Introduction
Wind engineering is a significant component in civil engineering. This is because winds can have devastating effects on different manmade and natural structures. Windstorms produce large amounts of damage every year (Holmes, 2007). Weather conditions such as thunderstorms generate local winds, whereas hurricanes and typhoons may generate very strong winds. Designing of buildings and structure takes into account the dead load of the structure plus the imposed load. However, in including a factor of safety for the structures the engineers have to factor in the wind load.
Types of Windstorms
Gales from Large depressions
These are strong winds generated in mid latitudes between 40 and 60 degrees. They are normally caused by extra-tropical cyclones and large and deep depressions. The gales are very large especially in the horizontal dimension and extend for about 1000 kilometers, therefore, have the capacity to affect large areas as they pass through various regions. Turbulence is quite high next to the ground. The direction of the wind remains almost the same for long hours.
Tropical Cyclones
Tropical cyclones occur frequently in tropical oceans during the end of summer and autumn. These cyclones will occur at a temperature of 26 degree Celsius, which contributes in generating the latent heat of the ocean that drives the cyclones. They are normally strong when they occur at a latitude of 10 degrees.
Thunderstorms
These are weaker compared to the tropical cyclones, but have the ability to generate severe winds via tornadoes and downbursts (Holmes, 2007). These are common in Australia, United States and South Africa. The energy in the thunderstorms is acquired from heat. This occurs when the warm moist air is convected upwards and mixed with the dry upper air. Consequently, through evaporation process there is rapid cooling that causes the air mass to lose buoyancy, thus sinks. Condensation will cause heavy rain that drags down in combination with the cold air. When strong downdraft reaches the ground, a strong wind is produced that will last for about 10 minutes (Holmes, 2007). Factors that contribute to the presence of severe thunderstorms include high humidity, presence of a cold front or a mountain range and instability in the atmosphere.
Tornadoes
This is one of the most damaging windstorms, which occurs in the form of a funnel-shaped vortex. Horizontal distance extent is about 100 meters, but can travel a distance of 50 kilometers.
Downbursts
These windstorms can produce damage extending to about 3 kilometers wide and 15 kilometers long (Holmes, 2007). The forward velocity of the storm determines the wind speed near the ground level.
Downslope winds
These winds occur in mountainous regions. According to Holmes (2007), they are caused by the thermal intensification of synoptic winds that are on the leeward sides of the mountains.
Damage Caused by Wind
Wind has caused a lot of damage to structures such as buildings and bridges. This has caused engineers especially when construction skyscrapers in areas that experience significant amounts of wind to take into account certain aspects in the design. Holmes (2007) provides some examples of damages that have been caused by wind action. These include the Ferrybridge in the United Kingdom that collapsed in 1965 and the Great Plains Life Building whose columns deformed, in 1970. Structures such as the masts normally signify one of the major applications of wind loading in design.
Wind Generated Debris
Windstorms usually contain a significant amount of debris. Because of the high velocity of the wind, the debris carried by the wind may have a significant amount of force that may cause damage to structures. Debris increases the pressure on the building walls and may even harm occupants in a building that is occupied. Debris may include, stones, sheet and rod objects.
Design Wind Speed and Structural Safety
Structural safety is ensured through the determination of the wind load that will result from the wind speed. Thus, it becomes important to determine the wind action. In designing a structure, the structural engineer will have to include the effect of the wind action to ensure that the structure does not overturn. Determination of the wind speed may be based on probabilistic approaches such as the extreme value analysis. Simulation techniques are used for determining the wind speeds of tropical cyclones. This is because they do not occur frequently hence using historical records of the winds speeds may provide inaccurate predictions.
The most common simulation technique used is the Monte Carlo approach (Holmes, 2007). This method makes use of satellite and information such as storm size and storm intensity in developing a computer based simulation of the wind speed. This approach helps to ascertain or come up with damage predictions that may occur in case, such a windstorm is experienced. Additionally, the engineer needs to take into account the direction of the wind as it influences the structural response of a structure. The structural engineer has to incorporate a factor of safety that takes into consideration the wind action after estimating the wind speeds.
Effect of Topography and Terrain on Wind Speed
The presence of manmade and natural topography can cause an increase in the wind speeds. According to Holmes (2007), this topography may include embankments such as those for dams, cliffs, hills, escarpments and ridges. Once the wind encounters a feature, the wind speed gradually reduces then increases to attain its maximum value at the crest of the feature. This is the case for shallow features. This design is based on a two-dimensional approach since the three-dimensional approach can be complex. This leads to the development or using a factor known as the topographic multiplier in the calculation of wind speed.
Topographic mutliplier=Wind speed at height z above the featurewind speed at height z above the flat ground upwind
Terrain can also influence or cause an increase in wind speed through altering the conditions in the boundary layer of the wind. For design purpose, the topographic multiplier becomes significant when it exceeds unity.
Calculating the Wind Action
The Australian/ New Zealand structural code AS/NZS 1170.2-2002 provides a procedure to be followed when determining the wind action. This procedure is as follows
- Determination the site wind speed
- Determination the design wind speed from the site wind speeds
- Determination of the design wind pressures and distributed forces
- Calculation of wind action
Determining the Site Wind Speed
Site wind speed denoted by Vsitβ can be obtained through this equation defined for 8 cardinal directions and at a datum height z above the ground:
Vsitβ= VRMd (Mz, calMsMt)
Where;
VR= regional 3s gust wind speed, in m/s, for the annual probability of exceedence of I/R as given in section 3.
Md = wind directional multipliers for the 8 cardinal directions (β), as given in section 3
Mz, cal = terrain/ height multiplier as given in section 4 of the codes AS/NZS 1170.2-2002.
Ms = shielding multiplier as given in section 4
Mt = topographic multiplier as given in section 4.
The wind speed will be determined at an average roof height h
Determination the design wind speed from the site wind speeds
The site wind speed is used in determining orthogonal design wind speeds for buildings (Vdes, θ). Thus, the Vdes, θ is taken as the Vsitβ linearly interpolated between the cardinal points within a sector ± 45 degrees. Where there are walls and lattice towers considering an incident angle of 45 degree, Vdes, θ shall be taken as the maximum value of the site wind speed Vsitβ in a sector ±22.5 degrees. Using the ultimate state design Vdes, θ shall not be lower than 30m/s for permanent structures and not lower than 25m/s for temporary structures.
Determination of the design wind pressures and distributed forces
The following formula is used to calculate the design wind speed (p) (AS/NZS 1170.2-2002)
p= (0.5 ρair) [Vdes, θ] 2CfigCdyn
Given that
p = design wind pressure in Pascal’s
(ρair) = Density of air taken as 1.2 kg/m3
Vdes ,θ= Building orthogonal design wind speeds {θ=0, 90,180, 270}
Cfig=Aerodynamic shape factor as given in section 5
Cdyn=Dynamic response factor as given in section 6{usually 1.0 unless where the structure is dynamically sensitive.
This pressure in conjunction with the area of the building that is exposed to the wind will be used to determine the force that is caused by the wind action on the structure. Thus, the total wind action will be the summation of the total resultant forces that act on the surface of a building.
Wind Tunnels
Wind tunnels are used to show the simulation of natural wind flow. Two types of wind tunnels layouts are in use, currently. These are the open-circuit type and closed circuit (Holmes, 2007).
The mean flow variations and reduction of turbulence is achieved through using the flow straightener and fine mesh screens. In cases where it is required to model boundary layers that are extremely turbulent, the contraction is usually ignored. The diffuser is used for preserving power by ensuring that the amount of kinetic energy, which is lost through the discharged air is reduced (Holmes, 2007).
Unlike the open circuit layout, in the closed circuit layout the air is continuously used in circulation without being expelled. These provides significant advantages such as there is considerable noise reduction compared to the open circuit, increased efficiency and the ability of integrate several test sections that have different characteristics (Holmes, 2007). The disadvantage of using this type of layout is that there are higher capital costs incurred and that because of the long period of operation, heating up of the air can make it difficult when using temperature sensitive instruments like the thermal anemometers, which operate using the cooling effect of the moving air.
Blockages in Wind Tunnels
Blockage may be caused by the existence of the walls and roof of the wind tunnel and is measured as a blockage ratio. The blockage ratio is the greatest cross-sectional area of the model at any section divided by the area of the wind tunnel cross-section (Holmes, 2007). The blockages can be removed through several methods. Firstly, the blockage problem can be reduced by ensuring the blockage ratio is less than 5%. This will help ensure that errors that occur are small. Secondly, corrections can be made if the blockage ratio is high. However, this presents some level of difficulty since the corrections factors that may be required are uncertain. Thirdly, the blockage problem can be addressed by designing the walls and roof of the test section in a way that it will reduce the blockage errors.
Computational Wind Engineering
There has been an increase in the need to apply computational fluid dynamic techniques in wind engineering. These techniques are aimed to make computations of wind flow especially around buildings much simpler.
Conclusion
The increase in technology has enabled the development of new techniques in the approach of designing buildings taking into account the wind factor. Engineers are likely to benefit from the increased use of techniques such as computational fluid dynamics, which will make the analysis of wind flow around buildings much easier and faster. Furthermore, engineers have to take into consideration a great number of factors and conditions when designing buildings. For instance, they have to have knowledge of the different windstorms that occur in an area where they intend to construct a building. Probability analysis is essential in determining the wind speeds in areas that do not experience extreme winds. For areas that experience less frequent turbulent cyclones, simulation techniques prove to be of valuable help in determining the wind speeds.
Bibliography
Holmes, J.D. 2007. Wind Loading of Structures. 2nd Ed. London: Taylor and Francis Group
AS/NZS 1170.2 – 2002, Australian and New Zealand Standard, Structural design actions,
Part2: Wind actions
Holmes, J & King, A. 2002. A Guide to A Guide to AS/NZS 1170.2 – 2002 – Wind Actions,
Warreen Publishing.
