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Every year millions of drivers use the Severn Bridge without giving it a second thought.

But when it was built 51 years ago, having been talked about for decades, it changed suspension bridge design forever.

Michael Parsons is the most senior surviving engineer to have worked on the Grade I listed structure, which carried the M4 for 30 years.

“It goes back to the failure of the Tacoma Narrows Bridge in America, which failed because of a phenomena called flutter,” the 89-year-old said.

“When we started building the Forth and Severn Bridges we realised we would have to solve that problem.”

Famous footage shows the Washington state bridge collapsing into Puget Sound on November 7, 1940. At the time it was the third longest suspension bridge in the world.

It fell victim to 40mph winds within months of opening on July 1, 1940, and the engineers here knew that given the fierce winds across the Severn Estuary they could not afford a similar failure.

The plan for the Severn Bridge was to use an open truss design that would strengthen the carriageway.

This was what was used on the Forth bridge, in Edinburgh. It was the dominant design in the US, where it can be seen on San Francisco’s Golden Gate bridge and New York’s Brooklyn Bridge.

Things did not go to plan.

Michael said: “During testing my boss, Gilbert Roberts, came in to my office and said, ‘Parsons, I am going to the National Physical Laboratory, because the model they were testing has broken from its mooring and has broken.’

“This was the model I had developed for Sir Gilbert on a shallow truss.”

Soon the project, being run by Freeman Fox & Partners and Mott, Hay & Anderson, had to go to tender.

Michael told his boss he had another design. It involved building the roadway from metal boxes.

“I had a drawing of a possible plated box,” Michael said. “I knew from calculations it would be safe and have torsional stability.

“He said, ‘Come with me and we’ll go down to the NPL.’”

When the pair arrived Roberts went to the wind tunnel to see the damage. He showed them Michael’s plans.

“He said, ‘I want to show you a possible alternative to the truss’ and showed them the drawing I had prepared.

“He said, ‘Do you think you can make that stable?’ and they said they thought they could.

“On the way back from London I got the impression he had decided he liked the box structure.”

The new design saw 88 separate boxes, made at Chepstow’s Fairfield shipyard, form the road.

The structure was able to maintain stiffness because they were designed to act like a plane wing.

Instead of trying to resist the wind they worked in harmony with the forces placed upon them.

And where the cables on other suspension bridges hung vertically, on the Severn Bridge they hang diagonally.

This helped prevent the swinging that signalled the end for the Tacoma Narrows span.

“To the non-specialist it’s not very much,” Michael said.

“But it was a turning point.”

The first boxes were lifted into the middle of the bridge, having been floated across the water.

This was no mean feat. The river has one of the largest tidal ranges in the world. Every day it rises and falls several times by 48ft.

As a result, it took 18 months to get them in place, because particular tides were required.

Ferries would pass beneath so everyone could see what was going on.

There was huge local interest.

“I feel I did a good job,” Michael said.

“But engineers don’t go around saying what marvellous guys they are.

“They just solve problems. They do that every day.

“I knew something would work and I had the confidence to tell my boss it would and it did.”

No where in the world was there a bridge like it.

The Second Severn Crossing opened in 1996. It is a cable stayed bridge, not a suspension bridge. The roadway on the original was renamed the M48 and the SSC now carries the M4.

“I’ve only been to the new Severn Bridge once,” Michael said.

“I was disappointed because I couldn’t see anything because of the wind barriers.

“When you go over the old Severn Bridge you feel as though you’re king of the world.

“You might as well be driving through a tunnel on the new one.

“There is no comparison between them. These cable stayed bridges, they all look as though they are temporary constructions.”

Chartered civil and welding engineer John Evans also worked on the bridge.

“The plated boxes meant it was significantly lighter than other bridges of the same span and roadway area,” he said.

“The towers did not have to be as strong. The cables are significantly lighter.

“We were not able to take full advantage of the fact that the anchors could have been lighter, because at the time we were building the foundations, we were still building for a truss bridge.”

The deck was only two-thirds of the weight of a comparable bridge.

“That is because of the fact that the deck is also the running surface,” John said.

“The top of the box is a structural element but it is also the running surface.

“The old truss design would have had a running surface that was about 30ft deep, but on the Severn Bridge it is only 10ft deep.”

John, who lives only a few miles from the bridge in Redwick, compared the manufacture of the boxes to shipmaking – which Gilbert Roberts had a background in.

“We now had a deck that was not only two-thirds the weight, but very efficient to manufacture,” he said.

“The icing on the cake was that at Fairfield in Chepstow there were shipyards.”

Keith Jones is director of the Institution of Civil Engineers Cymru.

“The deck was considered by some to be too thin but it has been there for over 50 years and it is a great testament to the design,” he said.

“They had to design it to counter the windload. The challenge was to make it an aerofoil shape so the windload acted the same as that passing over the wing of a plane.

“This was the first suspension bridge to be designed to do that. Before they were not designed to cope with the windload but to withstand the windload.”

This was the case with American suspension bridges before the doomed Tacoma Narrows bridge.

“The Severn Bridge was breaking new ground,” Mr Jones said.

“There are other bridges around the world that would have taken that design and used it themselves.

“It might only have one foot in Wales but we’ll claim it.”

He remembered crossing the Severn on the Aust ferry with his dad before the bridge was there.

“It really linked England to Wales and really brought us together, not just physically but economically,” he said.

“It is more than just A to B. People don’t measure distance so much in miles as they do in time.”

The last section of bridge went in on St David’s Day, at Beachley, in 1966. It was opened by the Queen on September 8, 1966.

After five decades the toll to cross it is to be scrapped this year.

“The bridge really brought Wales closer to England and London,” Mr Jones said.

“Can you imagine now going out to Gloucester to get to London?”

Civil engineer Norman Seward lectures at the University of South Wales.

“The engineers for the first Severn crossing would have been very concerned it did not blow down and did all sorts of wind tunnel tests,” he said.

“They came up with designing it like an aeroplane wing so it could take the forces placed up on it and not oscillate.”

No one has ever found a way of doing it better.

“It was a big leap forward in terms of suspension bridge design,” Mr Seward said.

The design was adopted by every subsequent long span suspension bridge.

That included the Humber and Bosphorus bridges, upon which Michael Parsons also worked.

“The first time I really felt good and proud about it, it was not to do with the Severn Bridge, but the Bosphorus Bridge in Istanbul, which I also worked on,” he said.

“I was going to India and I looked out of the window of the plane and I could see the Bosphorus Bridge shining in the sun.

“And I thought ‘There are not that many people who can say they can see something from up here they helped to create.’

“And it started with the Severn Bridge.”