Aerodynamic Modifications of Tall Buildings

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AERODYNAMIC MODIFICATIONS OF TALL BUILDINGS

Several rounds of wind tests were undertaken as the geometry of the tower evolved and were refined architecturally. The three wings set back in a clockwise sequence with the A wing setting back first.

After each round of wind tunnel testing, the data was analyzed and the building was reshaped to minimize wind effects and accommodate unrelated changes in the Client’s program. In general, the number and spacing of the setbacks changed as did the shape of wings. This process resulted in a substantial reduction in wind forces on the tower by “confusing” the wind. On the 1:500 scale models, tests were made both with and without vertical ribs that are a feature of the tower’s wall system in order to understand how much their effect was. At 1:500 scales the ribs were very small and thus had been left off for the main test program. (Peter and Baker, 2006)

  1. Pedestrian wind environment
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The comfort of pedestrians at ground level and on the numerous terrace levels was evaluated by combining wind speed measurements on wind tunnel models with the local wind statistics and other climatic information. Initial wind tunnel tests used 1:500 scale models. Subsequently three 1:250 scale partial models were employed to examine ground level areas, lower level terraces and higher level terraces in more detail, and to develop detailed mitigation measures.

Initial results from the thermal comfort study highlighted the need to introduce shade structures to avoid the strong adverse impact of solar radiation on thermal comfort in Dubai. A number of canopies and other types of shade structure were architecturally designed at ground level. Tests on the bare terraces indicated the potential for frequent uncomfortably strong winds, which can be protected by a combination of parapet walls, overhead trellises, and vertical screens.

  1. Conclusions:

The structural system of Burj Dubai is a buttressed core of high quality reinforced concrete. The weight of the building provided anchorage against lateral loads but was not sufficient enough to provide human comfort levels inside. The structure and weight of the building cannot be increased further as it was built on a sandy soil. The three main modifications made to reduce wind are: softening the corners of buttress or the nose, the setbacks and the reorientation of the building in the prevailing wind direction. The corner softening and changing the cross section of the building diverge the direct winds and reduce the direct load component of the wind on the building by using cutwater effect. It also reduces the vortex formation thus reduce its impact on cladding. The setbacks created a tapering effect on the building and disorganized the vortices which might have been formed in case of a straight form thus preventing stack effect. The most important modification was the changing of orientation of the building. It was reoriented such that the nose ends of the building face the prevailing winds than the tail ends. In this way the overall wind effect on the building (i.e. the base moments) got reduced significantly. The maximum displacement at the very top of the spire is 2 m which may occur once in 100 years. Even minor details were tested such as ribs and terraces were tested at larger scale models but were left off for overall design as they did not change the results significantly. The Burj as a whole can be considered as a highly tapered beam cantilevered from the ground.

  1. Taipei 101 : Secondary Case Study

Taipei 101 is designed to withstand thetyphoonwinds and earthquake tremors common in its area of the Asia-Pacific. Planners aimed for a structure that could withstand gale winds of 60m/s and the strongest earthquakes likely to occur in a 2,500 year cycle. The design achieves both strength and flexibility for the tower through the use of high-performance steel construction.

http://www.brianmicklethwait.com/culture/taipei101.jpg

  • Official Name:Taipei 101
  • Also Known As:Taipei World Financial Center
  • Built:2000-2004
  • Designed By:C. Y. LEE
  • Wind Engineer : RWDI Engineers
  • Type:Skyscraper
  • Stories:106
  • Maximum Height:508 Meters
  • Location:Xinyi District,Taipei,Taiwan
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The original corners of the façade were tested atRWDIinGuelph,Ontario,Canadaand revealed an alarming vortex that formed during a 3s 105mph winds at a height of 10 meters (a 100-year-storm) simulation. This was equivalent to the lateral tower sway rate causing largecrosswindoscillations. A double chamfered step design was found to dramatically reduce thiscrosswindoscillationresulting in Taipei 101’s unique "double stair step" corner façade. (http://en.wikipedia.org/wiki/Taipei_101). ArchitectC.Y. Leealso used extensive façade elements to represent the symbolic identity he pursued. These façade elements included the green tinted glass for the indigenous slender bamboo look, eight upper outwards inclined tiers of pagoda each with eight floors. A 660-metric-ton tuned mass damperstabilizes the tower against movements caused by high winds. The damper can reduce up to 40% of the tower's movements.

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  1. 151 Incheon Tower: secondary case study

The design of 151 Incheon Tower consists of two 151 storey towers connected by three sky bridges and basements to depths of 8.5m. The 151 Incheon Tower will stand 600m in height and be regarded as one of the world’s top five tallest towers.

This building design uses the openings or porosity through the buildings. Thus it provides an alternative route through the building. Therefore the building is designed as twin towers. The corners are modified as bleed slots through which wind can pass through without exerting much pressure on the building. This modification reduces base moments by almost 60%.

  1. CONCLUSION

We can see that wind affects large portion (ILGIN, 2006) of the building surfaces which created large lateral loads. It is quite unpredictable at lower levels. As the height increases they are constant and have a regular pattern according to the movement of sun. So for most of the buildings 40m to about 70m is the range of height that gets most unpredictable and rapidly changing wind forces. So this range of height needs to be taken care of properly.

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It is the wind study that is complicated. The solution to this comes from structural systems partly but more accurately by applications of fluid dynamics. Fluid dynamics is a very general subject that we use knowingly or unknowingly in our daily lives in applications from perfume sprays to flying an airplane. When we apply its principles and applications to wind it becomes aerodynamics. Aerodynamics was mainly and extensively used in design of planes and jets and other automobiles. However after the collapse of huge bridges and buildings its importance in design of buildings was seen as a solution that can be applied without complicating its structural systems further. Although it cannot completely eliminate wind effects, it brings down the vibrations to a comfort level.

Aerodynamics is application of principles on the outer part of a building or its skin in order to reduce its impact on the building. It is a technique to confuse the wind. It basically is a way to deflect or divert the wind to other way. Even a lateral movement of 1/200 of building height would mean a movement of 2m in a 400m high building which a huge strain on internal partitions and human comfort. Almost all of today’s tallest buildings are designed using this concept. It reduces the wind impact or base moments by about 40% to 35% with rather simpler modifications than changing its structure. The façade design and cladding members impose restrictions on these modifications in many cases.

If these ideas are incorporated into the design from the start then they will out very efficient design which require very little modifications later in the end. Apart from structural stability, these modifications provide aesthetically good buildings as can be seen in buildings like Petronas towers, Tapeii 101, Burj Khalifa, etc.

Ideas of channelizing the wind through buildings to run wind turbines are the next upcoming idea that would be soon seen in near future.

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