WIND LOADS. 5 



ll.it roofs were loaded with a snow load of more than 30 Ib. per sq. ft. and roofs with a pitch of one- 

 half carried the full snow load of 27 Ib. per sq. ft. of horizontal projection. 



A high wind may follow a heavy sleet and in designing the trusses the author would recom- 

 mend the use of a minimum snow and ice load as given in Fig. I for all slopes of roofs. The 

 m.ixiinuni stresses due to the sum of this snow load, the dead and wind loads; the dead and wind 

 loads; or of the maximum snow load and the dead load being used in designing the members. 



Wind Loads. The wind pressure, P, in pounds per square foot on a flat surface normal to 

 the direction of the wind for any given velocity, V, in miles per hour is given quite accurately 

 by the formula 



P = 0.004 V s (2) 



The pressure on other than flat surfaces may be taken in per cents of that given by formula 

 (2) as follows: 80 per cent on a rectangular building; 67 per cent on the convex side of cylinders; 

 115 to 130 per cent on the concave side of cylinders, channels and flat cups; and 130 to 170 per 

 cent on the concave sides of spheres and deep cups. 



Recent German specifications for design of tall chimneys specify wind loads per square foot 

 as follows: 26 Ib. on rectangular chimneys; 67 per cent of 26 Ib. on circular chimneys; and 71 

 per cent of 26 Ib. on octagonal chimneys. 



The official specifications for the design of steel framework in Prussia have recently been 

 amplified in the matter of wind pressures. For the wind-bracing, as a whole, the wind pressure 

 on the whole building is to be taken as 17 Ib. per sq. ft. For proportioning individual frame 

 members, girts, studs, trusses, etc., a higher value of wind pressure must be assumed, viz., 28 to 

 34 Ib. per sq. ft. 



It would seem that 30 Ib. per square foot on the side and the normal component of a hori- 

 zontal pressure of 30 Ib. on the roof would be sufficient for all except exposed locations. If the 

 building is somewhat protected a horizontal pressure of 20 Ib. per square foot on the sides is 

 certainly ample for heights less than, say 30 feet. 



Wind Pressure on Inclined Surfaces. The wind is usually taken as acting horizontally 

 and the normal component on inclined surfaces is calculated. 



FIG. 2. 



The normal component of the wind pressure on inclined surfaces has usually been computed 

 by Hutton's empirical formula 



P n = P'smA*- Meo "- 1 (3) 



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. (2). 



The formula due to Duchemin 



p .. p 2 sin A . . 



F l + sin' A 



where P n , P and A are the same as in (3), gives results considerably larger for ordinary roofs 

 than Hutton's formula, and is coming into quite general use. 

 The formula 



P, = P. A/45 (5) 



