04.04 Wind

Categories: Structural Systems

Introduction
Wind is generally the dominant form of loading on the facade, and will be dependent upon such variables as site location, local topography, ground roughness, building size and shape, and the number and location of openings within the building, as well as the size and nature of the component under consideration. The determination of wind loads on buildings is covered within BS 6399: Part2, which supersedes a previous document CP3: Chapter 5. It is not the purpose here to provide a detailed commentary on BS 6399, which is covered by numerous texts as well as CWCT Technical Note No. 3, rather to discuss the general principles and nature of wind load upon façade components.

The starting point for any wind load determination will be an understanding of the prevailing weather conditions appropriate to the site location, which will generally be based upon meteorological data gathered over a representative time period. It should be noted however, that not all such data are presented in the same standardised format, and any resultant design protocol will be dependent upon the initial data. This is illustrated within the two British Standard documents mentioned above. Both start from a basic premise of identifying a ‘wind speed’ for the site location and a wind speed contour map of the UK is presented in each. However CP3: Chapter 5 works on the basis of a 3 second gust, whereas BS6399 uses the mean basic wind speed, which brings BS6399 more into line with standardised meteorological data formats available elsewhere. The design process outlined within BS6399 will at some point include an appropriate factor to account for the gust condition. The CWCT document offers another option by presenting essentially the same data as pressures thus removing a conversion step from the subsequent design process.

Unlike gravity type loads, wind loads are applied normal to the incident surface, which needs due care and attention when applied to inclined surfaces.
 

Codification Applicability
Wind loading is a dynamic effect, which most codified processes simplify into a pseudo static condition and it should be appreciated that any such procedures will have limitations. The nature and form of the main building structure and possibly localised fixtures and fittings and their response to dynamic loading will influence the applicability of a pseudo static analysis. If in any doubt, wind tunnel testing may be a valid alternative, but should only be carried out by those well versed in the technology and its interpretation.
 

Generalised effects of wind load
Any structure that obstructs or deflects the wind will result in applied loads. Buildings generally offer a blunt obstruction to the wind flow and as such a positive pressure is exerted on the surface facing the wind direction. The numerical value of this pressure cannot be greater than the actual pressure of the wind itself. The wind flow is then deflected around and over the building, as well as through any openings in the façade. Due to the disruption to the airflow turbulence is induced which gives rise to negative suction pressures which will be generated on the sides, downwind surfaces and over roofs. Depending upon the relative disruption of the airflow these negative suction pressures can have a magnitude greater than the initial positive pressure of the smooth wind flow. Since the disruption and thus resulting turbulence is greatest around sharp edges the greatest suction loads are normally generated on the corners of buildings and along the roof eaves and ridgeline. A visual illustration of this turbulence effect is the suction induced on the leading edges of trucks pulling the flexible sides outward. Funnelling of wind flow through narrow passages can also lead to increased wind loads due to induced acceleration of the wind flow.
 

Site Location
As mentioned above, the start point for wind load calculations is to determine the basic site wind speed or pressure. Within the UK higher values would be expected the further north and west the location. Site exposure will also be a factor with wind speed increasing with altitude and proximity to coastline or large open areas of water resulting in an unobstructed fetch.

The knowledge of natural prevailing weather directions highlights the effect of orientation. In the UK the prevailing conditions are predominantly from the Southwest and peak wind loads would be expected from this direction. Thus the orientation of the building and overall design can have a significant influence upon the loads induced on the cladding components.
 

Local Topography and ground roughness
Any geographical features that disrupt or accelerate the wind flow will naturally influence the resultant wind loads. Wind speed increases with altitude and so the wind loading will increase if the building is placed on top of any high terrain or adjacent to cliffs etc. The nature of any slopes upwind of the building could also have an effect leading to increased wind speeds. Ground roughness is a term given to describe the nature of the surface over which the wind flowing. The rougher the surface the more disrupted the wind flow becomes and the wind speed is lowered by an effective increase in friction. Thus open exposed coastlines see higher wind speeds than sheltered inland areas, as there is far less disruption to the wind flow. This will also apply to large inland bodies of water. Man-made features will also disrupt the airflow. The nature of adjacent buildings should be accounted for both on a macro and micro scale.  A significant change in construction type could influence the wind flow within the immediate area and change the wind flow apparent on local buildings, whereas if the local pattern is repeated there may be minimal additional loads induced. Indeed if the building is well sheltered applied wind load may be minimal.
 

Building size and shape and the gust condition
The bigger the building the more disruption to the airflow and so generally higher wind loads would be induced. Clearly the higher the building the greater the wind speed and for very tall buildings this can become of critical concern, not just for the cladding but also for the main structure. Building sway of 2m and more is not uncommon in multi-storey situations, which can be increasingly uncomfortable for the occupants. Aerodynamic shapes will lead to less disruption of the airflow and thus lower induced wind loading, but orientation will be crucial. Buildings are static and will have to accommodate the wind blowing from all directions.

Wing loading itself is determined by the gust condition. Although the idea of a mean wind speed exists, it is a necessary numerical simplification. The wind rarely blows in a steady uniform manner, rather as a haphazard series of gusts. These gusts are themselves limited in size in terms of the area they affect.  Thus the bigger the surface being considered the lower the likelihood that the whole surface will have the maximum wind load applied. This can lead to confusion within the design process. The structural designers of the main building frame will be concerned with the building as a whole whereas those responsible for the cladding will be dealing with much smaller individual building components. Each could require different wind load criteria leading to different wind loads, with the cladding engineer normally having to consider larger values because smaller components will be susceptible to the full gust loading.
 

Openings
The resultant wind load on the building fabric is a combination of the external pressures and suctions combined with the internal pressure situation. Significant openings will not only affect the external local environment but also the internal pressure situation. Openings that connect external pressure differences need careful consideration.