Wind loads are essentially horizontal in nature and are exerted by wind pressure acting upon the external surfaces of the structure, and like a sailboat, the greater the surface area presented to the wind, the greater the force.
Wind pressure is directly related to wind velocity. At ground level, the wind velocity is virtually zero due to the friction created by the earth’s surface, velocity increases at higher elevations as the frictional resistance diminishes until it reaches ‘Gradient Height’. The gradient height will vary according to local conditions and will usually be specified in local or regional codes, but generally it will be approximately 300 m for flat ground, rising to 550 m for very rough terrain. At these elevations, the wind will reach its ‘Gradient Velocity’
This increase in velocity means that tall buildings are more susceptible to wind loads, and when you consider that a building is effectively a vertical cantilever anchored at its base, the overturning moment will increase exponentially in relation to its height.
Wind pressure is never constant, the proximity of other buildings or geographical features will disrupt the wind flow causing it to behave in unexpected and unpredictable ways, creating ‘swirling’ and ‘gusting’ conditions which can vary widely according to speed and direction.
This force can affect the structure in any number or ways, lateral displacement can cause the structure to lean or sway causing discomfort or anxiety to its occupants, while in extreme circumstances the entire structure could be subject to overturning.
On the windward side of the structure the wind pressure acts as a positive (or pushing) force, while on the leeward side it acts as a negative (or suction) force.
This negative force takes the form of alternating low-pressure vortices.
Wind Vortices will detach periodically from alternating sides of the structure. The structure will always tend to move in the direction of the low-pressure zone, thus causing what is commonly felt as the structure ‘panting’ or swaying from side to side. The low-pressure zones are determined by the wind direction.
This phenomenon is known as ‘Vortex-Shedding’ and if the frequency of the vortex-shedding matches that of the natural frequency of the structure, Resonance may occur.
On horizontal, long-span open structures such as bridges or elevated gantries, the wind passing over, under, and through the structure may cause a form of resonance known as ‘Aeroelastic-Flutter’, which when the frequency of varying oscillations between the high and low-pressure zones matches that of the natural frequency of the structure, they will amplify until they ultimately cause a collapse, a classic example of which is the spectacular collapse of the Tacoma Narrows bridge in 1940
Open-sided structures are particularly susceptible to positive uplift. If the wind flow through the structure is disrupted, or the exit on the leeward side is restricted, positive pressure will increase inside the building, which may cause the sidewalls or roof to ‘pant’, leading to the possibility that they may detach completely.
Generally, this condition is only a problem in industrial buildings where the sidewalls are partially open, with the remainder and the roof being ‘sheeted’ with profiled steel cladding.
To counter the effects of positive-uplift the structure should be designed so as to allow an uninterrupted airflow through the building and ensuring the sidewalls and roof are securely fixed to the steel skeletal structure.
Negative uplift is created by the ‘suction force’ of the wind passing over the structure. Roofs with a sloping profile are generally more susceptible.
Countering the Effects of Wind
The turbulence caused by wind vortices, particularly on tall structures can be mitigated by either; stiffening the structure, thereby increasing its natural frequency, or adopting the aerodynamic design approach.
Stiffening the structure is probably the most obvious and simplest approach, but on tall structures, making the structure stiff enough may render the design costly and uneconomic.
The aerodynamic approach is by far the more efficient which today is the most frequently applied approach. One method is to vary the cross-section of the structure by, ‘tapering’, ‘tiering’, or ‘stepping’ over its height, thus disrupting the uniformity of the vortex shedding.
Other effective techniques include; softening the sharp corners of the building, providing openings for the wind to pass through, or adding ‘spoilers’ to disrupt the vortices. An example of which is the ‘Helical Strake’ to be seen on tall chimney stacks.
The impact of wind pressure on structures is now often determined by simulations conducted in wind tunnels where scale models of the intended structure together with local features such as other buildings and obstacles which may affect the wind behavior are observed, monitored, and recorded.