Maduka Nweke; [email protected] 08034207864, 08118879331
Wind has great effects on structures but unfortunately, many people don’t consider its direction when building their houses. Several times, this has resulted in houses standing on the way of wind.
Those who build houses without considering wind direction do themselves much harm when windy seasons come. So, mounting anemometer (instrument to know wind direction) in and around your home will help so that you don’t waste great fortunes.
Effects of wind on structures can be classified as static and dynamic. Static wind effect primarily causes elastic bending and twisting of structures. And for tall, long span and slender structures, a dynamic analysis of the structure is essential. Wind gusts cause fluctuating forces on the structure, which induce large dynamic motions, including oscillations.
Wind flow is a dynamic and random phenomenon that is complex to model. It is composed of eddies of varying sizes and rotational characteristics that are carried along in a stream of air that moves relative to the earth’s surface. These eddies give wind its gusty or turbulent character, with speeds and forces that vary considerably in both time and space. In order to model wind flow, the wind vector at a point may be regarded as the sum of the mean wind vector (static component) and the dynamic or turbulent component due to wind speed variations from the mean.
Government of civilised countries always take cognizance of these effects and from the onset, the authorities that supervise and regulate structures do not waste time in correcting anomalies before the structures get high. In the lower levels of the atmosphere, the turbulence of strong winds largely arise from contact with surface features.
Therefore, wind flow is particularly gusty in urban areas. This is because as wind impacts a building, further chaos is added to the already unstable nature of wind by the separation of flow, the distortion of the mean flow, the formation of vortices, and the development of a wake.
These effects generate large, fluctuating wind pressures on the surface of the building that depend on the interactions of the flow characteristics (such as wind speed, wind height, ground surface features, air properties) with the building configuration (i.e its shape, location, and dynamic and physical structural properties). As a result of these pressures, large aerodynamic loads get imposed on the building that may cause significant structural excitation and vibration provided that the natural frequency is low enough (less than 1 Hz. Above this value the structure is considered to be dynamically ‘rigid’). Therefore, understanding the action of wind around dynamically sensitive buildings is of prime concern.
The authorities of government must be the one that understands the position of the wind so as to give veritable advice that will aid in supervision. In all intents and purposes, tall buildings and skyscrapers are becoming increasingly complex in overall design and scale which puts them at a greater risk to wind effects and induced forces on the structure. Nigeria and perhaps most African countries have not been experiencing the kind of fierce wind that is being discussed here but nothing stops them from taking precautionary measures to checkmate it in the event of occurrence. So the architectural and design engineers must ensure a safe, sustainable and cost-efficient design by utilising wind engineering studies. These studies are now an industry standard and are conducted to first evaluate the dynamic effect of wind on the structure and then to optimise the design to mitigate these effects. This is because for tall, high aspect-ratio structures, the analysis of unsteady vortex shedding is vital since this induces oscillating crosswind forces with certain frequency. If these coincide with the natural frequency of the structure, it could enhance the motion and either lead to damage or even failure of the structure.
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As previously stated, the overall response of a structure to wind loading depends on the structure’s natural fundamental frequency. The new generation of high-rise buildings are taller and more slender than those constructed in the past. Additionally, these modern structures are built with high-strength materials that have similar stiffness properties to conventional materials. They also contain fewer non-structural components (such as separation walls), which are often applied in such a way that they stand free with respect to small building motions. As a result, there is less damping in the building.
This combination of less weight, more flexibility, and lower damping lead to buildings that are more susceptible to wind action and dynamic behaviour. These buildings tend to have lower natural frequencies of vibration, which are more likely to coincide with the average frequencies of occurrence of wind gusts, hence large resonant motions are more likely to occur. As a result, the design of tall buildings against dynamic movement has gained importance. In the design of these buildings, two important factors must be taken into account: the design has to ensure both the safety of the building and the comfort in the building. This is also affected by the direction of the wind and where you face the front view of your home. With accurate directional position, the effect of the wind on the building could be minimised irrespective of the type of wind whether it is a dynamic or a static wind.
In the design of tall buildings for wind, it is essential to ensure that the structure has sufficient strength to resist the wind-induced forces, and that it has adequate stiffness to satisfy serviceability criteria in terms of lateral displacement. Furthermore, wind-induced motion should be limited to ensure the comfort of the occupants. Vibrating buildings can cause unsafe feelings and induce concern regarding the structural quality and integrity. Thus, it is considered that humans are surprisingly sensitive to vibration, to the extent that even motions that correspond to relatively low levels of stress and strain may feel uncomfortable. Therefore, fulfilling the vibration criteria for human comfort is decisive in the structural design of most tall buildings.
Under the action of wind flow, structures can experience two different responses, namely a static response and an aerodynamic (or dynamic) response. Aerodynamic forces on the structure include the drag (along-wind) force, which acts in the direction of the mean wind, and the lift (cross-wind or transverse) force, which acts perpendicular to the direction. These forces arise from different wind effects. It has been shown that the along-wind response of slender structures is mostly due to the action of turbulence buffeting and that these along-wind accelerations are larger for structures with lower fundamental frequencies. The cross-wind response is more likely to arise from vortex shedding or galloping. This response is likely to exceed along-wind accelerations if the building is slender about both axes.