3.2 Wind Pressure Effect on Building Walls and Roofs

Pressure coefficients used in practice have usually been obtained experimentally by testing models of different types of structures in wind tunnels. Commonly used coefficients refer to the average pressure or suction over a surface. Tangential forces are considered insignificant, so that the forces referred to act at right angles to the surfaces in question.

Roofs: The roof is usually the critical area in the wind design of low buildings, particularly residential structures. Where it is made up of light-weight components particular attention must be paid to anchorage details because of the suction condition prevailing over most, if not all, of it. A good example of such precautions is the time-honoured custom of weighting roofs in Alpine areas with large stones.

Critical Angle, Windward Slope: For every sloped roof there is a certain slope angle at which the suction coefficient over the windward slope reaches a numerical maximum. For low buildings this angle may be around 5 degrees; for average shapes such as the usual bungalow, from 5 to 15 degrees. For higher buildings with height to width ratios ranging up to 2: 1 the critical angle may be as high as 25 or 30 degrees. The average pressure coefficient for the critical angle is in the range -0.7 to -1.4.

Steep Roofs: As the roof slope increases beyond the critical angle the average pressure coefficient decreases numerically to zero; it then increases in a positive direction, indicating pressure, to a maximum of +0.8 or so for a slope angle of 90 degrees (i.e. in the plane of the windward wall). The angle at which the coefficient is zero varies from 20 to 45 degrees, increasing with the ratio of height to width of the building.

Leeward Slope: The effects of slope and building dimension ratios are much less pronounced on suctions on the leeward slope and for general purposes could probably be disregarded. Average values range from -0.5 to -0.8 for most building shapes and slopes.

Walls: For tall, slender structures the design of the walls and the frame, with regard to overturning moment, are likely to be critical. The trend toward high-rise buildings and curtain wall construction may lead to greater problems in limiting sway and in specifying the strength of fastenings for wall panels. Although average coefficients for leeward and side walls are only -0.5 to -0.7 high suctions occur just around the corners from the windward edges, and where stagnation pressures are high (near the top of tall buildings) these local suctions may be appreciable (possibly as high as -1.5).

Shielding: Model tests investigating the effects of building proximity have shown that where two buildings are close together rather high suctions may occur on the facing walls if the wind blows parallel to the "alley" between them. Average values of -1.0 have been measured, with local suctions along the windward edges as high as -2.0.

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