CROSS Safety Report
Local wind effects at high level
This report is over 2 years old
A reporter shares their experience on the wind speeds they encountered when carrying out high level works on a city centre building.
Key Learning Outcomes
For civil and structural design engineers:
Be aware of local wind effects and how suction pressures can be enhanced at corners, in gaps and on leeward faces which can affect finishes, glazing and cladding
Design should account for the surroundings presented to the building at all stages, including reasonable scenarios for future buildings
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Further to the Standing Committee on Structural Safety (SCOSS) alert Wind Adjacent to Tall Buildings published in December 2015 a reporter says that they had been involved in work on the outside face of a city centre building, but at a high level rather than at street level. From this, they know that the wind experienced on the leeward side are out of proportion to that experienced on the windward side.
In 5m/sec steady winds, gusting of up to 15m/sec can be felt on the leeward side. When he was working on the building, he needed to have data and help to assess the likely wind speeds to be encountered but was surprised at the lack of knowledge about what would actually happen.
It seemed to him that the structural engineers involved in the building's design had very little real idea about what local effects were likely to be. It illustrates to the reporter that more information is needed on the understanding of wind effects around tall buildings.
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On larger buildings it is usually for the engineer to define the peak velocity wind pressure, and a cladding consultant advises on the conversion of this to the pressure coefficients. For complex/tall buildings either computational fluid dynamics analysis or wind tunnel tests are used.
However as pointed out by the reporter there still seems to be a tendency for designers to concentrate on pressure when many problems are related to suction. Suction pressures can be enhanced at corners, in gaps and on leeward faces which can affect finishes, glazing and cladding. There is also a significant difference between everyday experience at ground level and that at moderate height.
The latter effect is not related to the well-known increase of wind speed with height but simply because in most cities, there is a common ‘roof height’ and above that level full exposure to wind exists. As pointed out in the SCOSS alert the design should account for the surroundings presented to the building at all stages, including reasonable scenarios for future buildings.