Why Queensferry Crossing windbreaks should be considered on future bridges

According to a bridge expert, it is possible that the windbreak used on the Queensferry Crossing will be used on other new bridges to prevent them from closing in bad weather.

Earlier this month it was revealed that wind protection has allowed Queensferry Crossing to remain open in strong winds on more than 100 occasions when it would otherwise have been forced to close. The landmark was reached during storms in early February, which brought wind gusts of up to 109 km/h (68 mph).

Speaking on a personal basis, Bear Scotland’s South East Unit Bridges Manager Chris Tracey said NCE that wind protection “should be considered” on “new long-span bridges that may be wind-sensitive”.

The establishment of this system, however, depends on the cost-benefit analysis.

“There is a cost associated with the wind protection itself, but your structure will also need to be sturdier,” Tracey said. “So the section sizes will have to be larger to withstand these loads and there will be more steel and concrete, which will increase costs.

“Although you could consider having a more aerodynamic profile – that’s one of the things they’ve done on Queensferry. It’s been designed to have a very aerodynamic profile so the bridge itself doesn’t have not too much impact on wind load.

The Queensferry Crossing is fitted with a specially designed 3.3m high wind deflector, which deflects gusts of wind and onto traffic. This allows it to remain open at significantly higher wind speeds than the unshielded Forth Road Bridge.

This approach has clear advantages: the Forth Road Bridge must close to winds of over 80km/h (50mph), while Queensferry can remain open at up to 112km/h (70mph).

“At that time you don’t have a lot of traffic on the road in those conditions anyway,” Tracey said. He added that it is mainly long-span bridges that are more sensitive to wind load, but the height of the crossing is also important.

Jacobs, as lead partner in the Jacobs Arup joint venture, supported Transport Scotland in the development of Queensferry Crossing. Jacobs’ global technology manager for decks, Nick Fuchs, also supports the wind protection approach.

“With windscreens now fitted to an increasing number of major over-water crossings such as the Prince of Wales, Queensferry and Mersey Gateway bridges, there is growing evidence of the effectiveness barriers to keep these valuable assets open in bad weather,” he said.

“This is particularly evident where new crossings have been constructed close to older crossings which have no wind protection allowing a direct comparison of the number of times the old bridge is subjected to restrictions on the passage of vehicles with high walls or even be closed.

“The adoption of a wind barrier on new build structures is relatively straightforward, but comes with additional capital cost implications due to the increased wind load the structure must withstand. “

Upgrading existing bridges is more of a challenge as these would likely need to be strengthened to cope with the increased wind load. It is the force of the wind against the structure. If the surface of a structure is changed, it will attract an increased wind load.

It was not possible to install a windbreak on the Forth Road Bridge, for example, due to the extra load this would have placed on the structure.

“The Queensferry Crossing has been designed with wind protection in mind, so it has been designed to withstand the additional wind load that wind protection brings onto the bridge,” Tracey said. “However, upgrading wind shields on structures can be problematic. You would have to assess the deck for the additional wind load.

“You would have to do an analysis of the wind to see what kind of loads it was going to attract, probably some modeling in a wind tunnel, and then do dynamic assessments of the effects on the structure. I imagine that in most structures some form of reinforcement would be required to accommodate this additional wind load. This is the difficult part.

Fuchs added: “Upgrading wind deflectors on existing structures is more difficult as reinforcement is likely to be required and aerodynamic behaviors may be altered.”

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