Scotland

Could a windshield keep the new Forth bridge open?

Lorry on Forth Road Bridge Image copyright Forth Road Bridge

Most of Scotland's major bridges were closed to high-sided vehicles on Wednesday as gales battered the country, but the new Queensferry Crossing will have a windshield to protect it from the elements. Will it work?

The new Queensferry Crossing, due to open in the summer, will not be closed by high winds, its designers have said.

The new bridge across the Forth between Edinburgh and Fife has been fitted with 3.5m-high baffle barriers to break up and deflect gusts of wind.

They have been modelled in a wind tunnel by Italian engineers and tested on sections of road near the bridge.

Bridge operators say the wind shields should "almost entirely eliminate the need for closures".

Image copyright Transport Scotland
Image caption The wind barrier is being tested on an approach road to the bridge

That is in stark contrast to the current Forth Road Bridge which is often closed to high-sided vehicles and HGVs as winds blow up the river estuary.

On Wednesday, the bridge was closed for the entire day as a lorry was blown over in the storm. The bridge had been officially closed to lorries at the time.

On Thursday, Scottish Tory deputy leader Murdo Fraser, a regional MSP for Mid Scotland and Fife, raised the issue during First Minister's Questions at Holyrood.

He said: "It will not have escaped the notice of my constituents that if the new Queensferry Crossing, with its windshielding, had been opened in December, as the first minister previously promised, they might well have been spared this disruption."

Ms Sturgeon replied that the contractual completion date for the crossing was June this year and it was "on track" for that date.

Image copyright Transport Scotland
Image caption The Queensferry Crossing will open in May
Image copyright Transport Scotland
Image caption An artist's impression of how the crossing should look

The new £1.4bn Queensferry Crossing is a 1.7 miles (2.7km) structure and will be the longest three-tower, cable-stayed bridge in the world.

Once it is opened, the current Forth Road Bridge will be used for public transport, cycling and walking.

The new crossing will take the vast bulk of the current load and it is essential that it remains open.

Designers say wind shields are being placed along each side of the bridge, which have been tested against the stormiest of conditions.

Transport Scotland project director David Climie told BBC Scotland: "We expect the wind shields to almost entirely eliminate the need for closures during the frequent periods of high winds in the Forth estuary, apart from in exceptional circumstances.

"This will increase the resilience of what will be the main crossing over the Forth estuary in future."

Image copyright PA
Image caption The Forth Road Bridge is frequently closed to high-sided vehicles

Mike Glover, technical director for the Queensferry Crossing Project, recently told BBC Scotland's Brainwaves programme the brief was that if people were able to get to the bridge they should always be able to cross it.

He says: "One solution would be to build a wind barrier, a solid thing, no wind would be able to get through.

"If you do that you will change the dynamics of the bridge quite dramatically and not positively, very badly.

"So what you have to do is create a wind screen which accepts the fact that air pressure will come through.

"You are creating a permeable screen through which some of the air comes but the rest is scooped up and over the bridge."

Bridge expert Alan Simpson, a past chairman of the Institution of Civil Engineers Scotland, agrees.

He says the barriers are "more like a thick hedge than a wall".

Mr Simpson says there is no doubt that the wind barriers will massively reduce the number of times the crossing will be closed to high-sided vehicles.

He says the design will have been tested in a wind tunnel to measure the pattern of the wind after it hits the barrier.

"It will certainly reduce incidents. In all normal circumstances people will be able to cross," he says.

The retired civil engineer, who was a regular consultant on the Forth Road Bridge for 40 years, says the older crossing has no wind barriers and many of the problems are caused by the fact that the towers give shelter and then vehicles emerge into the full force of the storm.

Some bridges have had wind shield baffling fitted retrospectively but this means extra force is added to the structure.

This was not something that was considered possible on the Forth Road Bridge, which has had problems with the corrosion of its supporting wires.

Other more modern long bridges in the UK, such as the Second Severn Crossing, opened in 1996, do have some wind shield protection, usually on either side of the bridge towers.

Latest technology

The Queensferry Crossing will have wind shields across its full length.

Mr Simpson says: "The wind on the Forth is so strong in comparison to the other major bridges. We are talking about a bridge at the same latitude as Labrador in Canada."

He adds: "When the Forth Road Bridge was planned and built, more than 50 years ago, no major bridges of that type had been built so far north in such an extreme environment."

Mr Simpson says the Queensferry Crossing is a modern bridge using the latest technology which is designed for the needs of the 21st Century.

However he warns that the designers of the previous road bridge did not foresee the levels of traffic or the size and weight of the vehicles.

He says: "What will happen in 50 years time? Nobody knows. Maybe we will look back and say they were really stupid not to design for x or y. You just hope you know what is going to happen."

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