How to calculate wind and snow loads

Written by sean butner | 13/05/2017
How to calculate wind and snow loads
Calculating total wind and snow loads enables you to design a structure to withstand environmental extremes. (Christopher Furlong/Getty Images News/Getty Images)

Engineers face the difficult problem of converting theory into practice. A thoughtful engineer anticipates structural burdens during planning, helping to prevent unexpected catastrophes. Budget restrictions forced the designers of the infamous Tacoma Narrows Bridge in the United States to implement a substructure that did not account for environmental wind loads. On 7th November 1940, the bridge's original design proved incapable of handling winds of 35 mph to 46 mph. The winds compromised the structural integrity of the bridge, which undulated, broke apart and collapsed. If you carefully calculate wind and snow loads and other environmental factors, you can help avoid similar engineering failures.

Determine the slope of the surface in question. Slope is expressed as a rise-over-run ratio. For instance, a roof that is 30 cm (1 feet) tall and 1.2 metres (4 feet) long has a slope of 1 to 4. Convert the slope to a ratio of 12 by dividing 12 by the run portion of the slope and multiplying the slope by the result. For instance, to convert the slope 1 to 4 to a ratio of 12, divide 12 by 4 and multiply 1 to 4 by 3 for a result of 3 to 12, a ratio equal to 1 to 4.

Calculate the pressure. Mark 0.093 square metres (1 sq foot) of snow, measure its depth, and weigh the snow. Convert the depth of snow to square metres and multiply the weight of the area of snow by its depth. The result is the pressure of the snow per square metre. Multiply .00256 by the wind speed in miles per hour squared to determine the pressure of the wind.

Find the slope factor or drag coefficient. Use the slope expressed as a ratio of 12 for determining the snow slope factor. Calculate the angle with the horizon by dividing the rise by the total length of the roof, which you can calculate by finding the square root of the sums of the squared rise plus the squared run. Divide the angle with the horizon by 90 to determine the drag coefficient for wind loads.

Multiply the pressure by either the slope factor or drag coefficient to find the total area load per square metre. Multiply this load by the total area of the surface to determine the total load the structure can sustain.

Estimate maximum typical pressure. Use meteorological statistics to determine the minimum and maximum snow levels and sustained wind speeds. Multiply the pressure by the minimum and maximum snow depths to determine minimum and maximum environmental loads. Use historical speed information to adjust the pressure of wind loads.

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