Walls, Wind Pressure and Bracing
One of the more interesting cases that we have recently investigated involved the collapse of a masonry wall for a commercial building. After a recent storm, the wall in question was found by the construction crew to have been toppled and as a result, completely destroyed. Upon piecing together the conditions of the previous night, the wind velocity was somewhere in the neighborhood of 20mph(29.33 ft/sec). Wind pressure derives its strength from velocity (i.e. the higher the velocity, the higher the pressure). In order to calculate wind pressure, the temperature of the air as well as the atmospheric pressure must be known. Using these two quantities, the density of the air is calculated. At 70 deg and an atmospheric pressure of 14.7 pounds per square inch, the density of air is approximately .075 pounds per cubic foot. Setting this quantity aside, the "velocity head" also has to be calculated. The velocity head is a term used to relate the pressure created at a specific wind velocity. The velocity head is then multiplied by the air density in order to obtain wind pressure in pounds/square foot. In this case the wind pressure exerted by a 20mph wind is 13.36 x .075 = 1psf. In other words, for every square foot of wall area, a one pound load is acting horizontally on that wall. In contrast, a 100mph wind at the same air density would exert a wind pressure of 25psf. Although these pressures are not large numbers, consider that they would act on large surfaces. A masonry wall 40' high and 100' long would have a surface area of 4000 sq ft. A pressure of 25 psf acting on a wall area of 4000 sq ft is equivalent to a 100,000 pound (4000 x 25) load acting horizontally at the center of the wall.
Since wind loads can vary substantially and unpredictably, bracing of the wall is imperative. The number of braces required is dependant on the dimensions of the wall and what distance the manufacturer will allow between braces. Braces must be anchored to the wall and ground by means of bolts anchored in concrete. It should be noted that manufacturers have procedures that are to be followed for selecting and applying wall bracing. The contractor is normally responsible for obtaining and installing enough braces to prevent wall collapse. However, since the occurrence of wind and wind loads is unpredictable, the number of braces to use becomes a "guessing game". If a wall is designed to withstand 90mph winds, contractors recognize that 90mph winds are not an everyday occurrence and that it would be a waste of money to brace for conditions with a low probability of occurrence during the construction phase. It makes more sense to brace for more realistic conditions such as wind loads imposed by a 20-30mph wind. However, wind gusts of 40-70mph may occur and the applied bracing may not be adequate to support the wall. Hence, the "guessing game".
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