Winds On Building - Report Example

Winds on Building 1.1 Winds on Building 1.1.1 Introduction Wind is a very complicated phenomenon which has very many situations of flows resulting from its interaction with various structures. Various eddies are produced by wind and have some characteristics that are rotational and are carried along in relativity to the surface of the earth. The eddies are responsible for the gusty nature of winds which in turn is responsible for the behavior of wind in low levels (Mendis et al., 2007). Structures, in particular the tall or the slender structures have a dynamic response to the effect of wind. There have been structures that have been known to collapse as a result of wind. Thus is however as a result of some other effects which are then accelerated by wind. An example of this is the “Tacoma Narrows Bridge” that collapsed in 1940 as a result of wind and consequent development of flexural and torsional oscillations (Holmes, 2001). Structures respond dynamically to wind as a result of various phenomena. Such phenomena include vortex shedding, buffeting, flutter and galloping. One problem that arises as a result of wind motion is the way humans respond to vibration and the way they perceive motion. In this regard, research has come to show that human beings are more responsive to vibration as opposed to motion (Hira and Mendis, 1995). This is a factor that influences the erection of tall structures where matters of serviceability surpass considerations of strength. 1.1.2 Development of Wind Wind is developed by the difference in the pressure of the air which is mainly due to unequal heating in the atmosphere. Creation of wind occurs as a result of: when the air is heated, there is a decrease in pressure. The rising of warm air creates some low pressure. Cool air rushes downwards to replace the warm air. This air is denser and causes increase in pressure. Wind is formed as air moves from the high pressure zones to the low pressure zones (Davenport, 1967). The Coriolis Effect in wind development asserts that the rotation of the earth causes movement of water and air to change direction. In the northern hemisphere, winds move to the right whilst in the southern hemisphere winds move to the left (Brabson and Palutikof, 2000). 1.1.2.1 Velocity Profile At high heights above the earth’s surface, the effects of friction are negligible and movements of air are mostly dictated upon by gradients of pressure in the atmosphere. These are as a result of the thermodynamic heating of the earth by the sun. The speed of the wind at this upper level is known as “gradient wind velocity”. When wind is travelling at a very close range to the surface of the earth, it is usually affected by a drag of air stream as a result of friction. After this terrain, there is a layer which acts as a boundary layer where the speed of the wind varies from very low speeds to a gradient wind speed at a gradient height. This layer depends on the terrain where the terrain is categorized in accordance with the length of roughness and can go up to a height of 3000m (Irwin, 1978). On a practical basis, it is always important to have a reference wind speed that is specified by the meteorological department of a certain country. The definitions of these reference wind speeds vary with countries. In a place like Australia for example, category 2 terrain has a 3 seconds gust speed at 10 meters. In this country, there is a wind model that was developed in Melbourne and was based on real data and on laws of logarithm. The following equation can be applied to areas that are non-cyclonic. Source: National Research Council Canada (2005) Vz (In the above equation) is the hourly speed of the wind at a certain height z. 1.1.2.2 Design Wind Speeds Design wind speeds for various directions and differing period returns can be derived by means of rigorous analysis through the incorporation of probability distributions for the speeds of the winds and the directions. A good example is the wind direction multiplier that is provided by AS/NZS1170.2 that varies from 0.80 for the eastern wind to 1.0 for the western wind. Design wind speeds are pertinent in assessing the risks that a building faces due to hurricanes and such winds. In definition of the design wind speeds, there are two basic scales that have been consistently used. Most use the “3 second gust wind speeds” and others use the “fastest mile wind speeds”. In order for applicability and ease of use, it is essential that one converts the “3 second gust wind speeds” to “fastest mile wind speeds”. This is done by subtracting 20 MP/H from the latter. The design wind speeds are determined on a basis of balancing the risks that are posed by high winds on every state area. Generally, the design wind speed is that speed of the wind that has a chance of 1% of being met or even surpassed in a certain year. With this speed in consideration, it means that a certain home or building is not guaranteed of not experiencing higher speeds of wind but the chance is less as compared to areas that have higher speeds. A design wind speed of above 120mph has higher chances of experiencing hurricanes, for between 100 to 120, the chances are moderate and for speeds between 90 and 100, there is a low risk of experiencing damages moderate water intrusion risk (Sachs, 1978). 1.1.3 Wind Pressures on Building 1.1.3.1 Conversion from Wind Speed to Wind Pressures Wind loads or pressures that are exerted on a building are dependent on the wind speed as well as the way that the structure interacts with the flow of the air. Taking the fact that wind is in motion, the pressure that it exerts on the structures is then a function of the kinetic energy. If it happens that all the kinetic energy is considered, then pressure exerted on the building becomes equivalent to the kinetic energy dissipated by the building and given by the equation: - Q = ½ ρ V2 Ρ = mass density V = velocity of the air The pressure that is given by this equation is referred to as “stagnation pressure”. The pressure can be defined as “maximum positive increase over ambient pressure that can be exerted on a building surface by wind of any given speed” (NCRC, 2005). This pressure is the basic reference point for all other pressures over a given structure. For the purpose of computation for design pressures, the wind speeds required are dependent upon the components of the building that is being designed. In regards to the structure, the maximum value is taken into consideration and varies with the geographical locality of the building. The values which give the most likely maximum are given by the meteorological department (NCRC, 2005). 1.1.3.2 Interaction of Wind and Structures Pressure distribution and suctions in a building is highly dependent on its disturbance of the air flow. The most prevalent datum for measurement of pressures as well as suctions is the ambient temperature in air flow that is not disturbed. When wind interacts with a simple structure like a free standing wall, the streamlines that are found be in line with that wall tend to move by the edges. This phenomenon cause a change in the direction and in the magnitude originally sustained by the wind is changed and consequently cause a pressure change (NCRC, 2005). There is production of stagnation pressure at near the wall edges. There is also an increase in the pressure gradient towards the wall edges. It is at this point that the flow of the wind gains back its velocity after the diversion by the wall. This happens in a parallel direction to the wall against the previous perpendicular direction (NCRC, 2005). Pressure on the windward face can be depicted by the diagram below: - Fig 1: Wind pressure on a building windward side Source: (Chien, 1951). There is a difference in the behavior of pressure behind walls. The air flow streamlines cannot be able to rejoin at once due to the inertia and some wake that is left between the wall and wind separation. Air in this region is contained by the air flow in the regions that are fast moving. This reduces pressure under the ambient pressure and creates a sought of suction. This condition is shown in Fig. 2. In order to get the pressure at certain surface points, the stagnation pressure is multiplied by the number of isobars. These factors are the coefficients of pressure that give a relation of the pressure and suctions on a given building to the design stagnation pressure or velocity pressure. Suction is indicated when the value is negative (NCRC, 2005). Pressure is never always constant over the surface of a structure. For simplicity of design, there is a specified coefficient for a certain surface. In order to get the total force for that particular surface, this coefficient is multiplied by the basic pressure and the area. The total force on a given wall is basically as a result of the windward pressure and the leeward suction of the said wall. In the case of a wall that is exterior to the building, net pressure is given by the difference by the pressures in and outside the building. The pressures inside the buildings are computed as functions of the openings in the walls (NCRC, 2005). 1.1.3.3 Flow around a Building Source: (Chien, 1951) Fig 2: Air flow around a building The above diagram depicts a simple building and the associated flow lines. For those buildings that possess flat roofs and roofs with low slopes, the wall that is subjected to pressure is the windward wall. The other surfaces of that building are in the wake and the pressures at that point are all be low the ambient pressure (NCRC, 2005). This is mainly so because the flow lines cannot be maintained along the sharp edges which makes the building and the wind to separate. The figure below shows a cross section that depicts how pressure is distributed in the building. Source: (Chien, 1951) Fig 3: Pressure distribution in a building This shows that pressure distribution is not constant in the building. There areas that have suctions, other pressure and all cases are not constant (NCRC, 2005). 1.1.3.4 estimating the Wind Pressure A) Australia Standard (AS 1170.2:2002) method “Aerodynamic shape factor” Cfig = Cpn Kp “Net pressure coefficient” Cpn = 1.3 + 0.5(0.3 + log10 (b/c]) (0.8 – c/h). (AS/NZS 1170.2, 2002) “Ultimate net wind pressure on free-standing wall” pnu = Cfig qzu “Ultimate free stream gust dynamic wind pressure” qzu = 0.0006 Vzu2 (AS/NZS 1170.2, 2002) B) British Standard (6399: Part 2:1995) method Dynamic pressure The value for the dynamic pressure by the standard method is given below: - Qs = 0.613Ve2 Where: Qs: - dynamic pressure (Pa) Ve : - effective wind speed External surface pressures The pressure acting on the external surface is given by: - Pe = qsCpeCa Where qs = dynamic pressure Cpe = external pressure coefficient for building surface Ca = size effect factor for external pressure Internal surface pressures Pi = qsCpiCa Where qs = dynamic pressure Cpi = internal pressure coeffie=cient for the building Ca = “size effect factor for internal pressures” Net surface pressures For enclosed buildings P = pe - pi pe is the external pressure and pi is the external pressure For free standing buildings and canopies P = qsCp Ca Where qs is the dynamic pressure, Cp is the net pressure coefficient for the canopy and Ca is the size effect factor for the canopy. 1.1.4 Summary This paper has extensively covered the various effects of wind loads to buildings. The discussion started with an introduction to winds and how they affect construction of structures, and then looked at the development of wind. There is an insight into wind speeds followed by various design considerations in wind speeds. Wind pressures and flows of wind around building have also been considered. Calculations of wind pressures have been slightly looked at starting with the Australian standard and then the British standard. The paper can thus serve as a good guide to the basics of effects of winds to construction of structures and in particular buildings. References AS/NZ1170.2 (2002) Australian/New Zealand Standard, Structural design actions, Part 2: wind actions, Standards Australia & Standards New Zealand. Brabson B. and Palutikof J. (2000). “Test of the Generalized Pareto Distribution for Predicting Extreme Wind Speeds”. Journal of Applied Meteorology, Vol. 39, 1627-1640. Chien et al. (1951), Wind tunnel studies of pressure distribution on elementary building forms, Iowa Institute of Hydraulic Research. Davenport, A. (1967) The dependence of wind loads on meteorological parameters. Proc. Int. Res. Seminar, Wind Effects on Buildings and Structures, Ottawa, Univ. of Toronto Press, 19 – 82 Hira A. and Mendis P. (1995) Wind Design of Tall Buildings. Conference on High-rise Buildings in Vietnam. Hanoi,Vietnam, February Holmes D. (2001). Wind Loading of Structures, Spon Press, London Irwin A. (1978) Human Response to Dynamics Motion of Structures. The Structural Engineer London. Mendis, P et al (2007), “Wind Loading on Tall Buildings” Loading on Structures 41-54. National Research Council Canada (2005), Wind Pressures on Buildings Retrieved 10.09.2011 from < http://www.nrc-cnrc.gc.ca/eng/ibp/irc/cbd/building-digest-34.html > Sachs P. (1978). Wind Forces in Engineering, Pergamon Press, Oxford Read More