Wind tunnelling
Liam Corcoran, National Highways
While replacing a steel barrier with a concrete central reservation is normally straightforward, the 5,128-metre iconic Prince of Wales bridge on the M4, provided a unique challenge during its new central reservation barrier installation due to its environment. The use of a wind tunnel, however, will help with the plans and will increase safety and reduce maintenance.
The proposed new type of barrier, which is safer and requires less maintenance.
Modelled by National Highways, it needed to understand the impacts of installing a concrete barrier, particularly on the 948m cable-stay section of the bridge, that is subject to strong winds and a challenging marine environment on the Severn Estuary. This section of the bridge can be particularly sensitive to windinduced oscillation – shaking – if changes are made to the road.
To ensure a concrete barrier is right for the structure and users of the road, an innovative feasibility study project was launched, aimed at modelling and assessing the aerodynamic stability implications of installing a new concrete barrier. This approach, as part of the project development stage, was pivotal because failure to correctly factor and address the impact of a concrete barrier could lead to changes in bridge behaviour during windy conditions.
By embracing innovation and working closely with engineering specialists, more efficient and safer projects can be undertaken. By embracing innovation and working closely with engineering specialists, more efficient and safer projects can be undertaken.
Modelling, trials and tests such as these, can ensure the right changes are made to see significant long-term benefits and improve the experience of roads for motorists with the barrier upgrades seeking to improve journeys and safety.
Its aim is to reduce the risk of vehicles crossing over from one carriageway to another, while reducing the duration of incident-related congestion.
Wind testing techniques
The project involved undertaking innovative wind testing techniques with a specialist consultant which also has experience with other well-known bridges across the globe, such as the Golden Gate Bridge. This allowed the proposed central reservation barrier design to be tested under different wind conditions and scenarios, whilst factoring in the specific dimensions and crosssection profile of the Prince of Wales Bridge.
The 1:70 scale model of the Prince of Wales bridge for the wind tunnel testing.
The project involved undertaking innovative wind testing techniques with a specialist consultant which also has experience with other well-known bridges across the globe, such as the Golden Gate Bridge.Taking place in Canada, the effects of wind on the bridge model could be observed virtually in a controlled and recorded bridge tunnel environment. It was determined that ‘the bridge with the new solid median will behave virtually identically to the bridge in its current state’, allowing steps to be taken towards onsite construction in 2025/26.
Results
The proposed concrete barrier is a significant improvement compared to the existing metal one and will reduce the risk of vehicles crossing over from one carriageway to another, improving safety and reducing the likelihood of incidentrelated congestion. In addition to being more maintenance-friendly, the concrete barrier was found to be able to last twice as long as the original metal barrier, with far less need for closures for routine repairs.
The current barrier requires partial closures of the bridge for ongoing maintenance, and these interventions can impact the bridge structure itself as the current barrier is bolted to the deck. Therefore the proposed barrier will require significantly less maintenance, as it is less prone to damage that requires interventions, and will not require mechanical fixing to the deck, reducing the impact on the bridge structure when maintenance is required.
Liam Corcoran, National Highways
All images courtesy of National Highways.