Modifying Aerodynamics Around Tall Buildings

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Modifying Aerodynamics Around Tall Buildings

ABSTRACT: Wind is a very powerful and unpredictable force affecting tall buildings worldwide. Its load increases with height and is more often sudden and in changing directions. Additionally it creates huge pressure differences on different sides of the building. It is becoming increasingly difficult to resist its force by structure alone. A better alternative is to understand the aerodynamics around it and design the form of the building in such a way that wind can induce the least impact. So today we can see that by using wind tunnel technology and other advanced technologies, the building forms are changing from straight rectangular blocks to more curved and streamlined forms. The principle behind this is to deflect or channelize as much wind as possible to reduce its impact on the structure of building. This can be achieved by corner modifications, tapering and setbacks, providing openings through the building or by sculpting the tops. Sometimes the aerodynamic study of the site helps in positioning of the building as in case of Burj Khalifa. Not only this study helps to mitigate the force on windward side but also reduces vortices on the leeward side thus minimizing wind shadow zone. Also care should be taken that the design does not produce vertical wind drifts which can cause pedestrian accidents. There are some proposals for channelizing these high speed winds at such heights to power the building. So the next generation skyscrapers will be an interdisciplinary product of architectural, structural and aerospace engineering fields. This is a new concept and is being successfully used in design of today tallest buildings. (ILGIN, 2006)

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KEYWORDS: Aerodynamics, Wind Tunnel Engineering, Vortices, Tall Building, Wind Excitation.

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TABLE OF CONTENTS

ABSTRACT………………………………………………………………………………………1

DECLARATION…………………………………………………………………………………2

ACKNOWLEDGEMENTS……………………………………………………………………....3

TABLE OF CONTENTS…………………………………………………………………………4

LIST OF FIGURES………………………………………………………………………………6

CHAPTERS………………………………………………………………………………………7

  1. IMPORTANCE OF AERODYNAMIC MODIFICATIONS …………………………….7
    1. Introduction to aerodynamic modifications…………………………………………..7
    2. Research question……………………………………………………………………..9
    3. Aims…………………………………………………………………………………..9
    4. Objectives ……………………………………………………………………………9
    5. Scope ………………………………………………………………………………10
    6. Limitations ………………………………………………………………………10
    7. Research framework ………………………………………………………………10
  2. TALL BUILDINGS ……………………………………………………………………11
    1. Definition of tall building. ………………………………………………………….11
    2. Development of tall buildings ……………………………………………………..11
  3. WIND LOADS …………………………………………………………………………..15
    1. Wind loading on structure ………………………………………………………….15
    2. Nature of wind ………………………………………………………………………16
      1. Variation of wind speed with height ………………………………………16
      2. Vortex-shedding phenomenon ……………………………………………17
        1. Along wind motion …………………………………………………18
        2. Across wind motion ………………………………………………….18
      3. Cladding pressures …………………………………………………………18
    3. Wind tunnel engineering ……………………………………………………………19
      1. Wind tunnel tests …………………………………………………………….20
      2. Pedestrian wind studies ………………..……………………………………21
  4. AERODYNAMIC MODIFICATIONS AGAINST WIND EXCITATION …………..24
    1. Sculpted building tops .…………………………………………………………….24
    2. Tapered form ...……………………………………………………………………..25
    3. Corner modifications ………………………………………………………………25
    4. Addition of openings through structure …………………………………………….25
  5. CASE STUDIES ………………………………………………………………………. 26
    1. Burj Khalifa: secondary case study ……………………………………………….. 26
      1. Wind climate study …………………………………………………………. 28
      2. Wind loading on main structure ……………………………………………. 29
      3. Pedestrian wind environment ………………………………………………. 30
      4. Conclusions…………………………………………………………………. 30
    2. Taipei 101: secondary case study ………………………………………………….. 31
    3. 151 Incheon tower: secondary case study …………………………………………. 33
  6. CONCLUSIONS ………………………………………………………………………. 35

BIBLIOGRAPHY …………………………………………………………………………. 36

PLAGIARISM REPORT …...……………………………………………………………… 38

LIST OF FIGURES

Figure 2.1. Monadnock Building, Chicago, USA. ……………………………… 11

Figure 2.2.Impact of wind along the height of the building ……………………… 12

Figure 2.3.Structural systems with increasing height. ……………………………..13

Figure 2.4. Fluid flow pattern around different basic shapes ………………………14

Figure 3.1. Wind pressure around a building ………………………………………15

Figure 3.2. Variation of wind speed with height ……………………………………17

Figure 3.3. Wind pattern around rectangular building ………………………………17

Figure 3.4. Wind pattern around rectangular building –vortex………………………18

Figure 3.5. Wind Tunnel Testing Of Petronas Towers …………………………….. 20

Figure 3.6.a Vortex Excitation on Tapered Spire –Mode1………………………… 21

Figure 3.6.b. Vortex Excitation on Tapered Spire –Mode1 ………………..……… 21

Figure 3.7. Design considerations for pedestrian wind studies: (a) downwash to street level; (b) high wind areas at the ground-level corners; (c) a large canopy; (d) large podiums; (e) recessed entry; (f) an arcade or an open columned plaza under a building; (g) corner entry ……………………………………………………… 23

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Figure 4.1. Corner modifications …………………………………………………… 25

Figure 5.1. Burj Khalifa …………………………………………………………..… 26

Figure 5.2. Plan View of Burj Khalifa Tower ………………………..……………. 28

Figure 5.3. Vortex Formation around the Tower ……………………………….. 29

Figure 5.4. Taipei 101 tower ……………………………………………………… 30

Figure 5.5. Plan View of Taipei 101 …………………………………………….. 31

Figure 5.6. Tuned Mass Damper in Taipei 101 Tower …………………………….. 32

Figure 5.7. Full Rendered View of Incheon Tower ………………………………… 34

  1. INTRODUCTION
    1. Introduction to aerodynamic modifications

Humans have always competed with each other to show their supremacy, power, talent, etc. in different fields with different types of expressions. One such pronounced expression is by buildings monumental buildings which have always grown vertically to signify their importance. Every advance in height comes with a new set of problems. Everytime a new technology or idea is required to cross the hurdle and each time it happens that a new crop of such structures using such technology are raised wherever economy permits. Earlier the tall and monumental buildings were meant for the purpose of worshiping (temple and cathedrals), gathering (public halls) and other purposes (like pyramids for burial). So the considerations were that of structural stability. But today, they are even used for commercial as well as residential purposes, so the challenges like the occupants comfort have added to the list.

‘As Greek temples and Gothic cathedrals are the representative building types of their respective periods, tall buildings and skyscrapers are seen as the best representative examples of industrialized society. They have compounded the human instinct to build ever higher, ego and competition, and the economic needs of coping with the density of urbanization.’ (ILGIN, 2006).

In today’s time, it is just impossible to imagine any major city without tall buildings shaping its skyline. They are most famous landmarks of cities (also because they can be located from far off), dominance of human ingenuity over natural world, confidence in technology and a mark of national pride; and besides these, the importance of tall buildings in the contemporary world is without doubt ever increasing despite their several undeniable negative effects on the quality of urban life.

The feasibility and desirability of tall buildings have always depended on the available materials, the level of construction technology, and the state of development of the services necessary for the use of the building. Therefore, advances in structural design concepts, analytical techniques, and a more sophisticated construction industry, in conjunction with the high-strength lightweight materials have made it possible to construct very tall, much more slender and lightweight buildings at a surprisingly low cost premium compared to conventional construction. (ILGIN, 2006) However, every advance in height comes with a new difficulty and hence the race toward new heights has its own challenges. Understandably, the increased flexibility and decreased weight do not provide sufficient anchorage and makes contemporary tall buildings much more vulnerable to environmental excitations such as wind, which leads to horizontal vibration.

Since wind can create excessive building motion, the dynamic nature of wind is a critical issue, negatively affecting occupancy comfort and serviceability. Excessive building motion can, create noise and crack partitions, damage non-structural elements such as curtain walls, cause glasses to break, reduce fatigue life, malfunction of the elevators and equipment, and result in structural damages or even collapse.

Therefore, the extreme vibration is a greater concern for both users as well as designers of modern tall buildings, and excessive acceleration experienced at the top floors during frequent windstorms should be kept within acceptable limits to minimize discomfort for the building occupants and to avoid these kinds of undesirable events.

Many researches and studies have been done in order to mitigate such an excitation and improve the performance of tall buildings against wind loads. Hence, different design methods and modifications are possible, ranging from alternative structural systems to the addition of damping systems in order to ensure the functional performance of flexible structures and control the wind induced motion of tall buildings.

An extremely important and effective design approach among these methods is aerodynamic modifications in architecture. It comes into play when the structural part of the building can no longer resist lateral wind forces without any major structural modifications and design and at the same time without significantly increasing the cost of the project. Aerodynamic modifications include modifications of building’s cross-sectional shape and its corner geometry, sculptured building tops, horizontal and vertical openings through-building to allow wind to flow past the buildings with effects on the building structure and skin. In this study we will look on some of tall buildings and how their design was modified by aerodynamic studies.

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By changing the flow pattern around the building, i.e. an appropriate choice of building form, moderates wind responses when compared to original building shape. As far as wind loading and resulting motions are concerned, for tall and slender buildings, the shape is critical and a governing factor in the architectural design. Understandably, tall building design requires a unique collaboration particularly between the architect and the engineer. This interdisciplinary approach to resolving building planning, construction, and usage issues plays a vital role.

Moreover, wind safe tall building design begins with the architect, and so, the influence of the wind action must be considered from the very beginning of the architectural design process of tall buildings. Designs created by the architect should be such that it allows for the aerodynamic modifications to take place without compromising other aspects of design especially its area. Therefore, skyscrapers of the next generation should be the products of collaboration, in particular between the architectural, structural and aerospace engineering fields without victimizing the architectural design. But first we will understand the nature of wind and its importance at higher levels from the ground and also some basic principles of fluid dynamics (as wind is a fluid).