Prof. Dr. Zahid A. Siddiqi, UET, Lahore WIND LOAD

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1 WIND LOAD Wind load is produced due to change in momentum of an air current striking the surface of a building. A building is less likely to experience the other design loads in its life but it is almost certain that the building is likely to be subjected to the design wind loads. If the building is very tall, the wind velocity varies along the height and sophisticated codes account for this effect.

2 In designing for wind, a building cannot be considered independent of its surroundings. The influence of nearby buildings and of the land configuration can be substantial. The horizontal swings may not be dangerous but may cause motion sickness in the occupants. The modern skyscraper, which uses lightweight curtain walls, dry partitions, and high strength materials, is more prone to wind motion problems than the early skyscrapers, which had enormous weight of the masonry partitions, heavy stone facades, and massive structural members.

3 Keeping the movements in the upper levels of the building to acceptable human tolerances is the goal of the structural engineer. Wind loads change rapidly and even abruptly, creating effects much larger than if the same loads were applied gradually. In designing tall buildings to withstand wind forces, the following are important factors that must be considered:

4 1. Strength and stability requirements of the structural system. 2. Fatigue in structural members and connections caused by fluctuating wind loads. 3. Excessive lateral deflection that causes cracking of partitions and external cladding, misalignment of mechanical systems and doors, and possible permanent deformations. 4. Frequency and amplitude of sway that can cause discomfort to the occupants.

5 5. Possible buffeting that may increase the magnitudes of wind velocities on neighboring buildings. 6. Effects on pedestrians. 7. Annoying acoustical disturbances. 8. Resonance of building oscillations with the vibrations of elevator hoist ropes. The variations in the speed of prevailing and seasonal winds are referred to as fluctuations in mean velocity. The variations in the local winds, which are of a smaller character, are referred to as gusts.

6 Wind speeds of 30 to 110 km/h are typically reached in a thunderstorm and are accompanied with swirling wind action exerting high suction forces on roofing and cladding elements. In a fully developed hurricane, winds reach speeds up to 110 to 130 km/h, and in severe hurricanes can attain velocities as high as 325 km/h. Tornadoes develop within severe thunderstorms and hurricanes and consist of rotating column of air, accompanied by a funnel-shaped downward extension of a dense cloud having a vortex of several meters, typically 60 to 245 m in diameter whirling destructively at speeds up to 480 km/h.

7 The pressure at the center of a tornado is extremely low, so that as the storm passes over a building, the pressure inside the structure is far greater than that outside, causing the building to literally explode. The average or mean wind speed used in many building codes of the United States is the fastestmile wind, which can be thought of as the maximum velocity measured over one mile of wind passing through an anemometer. The only accurate way to determine the gust factor is to conduct a wind tunnel test.

8 It is surprising for a beginner the wind also produces forces in a direction perpendicular to it. There appear to be three distinctly different reasons why a building responds in a direction at right angles to the applied wind forces; these are: 1. The biaxial displacement induced in the structure because of either asymmetry in geometry or in applied wind loading. 2. The turbulence of wind. 3. The negative-pressure wake or trail on the building sides.

9 For tall buildings it appears that the crosswind response is caused mainly by the wake. Consider a cylindrically shaped building subjected to a smooth wind flow. The original parallel stream lines are displaced on either side of the cylinder, and this results in spiral vortices being shed periodically from the sides of the cylinder into the downstream flow of wind which is called the wake.

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12 At higher speeds, the vortices are shed alternately first from one and then from the other side of the cylinder. When this occurs, there is an impulse in the alongwind direction as before, but in addition, there is an impulse in the transverse direction. The transverse impulses are, however, applied alternately to the left and then to the right. This kind of shedding, which gives rise to structural vibrations in the flow direction as well as in the transverse direction, is called vortex shedding or the Karman vortex street.

13 It has become routine to obtain the design information concerning the distribution of wind pressures over the surface of the building by conducting wind tunnel studies on small scale models. In the design of tall buildings it is recognized that use of the wind tunnel approach is a more refined method for arriving at design wind loads. Wind tunnel model studies generally indicate lower wind loads than prescribed in the codes and lead to more cost-effective designs.

14 Buildings with unusual aerofoil shapes and which are torsionally flexible need to be wind-tunneltested even when height is not a major design consideration. Prismatic shapes, as a rule of thumb, can be considered as candidates for wind tunnel test when the height exceeds the range of 40 to 50 stories. Majority of wind tunnels range in size from 1.5 to 6.0 m and the essential features of a boundary layer wind tunnel are the following:

15 1. Be large enough to contain models with enough clearance to facilitate easy installation and removal. 2. Be large enough to generate a natural wind profile over a wide range of speeds. The turbulence generated should match that found in the atmosphere. 3. Provide for a rotating model.

16 4. Be capable of employing a wide range of testing techniques with very low wind speeds of 1.5 to 3.25 km/h for smoke visualization and higher speeds of 90 to 110 km/h for wind pressure measurements and to meet the dynamic similarity parameters. A relatively stable turbulent boundary wind layer with duplicated mean velocity profile can be achieved in a 20 to 30 m long tunnel. The floor of the tunnel is covered with appropriately located roughness elements consisting of blocks of Styrofoam or other suitable material.

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21 Prof. Dr. Zahid A. Siddiqi, UET, Lahore

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24 At the test section located at the end of a long fetch, a test model is installed in a surrounding consisting of duplicate models of the actual buildings that are around the building being tested. The structural engineer engaged in the design of tall building relies on the wind engineer to obtain the following quantitative data for use in the building design: 1. The intensity and scale of pressure fluctuations on exterior panels and glass surfaces.

25 2. The overturning moments and shears that should be used in the building design. 3. The oscillation response of the structure, in terms of becoming a human comfort problem. 4. The change in the wind environment at the ground in terms of it becoming uncomfortable or even dangerous to pedestrians.

26 Basic wind speed is defined as the fastest wind speed in km/hr having a probability of occurrence of 0.02 and measured at a point 10m high above the ground under exposure category-c conditions, defined later. The factors affecting the wind pressure in addition to the basic wind velocity are: a) Gradient of wind velocity with height above ground. b) Local variations of pressure due to vortices.

27 The pressure is the highest at the corners, relatively high at the edges and low at the center of the building. This affects cladding or minor element design. Highest Lowest Higher Figure. Local Variation of wind Pressure.

28 c) Exposure of the structure. For example, the coastal areas will have more wind loads. Buildings surrounded in other tall buildings will experience less wind pressures. The wind can be just a gust of wind or long wind periods. There are three Exposure Categories defined in the UBC-97 Code.

29 Exposure B has terrain with buildings, forest or surface irregularities, covering at least 20 per cent of the ground level area and extending 1.6 km or more from the site. Exposure C has terrain that is flat and generally open, extending 0.8 km or more from the site in any full quadrant. Exposure D is the most severe exposure in areas with basic wind speeds of 129km/hr or greater and has terrain that is flat and unobstructed facing large bodies of water over 1.6 km in width relative to any quadrant of the building site.

30 Exposure D extends towards the land from the shoreline 0.4 km or 10 times the building height, whichever is greater. d) Internal pressure. When the wind enters the building from the windward side and the leeward side is relatively closed, internal pressure is developed that acts like negative pressure. Similarly, when high-speed wind passes by a building, it produces a vacuum on the leeward side. This vacuum results in internal suction producing negative pressure fro the structure.

31 Wind Windward side Building Leeward side (a) Windward and leeward sides. wind Internal Pressures Internal Suction wind (b) Internal pressure. (c) Internal suction.

32 UBC-97 WIND LOADS Prof. Dr. Zahid A. Siddiqi, UET, Lahore According to UBC and , there are two methods for the calculation of wind pressure. Method 1 Normal Force Method may be used for any structure but is the only method for the design of gabled rigid frames. In this method, the wind pressures are simultaneously applied normal to all exterior surfaces. Hence, the wind loads will be inclined for parts of the structure at an angle to horizontal and vertical planes.

33 Method 2 Projected Area Method has two restrictions: Firstly, it is not applicable to gable rigid frames. Secondly, it is not applicable if the height of the structure is more than 60m. In this method, only horizontal and vertical pressures are applied. The horizontal pressure is applied over the full vertical projected area of the structure and vertical pressure is applied over the full horizontal projected area.

34 Wind pressure, P = q s A C e C q I w q s = wind stagnation pressure = ½ ρ V V 2 ρ V A C e = air density = basic wind speed (km/hr) = effective exposed area = combined height, exposure and gust factor coefficient (Table 16-G of UBC)

35 C q = pressure or shape factor coefficient for the structure or its portion under consideration (Table 16-H of UBC) I w = importance factor (Table 16-K of UBC) = 1.15 for essential and hazardous facilities like hospitals, fire and police stations, disaster centers, water tanks and buildings with occupancy more than 300 people.

36 I w = 0.87 for buildings and other structures that represent a low hazard to human life in the even of failure, such as agricultural facilities. = 1.0 for all other buildings. Combined Height, Exposure And Gust Factor (C e ) The values of this coefficient are given in Table.

37 Height Above Average Level of Adjoining Ground (m) UBC Table. Values of Coefficient (C e ). Approximate Exposure C Exposure B

38 Pressure Coefficient (C q ) Prof. Dr. Zahid A. Siddiqi, UET, Lahore The values of this coefficient for various parts of the building are given in following Figure and Table. C p = 0.7 Wind Direction C p = +0.8 C p = 0.5 h Elevation Fig. Typical Values Of Pressure Coefficient.

39 Part of Structure Windward roof Leeward or flat roof Windward walls Leeward walls Table. Pressure Coefficient (C q ). Angle 0 to to to 45 > outward 0.8 inward 0.5 outward Prof. Dr. Zahid A. Siddiqi, UET, Lahore C q 0.7 outward 0.9 outward or 0.3 inward, which ever is more critical 0.4 inward 0.7 inward Chimneys, tanks and solid towers square or rectangular hexagonal or octagonal round or elliptical Signs, flagpoles, light poles or minor structures Roof eaves without overhangs Overhangs at roof eaves and canopies < to any direction 1.1 any direction 0.8 any direction 1.4 any direction 2.3 upward 2.6 upward 1.6 upward 0.5 added to above values

40 Other UBC Wind Load Provisions Prof. Dr. Zahid A. Siddiqi, UET, Lahore 1. Wind should be assumed to come from any horizontal direction. No reduction in wind pressure shall be taken for the shielding effect of adjacent structures. 2. The base overturning moment for the entire structure, or for any one of its individual primary lateral resisting elements, should not exceed two thirds of the dead load resisting moment. Referring to Fig., the overturning of structure about point A may be investigated as follows:

41 H a W A b / 2 Fig Safety Against Overturning.

42 M w = overturning moment due to wind = H a M DL = resisting moment due to DL for overturning = W b / 2 M w 2/3 M DL FOS = 1.5 or 3/2 against overturning

43 where, H = resultant horizontal wind load a = height of resultant H from the base W = resultant of dead loads of the structure and b = width of the structure For an entire structure with a height-to width ratio of 0.5 or less in the wind direction and a maximum height of 18m, the combination of the effects of uplift and overturning may be reduced by one third.

44 3. For outward acting pressures on roofs and leeward walls, C e shall be evaluated at the mean roof height and it is applied for the entire height of the structure for the leeward walls. 4. A building structure or story shall be considered open when 15% or more of the area of the exterior wall on any one side is open (doors & windows, etc.)