Localised wind barriers
Last updated on 31 Jan 2012
A study has been carried out to examine the feasibility of providing localised wind barriers on the bridge at the main towers. This was one of the recommendations of a working group set up by the Bridge Authority to look at ways of mitigating the impact of high winds on traffic crossing the bridge.
The localised barriers would be provided in order to:
- Prevent sudden gust-induced swerving of wind susceptible vehicles when passing by the towers
- Reduce the likelihood of a loss of control/overturning event due to the above
- Reduce the buffeting at the towers to no more than would be experienced in crossing the remainder of the unprotected span
- Be used in conjunction with the other measures recommended by the High Wind Working Group.
The consulting engineers, Flint and Neill Limited, were appointed to carry out the study and have produced a first draft report. Their preliminary work has shown that the erection of localised wind barriers at the towers could provide significant localised shielding.
The design concept used by Flint and Neill is based on that used for the M48 Severn Bridge solution. That system was wind tunnel tested and proven by additional vehicle trials, and has now been in successful operation for nearly 20 years. It has reportedly reduced the risk of overturning at the towers to the same level as on the rest of the bridge.
Vertical slats are proposed to provide the shielding within the barriers which will allow greater transparency for drivers as they pass them, and is considered to provide a more aesthetically pleasing effect on the bridge when viewed from afar. The target level of shielding is set so that the lateral pulse that occurs at the tower would be less than the peak lateral buffeting force on normal crossing of the unshielded span. The wind barrier system will smooth the changes past the tower, and in particular the vanes act to prevent flow reversals (eddies) that could occur immediately behind the tower legs.
The barriers would be feathered down to achieve a lateral shielding effect in order to avoid sudden changes in sideways force being applied to passing vehicles. This is done by ramping down the height and increasing the porosity.
The panel arrangement proposed is for 4.3m high panels ramping back down to 1m either side of the tower over a 40m distance.
Initial discussions have taken place with both local authorities and Historic Scotland over the form of the localised shielding and a formal planning application will be made once the scheme is developed further.
The design loading on the barrier needs to be transmitted into the deck’s lateral bearings and the next stage is for the assessment of these bearings to be made under various load combinations.
If loads are too great for the deck bearings, a more rigorous wind tunnel test may have to be carried out to refine the analysis and design.
Implementation of the scheme will depend on budget, approval from Historic Scotland, and a determination of whether the scheme will deliver value for money in light of plans to remove all traffic except public transport from the bridge when the new crossing opens in 2016.
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