WIND PRESSURE EFFECTS ON HIGH RISE BUILDINGS

ASHWIN NAYANAR113701164DISSERTATION 2014MANIPAL UNIVERSITY

1.2 AIMTo study, analyze and control the wind-pressure effects on high rise buildings.

1.3 OBJECTIVES To understand the basic concept of tall building and its planning. To have a complete study of wind effects on tall buildings, their extents and materials. To understand their applications and typology of buildings where these designs have been used through case studies. Understanding different types of aerodynamic modifications.

1.4 METHODOLOGY Case studies to understand the concepts and technologies in detail. A varieties of different secondary sources were used to provide information regarding the topic. Study from books, various references, journals and through online search.

1.5 SCOPETo show that aerodynamic shaping of a building has a great importance in the design of a tall buildings and architects must be aware of this fact when designing a tall building. Thus, at the early stages of the planning of tall buildings, it must be certainly integrated with the other design disciplines.

1.7 FOCUSTo get a clear understanding of the how the wind-pressure affects the high rise buildings and what are the adverse effects caused by it on the building, occupants and also at pedestrian level. Moreover, to prevent and to take measures so to avoid problems by implementing proper design measures collected by various study and research.

1.6 LIMITATIONSThe only limitation with this study is the availability of primary case study. There aren't any tall buildings in India which has aerodynamic form and techniques as its primary concept. With the limited time period in mind, doing a case study outside India is not possible. Hence I will be depending on the secondary sources.

INTRODUCTION:The tall buildings are designed chiefly to serve the needs of the occupancy, and, in addition to the satisfied structural safety, one of the leading design requirements is to meet the necessary standards for the ease of the building users and the serviceability. In this context, since wind can create extreme building motion, the active nature of wind is a critical issue, negatively affecting occupancy comfort and serviceability. Moreover, the human response to building motion is a very complex phenomenon concerning both physiological and psychological features. Furthermore, extreme building motion can, create noise and crack partitions, damage non-structural elements such as curtain walls, reason for 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 anxiety for both users as well as designers of modern tall buildings, and extreme acceleration experienced at the top floors during recurrent windstorms should be kept within acceptable limits to minimalize discomfort for the building occupants and to avoid these kinds of undesirable events.LITERATURE REVIEW: Definition of Tall BuildingThere is not a convinced description for tall building, high-rise building and skyscraper in terms of height, or number of stories. Although the terms all mean the same group of building which is built extremely high, there is an implicit difference among them. Respectively, skyscraper is a more self-confident term when compared to tall building and high-rise building. On the other hand, according to the authors: A building named as a tall building must satisfy all of the following conditions: Its height has to exceed significantly its plan dimensions. In other words, this condition is directly related to the slenderness ratio. It has to be much taller than the local (in which it is situated), but not necessarily the global context.WIND EFFECTS OF BUILDINGS:Nature of windWind, which is created by temperature differences, could be labeled as an air motion, generally applied to the natural horizontal motion of the atmosphere. The vertical motion, on the other hand, is called as a current. Air close to the surface of the earth moves three dimensionally, in which horizontal motion is much better than the vertical motion. While the vertical air motion is significant particularly in meteorology, the horizontal motion is significant in engineering. The surface boundary layer about the horizontal motion of wind extends upward to a certain height above which the horizontal airflow is no longer pretentious by the ground effect. Most of the human activity is achieved in this boundary layer, and hence how the wind effects are felt within this zone is of great anxiety in engineering.

Vortex SheddingAlong wind and across wind are two important terms, used to clarify the vortex-shedding phenomenon. Along wind or simply wind is the term used to refer to drag forces. The across wind reaction is a motion, which happens on a plane perpendicular to the direction of wind. When a building is subjected to a wind flow, the initially parallel wind stream lines are displaced on both transverse sides of the building, and the forces produced on these sides are called vortices.

Cladding pressuresThe cladding design for lateral loads is a very significant subject for architects and engineers.Even though the wrecked glass resulting from the exterior cladding failure may be a less significant consideration than the structural collapse during an earthquake, the cost of replacement and risks for pedestrians require cautious concentration in its design. Wind forces play a major role in glass breakage, also affected by solar radiation, mullion and sealant details, tempering of the glass, double or single glazing of glass, and exhaustion. Breaking of large panels of glass in tall buildings can badly damage the neighboring properties and injure the pedestrians. Tall buildings cause accelerated wind at ground level, which may affect the comfort and safety of the pedestrians. The general massing of the building and its orientation towards the prevailing wind are serious factors that dictate how much the impact will be. Pedestrian wind studiesWith the introduction of tall buildings, the wind environment around them has become a significant technological and social problem. The shape of the building or structure may create inhospitable or even dangerous wind environmental conditions for pedestrians at street level.

Types of Wind Excited Motion

Along wind motion Across wind motion Torsional motion

Methods to Control Wind Excitation of Tall Buildings

The wind induced dynamic retort of tall buildings can be controlled by global design modifications. These are:-increasing building mass (not feasible or practical because of the resulting magnification of the seismic force, and the great additional cost),-increasing stiffness by using an effective structural system, aerodynamic modifications in architecture, -accumulation of damping systems including passive, active, hybrid and semi-active control. Besides these, if suitably designed, claddings, which are selected for weather resistance quality and pleasing appearance, can also deliver a significant amount of damping.

Aerodynamic modifications in architecture:The wind induced motion of a tall building can be measured either by reducing the wind loads or by reducing the response. A proper selection of building shape and architectural modifications can result in the decrease of motion by altering the flow pattern around the building. A building can be planned with smooth lines and curves so that it, like a plane, is highly aerodynamic, and that the wind will just move smoothly over it, without pushing too much. Some modifications such as, modifications of cross-sectional shape of the building, its corner geometry, sculptured building tops, openings through building are also an tremendously important and active design tool to mitigate wind induced motion.

Addition of damping systems:In the design of tall buildings, engineers must accept a level of the natural damping in the structure to measure the building habitability during frequent wind storms. The actual damping in building structures is a difficult quantity to measure and differs according to the response levels, type of structural systems, cladding system and materials used for construction.Recognizing this ambiguity associated with estimating the natural damping in structural systems, engineers have presented energy dissipating systems into the design of buildings. These devices are called dampers in short and like the dampers used for slowing down the closing of the doors they dampen the motion of the building. The addition of damping is then another approach towards the reduction of the effects of the wind induced motion on a tall building. Damping systems can be mainly classified into two groups:1- Soil damping;2- Auxiliary damping.

Wind resistant structural systemsOutrigger-belt, framed tube, exterior braced and bundled tube systems are the most effectual structural systems alongside wind loading. Basically there are three types of buildings.

i. Steel Buildings Most of the tall buildings have steel structural system due to great strength to weight ratio ease of assembly and field installation economy in transport to the site, accessibility of various strength levels and wider assortment of sections.ii. Reinforced concrete buildings The invention of reinforced concrete amplified the significance in use of concrete in the construction industry to a great extend mobility characteristics and its structural fireproof properties.iii. Composite buildingsComposite construction, essentially described by steel frame stabilized by reinforced concrete. Types of structural systems-Rigid frame systems-Braced frame systems-Shear walled frame systems-Outrigger systems-The frame tube systems Modification of tall buildings against wind excitation-Addition of Openings-Effects of Fins and Vented fins-Effects of Slotted corners, Chamfered corners and Corner recession

-Modifications to Building Shape-Modifications to Corner Geometry-Effects of Roundness of Corners-Effects of Tapering and Changing the cross Sectional Shape along the Height-Effects of Twisting or Rotating of Buildings

WIND TUNNEL TEST

Wind Tunnel Analysis is conducted so as to calculate the different velocity and intensity of wind-pressure effects on a high rise at different points of the building.This gives us a better understanding of the flow of wind patterns around a building.

TUBE_1 (mm)TUBE_2 (mm)VELOCITYRPM

2.42.2INITIAL READINGINITIAL READING

2.42.21 m/s60 rpm

2.42.3100 rpm

2.42.4150 rpm

2.52.6250 rpm

2.93.0300 rpm

3.53.9400 rpm

5.04.815 m/s500 rpm

Where;

Test Section Size : c/s = 600mm x 600 mm length = 200 mmMaximum speed: 45m/s(But required on 15m/s 17m/s for test project)Honey comb size : 50mm x 50mm x 450mmPower : 22KW/30HP AC motor

To calculate the Coefficient of pressure; To calculate the Velocity at a particular point;

CASE STUDIES:5.1 PETRONAS TWIN TOWERS, Kuala Lumpur5.2 TAIPEI 101, Taipei5.3 BURJ KHALIFA, Dubai5.4 IMPERIAL TOWERS, MumbaiHEIGHT/STORIESCLIMATE/WIND SPEEDSTRUCTURAL SYSTEM/MATERIALMATERIALSMODIFICATIONS

PETRONASTOWER,KUALA LUMPUR451.9m/88 STORIESTROPICAL RAINFOREST CLIMATE

OUTRIGGER/COMPOSITECURTAIN WALLSUSE OF TMD AND THE EXTERIOR IS DESIGNED SUCH A WAY THAT IT CONFUSES THE WIND.

TAITEI 101, TAIPEI508m/101 STORIESOUTRIGGER/COMPOSITECURTAIN WALLSCORNER SOFTENING AND THE USE OF TMD AND HYDRALIC DEVICES TO CONTROL ALL WIND LOADS.

BURJ KHALIFA, DUBAI828m/163 STORIESDESERT CLIMATE

CONCRETE WITH STRUCTURAL STEEL SPIRE/ STEEL & CONCRETECURTAIN WALLSY SHAPED PLAN, SO IT HELPS IN CHANALISING THE WIND.

IMPERIAL TOWER, MUMBAI254m/60 STORIESHOT AND HUMIDCONCRETE SHEAR WALL AND STEEL RIGID FRAME/ STEEL AND CONCRETE.CONCRETE, GLASSSCULTUPERD HEAVY TOP, AND SETBACK EXTERIOR.

WILLIS TOWER, CHICAGO442.1m/108 STORIESCOLD AND HUMID13kph-20kph max9 SETS OF BUNDLED TUBE/ STEELCURTAIN WALLSITS STRUCTURAL SYSTEM ITSELF BINDS UP THE WHOLE BUILDING.

5.5 WILLIS TOWER, Chicago

Tall buildings are always very stimulating to design, in terms of safety, occupant luxury, structurally etc. Any building which is tall will have to take wind as a chief factor as a lateral load. These wind loads can be controlled by certain methods i.e. Using aerodynamic shape and form altering of the building according to the site conditions. Designing the building that is structurally stable to endure these forces etc.

In some cases we might have to include both these factors i.e. Architectural modifications Structural modificationsto make the building stable from wind loads and other forces acting on tall structures.

Architectural modifications to corner geometry, such as chamfered corners, slotted corners, rounded corners, corner cuts, can also significantly reduce wind induced response of buildings. Addition of openings completely through the building, particularly near the top, is another very useful way of improving the aerodynamic response of that structure against wind.

The following table shows the type of modifications that can be done for tall buildings according to the height.HEIGHT(m)MODIFICATIONS

ARCHITECTURALSTRUCTURAL

0 - 200m FORM HAS IMPORTANCE AT THIS HEIGHT ALSO. DEPENDS ON THE SITE AND CLIMATIC CONDITIONS MODIFICATIONS TO THE BUILDING FORM CAN BE CHANGED FOR SAFETY AND FOR NATURAL VENTILATION. RIGID FRAMES, SHEAR WALLS ETC, USE OF PASSIVE DAMPING CAN BE USED.

201 - 400m AERODYNAMIC FORM HAS IMPORTANCE AT THIS LEVEL. BUILDING SHOULD BE DESIGND SUCH THAT IT SHOULD NOT CAPTURE THE WIND AND CAUSES DAMAGE TO THE BUILDING. ARCHITECTURAL MODIFICATIONS SUCH AS CORNER SOFTNEING, SCULPTURED TOP, AND OPENINGS AT THE TOP ETC SHOULD BE CONSIDERD WHILE DESIGNING. BUNTTLED TUBE, MEGA COLUMNS, OUTRIGGER USE OF TUNED MASS DAMPERS WILL GIVES ADDITIONAL CONTROL AND SUPPORT FOR THE STRUCTURE FROM WIND LOADS.

401 and above AERODYNAMIC FORM IS VERY IMPROTANT FACTOR WHILE DESIGNING. DESIGNING SUCH TALL STRUCTURES SHOULD BE IN SUCH A WAY THAT THE FORM CAN CHANALIZE THE WIND. ANY BUILDING WHICH IS 400 OR 500m ABOVE THE SEA LEAVEL WILL EXPERIENCE A CONSTANT WIND LOADS ON THE BUILDING EXTERIOR. A COMBINATION OF BOTH ARCHITECTURAL AND STRUCTURAL COMPONENTS WILL MAKE THE BUILDINGS AT THIS HIGH STABLE FROM WIND FORCES. DEPENDING ON THE WIND SPEED AND OTHER NATURAL FORCES THE MATERIALS ARE ALSO IMORTANT, FOR CLADDING, INTERIOR ETC. OUTRIGGER STRUCTURES, BUTTRESSED CORES DEEP PLIE FOUNDATIONS DAMPIND DEVICES, HYDRAULICS ETC

REFERENCES

(1) 780 Third Avenue Building, http:// www.780third.com, accessed November 15, 2005.(2) Ali, M., and Armstrong, P., Architecture of Tall Buildings, Council on Tall Buildingsand Urban Habitat Committee, McGraw-Hill Book Company, New York, 1995.(3) Kim, Y., You, K., and Ham, H., Aeroelastic Responses of Tall Building to Wind LoadsUsing TLD, CTBUH 2004, p. 510-515, Seoul, Korea, 2004.(4) Taranath, B., Structural Analysis, and Design of Tall Buildings, McGraw-Hill BookCompany, New York, 1988.(5) William J. LeMessuriers Super Tall Structures: A Search for the Ideal,Architectural Record, Vol. 173, p. 144-150, 1985.(6) Baker, W., The Worlds Tallest Building-Burj Dubai, U.A.E., CTBUH 2004, p. 1168-1169, Seoul, Korea, 2004.(7)Ali, M.M. (2005). The skyscraper: epitome of human aspirations. In Proceedings of the 7th World Congress of the Council on Tall Buildings and Urban Habitat: Renewing the Urban Landscape [CD-ROM]. Chicago