3.4 Structural Design of Steel Buildings for Winds Loads

Wind forces fluctuate with time but for many structures the dynamic effect is small and the wind load can be treated using normal static methods. Such structures are defined as 'rigid' and Eurocode 1 provides guidance on this classification. For slender structures the dynamic effect may be significant. Such structures are classified as 'flexible' structures and their dynamic behaviour must be taken into account.

The most important parameter in quantifying wind loads is the wind speed. The basis for design is the maximum wind speed (gust) predicted for the design life of the structure. Factors which influence its magnitude are:

  • Geographical location; wind speeds are statistically greater in certain regions than others. For many areas considerable statistical data is now available and basic wind speeds are published usually in the form of isopleths which are lines of equal basic wind speed superimposed on a map. The basic wind speed is referred to in Eurocode 1 as the reference wind speed and corresponds to the mean velocity at 10m above flat open country averaged over a period of 10 minutes with a return period of 50 years.
  • Physical location; winds gust to higher speeds in exposed locations such as coasts than in more sheltered places such as city centers, because of varying surface roughness which reduces the wind speed at ground level. This variation is taken into account by a roughness coefficient which is related to the roughness of the terrain and the height above ground level.
  • Topography; the particular features of a site in relation to hills or escarpments are taken into account by a topography coefficient.
  • Building dimensions; height is important in particular because wind speeds increase with height above ground level.
  • The mean wind velocity is determined by the reference wind velocity factored to account for the building height, ground roughness and topography. The wind pressure is proportional to the square of the mean wind speed. In addition the following parameters are important:
  • Structural shape; it is important to recognise that wind loads are not simply a frontal pressure applied to the facade of a structure but are the result of a complex pressure distribution on all faces due to the movement of air around the whole structure. The distribution is further complicated by adjacent structures and natural obstructions/variations such as hills, valleys, woodland which may all influence the pattern of air movement and associated pressure distribution.
  • Roof pitch; this parameter is really a special aspect of structural shape. It is worth noting that roofs with a very shallow pitch may be subject to uplift or suction, whilst steeper roofs - say greater than about 20° - are likely to be subject to a downwards pressure.
  • Wind direction; pressure distributions will change for different wind directions.
  • Gust response factor; this factor is used to take into account the reduction of the spatial average of the wind pressure with increasing area due to the non-coincidence of maximum local pressures acting on the external surface of the structure. Thus small parts of a building, such as cladding units and their fixings, must be designed for higher wind pressures than the whole structure. The gust response factor is related to an equivalent height, which corresponds approximately to the centroid of the net wind force on a structure.

Tabulated procedures enable the above parameters to be accounted for firstly in calculating the design wind speed, and secondly in translating that wind speed into a system of forces on the structure. These equivalent static forces can then be used in the analysis and resistance design of the structure, as a whole. However, certain additional features of wind should also be taken into account:

  • Local pressures, particularly at corners and around obstructions in an otherwise 'smooth' surface, can be significantly higher than the general level. High local pressures particularly affect cladding and fixing details, but can also be a consideration for structural elements in these areas.
  • Structures sensitive to wind should be given a more sophisticated treatment. It might involve wind tunnel testing and include the influence of surrounding buildings. Structures which might need to be treated in this way include high-rise buildings, long or slender bridges, masts and towers.
  • Aerodynamic instability may be a consideration for certain types of structure or component, for example chimneys and masts. Vortex shedding can normally be avoided by the use of strakes. Galloping may be a problem in cables.
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