'Mini-Bridges' to Keep Forth Traffic Flowing

‘Mini-Bridges’ to Keep Forth Traffic Flowing

Engineers at the Forth Road Bridge have unveiled dramatic plans for “mini-bridges” designed to slash the impact of major roadworks scheduled for next year.

Last November a feasibility study commissioned by the Forth Estuary Transport Authority (FETA) reported that a continuous contraflow of at least eight weeks would be required to replace the bridge’s main expansion joints, resulting in unprecedented disruption to traffic.

However, FETA set up an engineering project team to carry on looking for innovative ways of limiting the period of closures. Now the bridge authority has revealed a scheme to build two large temporary ramps – dubbed “mini-bridges” – over the main expansion joints, allowing workers to rebuild the worn-out components piece by piece while traffic continues to flow overhead.

The custom-built ramps would be significant structures in their own right, measuring at least 80 metres long and over two metres high. They would be positioned under each of the Forth Road Bridge’s towers. A 30 mph speed limit would be in place and all vehicles except HGVs would be able to use them.

The ramps would take several days to construct and dismantle, so week-long contraflows would still be required at the beginning and the end of the project. Similar restrictions would be required the following year when the joints on the opposite carriageway were replaced. However, at least six weeks of continuous contraflow would be avoided each year.

A new report to the Forth Estuary Transport Authority states that estimated costs for the scheme are still to be finalised, but there would be a significant increase over the original £3.5 million estimate for replacing the main expansion joints. Additional funding would be required from the Scottish Government.

Barry Colford, FETA Chief Engineer & Bridgemaster, said: “These joints have reached the end of their design life and need to be replaced. However, eight weeks of continuous contraflow on the Forth Road Bridge would bring almost intolerable pressure to bear on the transport network. That’s why we kept on looking for innovative ways of reducing the period of closures required.

“We’ve worked up the design of these ramps and carried out the calculations, and we’ve now confirmed that this is technically possible. Some strengthening of the bridge deck will be required and the overall cost and length of the project will increase.

“The additional work required would delay the project until after our original preferred start date of April 2009. However, with the ramps in place the timing of the works is not so critical.” Councillor Tony Martin, FETA Convener, commented: “These ramps will add significantly to the cost of replacing the main expansion joints. However, recent economic studies indicate that this is good value for money compared to the delays and costs to business of a continuous eight-week contraflow.

“Initial feedback from our major stakeholders has been very positive, and they have indicated that two separate one-week closures would be far easier to cope with than a continuous eight-week period.

“HGVs would probably not be able to cross the bridge in one direction during the works, but this only represents 3% of the bridge’s total traffic. Feedback from the haulage industry is positive as this scheme would prevent the alternative routes becoming gridlocked.”

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