LOADS IN STRUCTURES 



stare on a cylindrical surface is about one-half the pressure 

 on a flat surface having the same 

 width as the diameter of the cyl- 

 inder and the same height. 



If p'. Fig. 1, represents the 

 direction and strength of the 

 wind pressure against the roof abc, 

 it is the normal component p that 

 must be ascertained in order to 

 calculate the total pressure nor- 

 mal to the roof, or to determine 



the stresses in the members of a roof frame or truss. The 

 other component pp is acting upwards and in a direction 

 parallel with the slope. The latter force is not taken into 

 consideration. The wind, which is usually supposed to 

 exert a horizontal pressure of 40 lb., strikes the roof at an 

 angle; consequently, the pressure p, normal to the slope, is 

 less than 40 lb. 



The full discussion of the relation between p' and p is 

 somewhat more complex than the one given here, however. 

 What has been said shows in a general way why p is more 

 nearly equal to p' when a roof is steep than when a roof is 

 flat. In the design of roof trusses, a horizontal wind pres- 

 sure of 40 lb. is usually assumed. 



NORMAL WIND PRESSURE FROM HORIZONTAL PRES- 

 SURE OF 40 LB. PER SQ. FT. 



