14 WIND LOADS 



The normal component of the wind pressure on inclined surfaces 

 has usually been computed by Hutton's empirical formula 



P n =Psin^ L842 cos ^-1 (4) 



where P n equals the normal component of the wind pressure, P equals 

 the pressure per square foot on a vertical surface, and A equals the 

 angle of inclination of the surface with the horizontal, Fig. (5). 

 The formula due to Duchemin 



2 sin .4 (5) 



1 + sin 2 A 



where P n , P and A are the same as in (4), gives results considerably 

 larger for ordinary roofs than Hutton's formula, and is coming into 

 quite general use. 

 The formula 



P 2- A (6) 



rn= 45 A 



where P n and P are the same as in (4) and (5), and A is the angle 

 of inclination, of the surface in degrees (A being equal to or less than 

 45), gives results which agree very closely with Hutton's formula, 

 and is much more simple. 



FIG. 5. 



Hutton's formula (4) is based on experiments which were very 

 crude and probably erroneous. Duchemin's formula (5) is based on 

 very careful experiments and is now considered the most reliable form- 

 ula in use. The Straight Line formula (6) agrees with experiments 

 quite closely and is preferred by many engineers on account of its 

 simplicity. 



The values of P n as determined bv Hutton's, Duchemin's and the 



