GRAPHIC STATICS 227 



Wind Force 



It was stated that when the slope of a roof is less than 30 

 degrees it is customary to assume the wind load as acting hori- 

 zontally. When the slope is greater than 30 degrees the wind 

 is an important matter and the exact amount and direction must 

 be considered. When the forces in a roof are treated graphically 

 the best practice is to obtain the exact forces caused by wind, no 

 matter what the slope of the roof. 



A number of formulas for wind are in use, but the most modern 

 is that of Duchemin. It is based on very careful experiments 

 and is considered the most reliable wind pressure formula now 

 in use. 



Let P = horizontal wind pressure in pounds per square foot. 

 P n = wind pressure normal to the surface of the roof, in 



pounds per square foot. 



A = angle of the surface of the roof, with the horizontal, 

 expressed in degrees. 



D 2 sin A 



then P n - P -. r-r-j- 



1 + sin?A 



All designers like simple straight-line formulas, so the following 

 is used by a number of men. It gives values somewhat lower 

 than those given by the Duchemin formula, but agrees fairly 

 well with some experiments. For roofs having a slope exceeding 

 45 degrees the full horizontal pressure is used. When the angle 

 is less than 45 degrees, the straight-line formula is 



P n = P(A + 45). 



A number of years ago Professor Karl Pearson proposed that the 

 pressure on a roof, normal to the surface, be taken as equal in 

 pounds per square foot to the slope of the roof expressed in degrees, 

 up to a maximum of the number of pounds horizontal pressure, 

 after which the normal pressure should be equal to the horizontal 

 pressure. For example, when the angle of the roof with the 

 horizontal is 20 degrees, the normal pressure will be 20 Ibs. per 

 square foot. Expressed as a formula, using a maximum of 50 

 Ibs. per square foot, the horizontal pressure used in early days, 

 it appears 



P P * A 

 ^"" ~50~ 



