firth of Forth and Dundee to the north, while the nearest crossing is 35km upstream. With over 24M vehicles crossing every year, even a partial closure of the bridge has a tremendous economic impact.
A cost effective solution needed to be found, one that would keep the traffic flowing. “We looked at the possibility of augmentation, but it hasn’t been done on this scale,” says Forth Estuary Transport Authority (FETA) chief engineer and bridgemaster Barry Colford.
“The best way would have been to replace the cable, but for that we would need to shut the bridge to traffic to eliminate the risk to the user. It would take three years of bridge closures. We could replace it but wouldn’t want to without a second road crossing in place [nearby].”
The Scottish Parliament agreed that a new bridge should go ahead in December 2007 and earlier this year images were revealed of the new cable stayed bridge. Designed by a joint venture of Arup and Jacobs, the new bridge features three sleek single pylons with two main spans each about 650m long and an overall length of 2.7km, including the approach viaducts. It will sit just west of the existing Forth Road Bridge and the Forth Rail Bridge in the Firth of Forth, creating a spectacular array of three major bridges.
The cables, which fan out from the pylons downwards to support the deck, cross over cables from the next pylon along as they reach the deck, creating a triangulated cats cradle effect. The crossed cables add stiffness, which helps the stability of the multispan cable stayed bridge and allows effective wind shielding to be added to the bridge.
“If you have a two pylon [cable stayed] bridge, the back span is anchored and the pylons don’t need great stiffness,” said Jacobs/Arup project manager Mike Glover. “With three pylons, the middle [pylon] is not anchored and it relies on the ones either side for stiffness. Stiffness in the towers is what you need, but if you can get stiffness some other way [in the deck] then the tower doesn’t need to be so stiff.”
Design is still being finalised, but the deck is likely to be a steel, or composite steel and concrete, box section. The pylons will be slipformed reinforced concrete with the first section of the pylon installed on top of precast piles. Pile caps will include the first section of the pylons so that once submerged the pylon base will protrude above the high tide level. The pylons will then be constructed by conventional slipforming to the tip. “The towers are ellipsoidal and about 16m wide at the base. By the time they reach the deck, 50m in the air, they are about 8m wide,” says Glover.
The new bridge is named the Replacement Forth Crossing, but it is actually augmenting rather than replacing the old crossing. In 2008, the findings of a second internal inspection on the cables of the Forth Road Bridge were cautiously optimistic that the rate of deterioration was at the slower end of expectations. It opened up the possibility of using the existing bridge as a public transport corridor and meant the width of the new crossing could be reduced.
“The project was going to cost between £3.5bn and £4.2bn,” says Transport Scotland interim project director John Howison. “Now it will cost around £1.7bn at outturn costs including VAT. We’ve always been dealing with the uncertainty of the bridge being available [in the future]. The continuing work of FETA means yes it can be available.”
Once the new crossing is operational the load on the bridge of public transport, cyclists and pedestrians will be much lighter than the heavy traffic it is currently carrying. The reduction in load helps to counterbalance the reduction in strength “With the existing bridge, 85% of its strength is keeping it up and 15% of its strength is for the traffic,” says Howison. “We are balancing load reduction with the loss in strength [from corrosion].” The reduced loads also reduce the risks of damage from vehicle impact. “We were going to look at vehicles impacting the tower and how to strengthen them,” says Colford. “However, if by 2016 heavy goods vehicles are using the other bridge, it’s a balance of risk against cost.”
The best way would have been to replace the cable, but for that we would need to shut the bridge to traffic to eliminate risk to the user
Barry Colford, FETA
The presence of an alternative crossing takes the burden of being the sole road crossing off the existing bridge, opening up the possibility of closures for maintenance and reducing maintenance costs.
“When the government gave a firm commitment, it gave us an impetus to carry out a review [of our planned maintenance],” says Colford. “The method of funding [from the government] gives certainty of delivery and the new bridge will reduce maintenance costs on the existing crossing.”
For example the expansion joints need replacing on the bridge (NCE 19 February). To avoid bridge closure, an elaborate set of temporary ramps were proposed that would take the traffic up and over the joint replacement works. However, postponing the works by eight years until the new bridge is open eliminates the need for complicated temporary works and will save around £6M or more.
“There are always risks, joints are getting older,” says Colford. “There are no guarantees. However, we’ve gained confidence that the joint could last eight years. If a major part failed, it could be replaced overnight or on the weekend.”
The wind shields on the new crossing mean that high winds are no longer an obstacle to crossing the Firth of Forth. Even if the existing bridge is closed due to high winds, buses will be diverted onto the hard shoulder of the new bridge, assuring a constant service independent of the weather.
“The new crossing will have 3m high wind shields,” says Glover. “If a vehicle can get to the bridge, it will go across. A wind shield doesn’t stop wind, it creates a pressure difference. It allows wind to go through, but at a lower pressure. Otherwise the forces on the bridge would be excessive.”
A number of measures are being taken to try and reduce congestion on the bridge, including park and ride schemes, which are being implemented or improved, predominately on the north side of the estuary.
“We’re not producing step change in capacity and particularly not for single car commuters,” says Howison. “We’re looking for the growth to be taken up by public transport.”
Intelligent Transport System technology is also being used to try and reduce congestion; signs that give drivers information such as the time until the next junction, car parking spaces and when the next bus is due.
“We want to use technology rather than tarmac – to use technology to take the cap off the peak,” says Glover. The tender process for the design and build bridge contract is likely start this winter, but the contract is not scheduled to be awarded until spring 2011. A five year construction period should then see the route across the Firth of Forth secured by 2016.