The Impression of Hurricane Readying to Houses in Australia and UAE - Research Paper Example

Running Heading: Effect of wind loading to buildings in Australia and UAE Effect of wind loading to buildings in Australia and UAE Name: Tutor: Course: Date: Wind can be basically defined as air in motion. This movement of air is caused by the uneven heating of the Earth's surface by the sun and pressure gradients in the atmosphere. Wind is a natural force that is highly unpredictable and very complex due to the many flow situations arising from its interaction with structures. According to Holmes(2001), Wind is composed of a multitude of eddies of varying sizes, and rotational characteristics carried along in a general stream of air moving relative to the earth’s surface. These eddies give wind its gusty or turbulent character. The gusty nature of wind, especially in the lower atmosphere, is as a result of its interaction with surface structures. In normal conditions, the average wind speed tends to increase with height above the earth’s surface, while the gustiness decreases with height. Structures that are slender and tall have very dynamic responses to wind. A popular example of a structure that collapsed due to effects of wind is the Tacoma Narrows Bridge in 1940. The bridge collapsed at a wind speed of only 19 meters per second after developing a coupled torsion and flexural mode of oscillation. Such disasters justify the billions of dollars spent on research on structural effects of wind. Taranath(1988), emphasizes that the designing of very tall buildings cannot depend on the simple quasi-treatment of wind loading that is universally accepted when designing low to mid-rise structures. When erecting tall structures, other factors such as effects of resonance, stiffness, damping, wind acceleration, interference from other tall structures, wind direction and cross-wind response should be carefully considered. Slender structures are very receptive to vibrant reactions in line with the wind course as a result of turbulence buffeting. When building a wind sensitive structure, three basic wind effects should be carefully considered. The first thing is the wind effect on the surrounding environment caused by erecting the structure. These could be the effects on pedestrians, other buildings, motor vehicles and other architectural features. The second thing is that engineers must consider is the wind pressures all over the surface area of the structure in order to design the cladding system. As a result of the momentous cost of classic facade systems in proportion to the overall cost of giant structures, engineers cannot afford the luxury of conservatism in assessing design wind loads. The final problem is to determine the design wind load for designing the lateral load resisting structural system of a structure to satisfy various design criteria. Hira and Mendis (1995) define the wind vector as the intensity and direction of the wind. He generalizes that at any point on the earth; wind vector is calculated as the sum of the mean wind vector and a dynamic or turbulence component i.e. V (z, t) = V (z) + v (z, t) According to Hira’s formula, dynamic loading of a structure depends on the turbulence and the size of eddies. Large eddies, whose dimensions are comparable with the structure, give rise to well correlated pressures as they envelop the structure. Small eddies; on the other hand, result in pressures on various parts of a structure that become practically uncorrelated with distance of separation. The eddies and turbulence exerted on a building may be as a result of the location of another structure. Roughness differs from one terrain to another depending on the structures as shown in the table below. Terrain category Roughness Roughness length, zo, (m) 1. Exposed open terrain with few or no obstructions 0.002 and water surfaces at serviceability wind speeds. 2. Water surfaces, open terrain, grassland 0.02 with few, well scattered obstructions having heights generally ranging from 1.5 to 10 m. 3. Terrain with numerous closely spaced obstructions 0.2 3 to 5 m high such as areas of suburban housing. 4. Terrain with numerous large, high (10.0 m 2 to 30.0 m high) and closely spaced obstructions such as a large city centre and well-developed industrial complexes. (Australian/New Zealand wind code, AS/NZS1170.2, 2002.) The above table, adapted from the Australian/New Zealand wind code, AS/NZS1170.2, 2002, shows that the roughness length and turbulence increases with an increase in surface structures. The features of wind pressure on a building are a function of the characteristics of the oncoming wind, the geometry of the building, proximity of the structures upwind. The pressures are never constant, and tend to fluctuate, partly as a consequence of the gustiness of the wind, and as a result of local vortex shedding at the edges of the buildings themselves. The changing pressures can cause fatigue damage to structures, and dynamic excitation, if the structure is dynamically wind sensitive. The pressures are also not homogeneously spread over the face of the building but change with position. Holmes (2001) insists that the intricacies of wind loading should be thoughtfully considered when applying a design document. As a result of the many doubts involved, the utmost wind loads experienced by a structure during the course of its existence, may differ greatly from those assumed in the design stage. The idea here is that the collapse or stability of a structure in a wind storm can not necessarily be assumed as an indication of the non-conservativeness, or conservativeness, of the Wind Loading Standard. The Standards do not apply to buildings or structures that are of extraordinary forms or location. Wind loading directs the design of some types of structures such as slender towers and tall buildings. It usually becomes attractive to make use of experimental wind tunnel data in place of the coefficients given in the Wind Loading Code for these structures. Denny (2010) emphasizes the significance of the wind tunnel method which is used by structural engineers when analytical test data cannot be completely relied upon to estimate certain types of wind loads and the expected response by the structures. For instance, buildings with strange aerodynamic shapes or very flexible structures may require more accurate estimates of wind effects on buildings. These can be obtained through aero-elastic model testing in a boundary-layer wind tunnel. Wind testing is a powerful tool used by structural engineers to determine the nature and intensity of wind forces on complex structures. The process involves Wind tunnel testing involves forcing air through the building model under consideration and its surroundings at various angles, and velocities relative to the building orientation reflecting the expected wind directions. This is normally achieved by placing the complete model on a rotating platform within the wind tunnel. Once testing is finished for a chosen direction, the platform is basically rotated by a preferred increment to symbolize a new wind direction. In order to use wind tunnel results to aid in the prediction of wind forces acting on the full-scale structure, the behavior of the natural wind must be satisfactorily modeled by the wind tunnel. Hira and Mendis(1995) instruct that in order to create a model that completely reflects the effects of natural wind and accurately shows the dynamic similarity between the model and full-scale results, some non-dimensional parameters must kept as close to the real wind as possible. These include “…the velocity profile U(z) /U( zo ) , that is the variation of velocity with height normalized with respect to the values at height zo , the height of the building under investigation; the turbulence intensity U /U ; and the normalized power spectral density, nSU (n)/σ 2U , which defines, the energy present in the turbulence at various frequencies.”(Hira 1995, pp 367). Wind tunnel testing is widely accepted all over the world during the design stage of tall structures. Owners of buildings also consider the cost of wind tunnel testing as insignificant compared to the risk of building an unsafe building that could collapse anytime. Wind tunnel testing also reduces the amount of money used in building and significantly avoids expensive maintenance costs connected with malfunctions as a result of leakage and / or structural failure. Taranath (1988) adds that when designing a tall building, the engineers must consider the stability, serviceability, strength and human comfort in the building. Stability refers to the ability of the building to withstand forces that may cause uplift, sliding or overturning of the structure as a whole. Strength means that the structural components should be sufficient to withstand the imposed loading throughout the life span without failure. Denny (2010) explains that the design of structures has evolved from concrete, to steel and now to the age of lighter, stronger alloys of steel. Building very tall structures using concrete was inefficient and very costly. The structures were also very rigid and faced the risk of easy collapse during earthquakes and strong winds. According to the author, the oldest and most obvious way to increase the stability and reduce sway is by stiffening the structure. This could be done by adding more steel members to a steel truss foundation. Traditionally, a huge tank of water was placed on top of a building. The sloshing and movement of water around considerably reduced swaying and thus created more stability. This ancient technology led to the development of today’s sophisticated dampers. Very tall buildings have tuned mass dampers. This is a computer-controlled hydraulic system that pushes a huge block weighing thousands of tons on the top floor of a building. The computer-controlled hydraulic system receives feedback regarding the wind loading then; it automatically calculates the best position for the block. Some buildings have a giant pendulum suspended in the top floors. The pendulum moves according to the swaying movement of the building and creates stability. Riera and Davenport (1998) explain the new way to control vibrations and wind loading is through the use of dampers. Damper systems can be classified into two depending on the degree of human influence. Damping systems that don’t depend on any human influence are known as passive dampers. These include distributed viscous dampers, impact type dampers, friction dampers, tuned-mass dampers, visco-elastic dampers etc. semi-active dampers require some human or computer-controlled mechanism to work. These include hydraulic dampers, controllable fluid dampers, variable friction dampers, variable stiffness dampers etc. Further developments in science and technology have led to the innovation of a high-tech approach to damping using magnetorheological (MR) fluids. These are fluids that have the strange ability to turn from a liquid state to a solid state when subjected to a strong electromagnetic field. The MR fluids are filled in specific areas of the building and in case of an earthquake or strong winds the fluid is magnetized, and it immediately turns into solid state. This stiffens the building and makes it stable. Kassimali reminds engineers of tall structures, especially buildings that they must have in mind the comfort of the people who will live in the building. According to him, human beings are surprisingly alert and aware of vibration and perception of motion. Human beings can feel even the slightest level of stress and strain in a building. This rule especially applies as the distance from the ground increases. People living in lower floors may not be as responsive as those living in upper floors. The builders of these structures also consider other things such as elevator systems and their speed. Advancement in technology has greatly reduced the limitations of building tall structures. The ultimate limitation that remains is the source of funds, since such constructions are extremely expensive. References Hira A. and Mendis P. (1995). Wind Design of Tall Buildings Conference on High-rise Buildings in Vietnam. Hanoi, Vietnam, February. Holmes D.J. (2001). Wind Loading of Structures. London, Spon Press. Taranath B.S. (1988). Structural Analysis and Design of Tall Buildings. McGraw-Hill Book Company. Denny, M. (2010). Super Structures: The Science of Bridges, Buildings, Dams, and Other Feats of Engineering. JHU Press. Holmes D.J (2007). Wind loading of structures. 2nd Edition. Taylor & Francis. Cermak, E. J. (1980). Wind engineering: proceedings of the fifth International Conference on Wind Engineering, Colorado, USA, 8-13 July 1979. Pergamon Press. Council on Tall Buildings and Urban Habitat and Robertson, L. E (1980). Tall building criteria and loading. ASCE Publications. Council on Tall Buildings and Urban Habitat and Beedle, L.S. (1983). Developments in tall buildings. Hutchinson Ross Pub. University of Southampton, Civil Engineering Research Association. (1967). Tall buildings: the proceedings of a symposium on tall buildings with particular reference to shear wall structures. Symposium Publications Division and Pergamon Press. Riera, J. D. and Davenport, A. G. (1998). Wind effects on buildings and structures: proceedings of the Jubileum Conference on Wind Effects on Buildings and Structures, Porto Alegre, Brazil, 25-29 May 1998. Taylor & Francis. Kassimali, A. (2006). Structural Analysis. 4th Edition. Cengage Learning. Ghali, A. and Neville, A.M. (2007). Structural analysis: a unified classical and matrix approach. 4th Edition. Taylor & Francis. Read More