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From the sand of the Arabian Desert, rises the tallest man made structure on earth. A city in the sky, nearly half-a-kilometer tall, the Burj Khalifa is a pinnacle of engineering. Building a skyscraper such tall would face many problems. Among them the pressure of wind was a pretty big headache for the engineers. Tall slender structures like these are susceptible to wind induced vibration.
In a steady flow of air at lower speed, the net force of a cylinder will be same direction. The cylinder will bend in the direction of air. The speed and direction may fluctuate but the cylinder will not vibrate consistently (fig:2-A) . At lower speed this is the case, but as the speed increases the air begins to separate from the surface of the cylinder. Creating two different eddies (fig 2-B) (an eddy is the swirling of a fluid and the reverse current created when the fluid is in a turbulent flow regime). As long as this eddies are symmetrical the force will remain in the direction of the fluid flow. But there is a critical moment when the system loses its stability, the energy gradient from the main stream and the slow moving eddies become too high, eddies begins to oscillate (fig:2-C) . As this point phenomenon called vortex shedding occur, and the resultant forces is no longer downstream. This can become a massive issue if the frequency of the shedding matches the resonant frequency. That means that the direction of the sway and the direction of the force become synchronized and the amplitude of the swaying is allowed to grow as the energy is being stored between each cycle. Every building dissipates some of that oscillation energy through natural dampening, through materials and through its friction at the joint. But in the case of skyscraper as tall as this, these are not enough. The world former tallest building “Taipei 101 Tower” used mass dampener to oppose the oscillation. Engineers would tune the damper to the same frequency of the oscillation. So, when the building moves right, the damper moves left. Thus the kinetic energy is dissipated and the magnitude of resonant motion is reduced. But for structures made as commercial use as Burj Khalifa it will cost money and space. So the scientists rely on simply aerodynamics to stop this from happening.
The design of the tower is such that it confuses the wind force. The wind force increases as the height of the tower increases, and if the building height is like that of Burj Khalifa, than the allowable deflection is 3m or so which can make the people inside the building uncomfortable. So what they have done is, they have regularly changed the profile of the building so that the boundary layer of wind that is formed around the building is the turbulent one. Also the wind that is flowing will get confused with height because of the change in the shape of the structure. This reduces wind force drastically and helps the tower to remain upright. The lateral forces are simply carried by the tower frame and spinal wall system. The successive tiers of the Burj Khalifa work like a kind of spiral, with the different arms of its Y-shaped base gradually getting shorter and shorter, one by one. The shape of the structure does not let the wind force to make vortices and eddies like a cylinder. And thus it does not move so much though it is a desert storm outside. The building has essentially six important wind direction. The principle wind directions are when the wind is blowing into the “nose”/”cutwater” of each of the three wings (corners of the building). The other three directions are in between two wings, can be termed as tail directions. It was noticed that the force spectra for different wind directions showed less excitation in the important frequency range for winds impacting the pointed or nose end of a wing than from the opposite direction, tail. This was kept in mind when selecting the orientation of the tower relative to the most frequent strong wind directions for Dubai and the direction of the setbacks.
Today, the total urban population has already surpassed 50% of the whole population; in 2050 it will be 80%. And to meet the demand some day we will see buildings like this everywhere. And this very building, the Burj Khalifa will be the pioneer of that era.
Source:
William, D. Stnton, Lawrence C.
“Engineering the worlds tallest- Burj Dubai”
Official website of Burj Khalifa- http://www.burjkhalifa.ae
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