WIND PRESSURE 13 



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. 



The pressure on tne vertical sides of buildings is usually taken at 

 30 pounds per square foot, equivalent to P equals 37^ pounds in 

 formula (3). This would give a \elocity of 96 miles per hour, which 

 would seem to be sufficient for all except the most exposed positions 

 The velocity of the wind in the St. Louis tornado was about 120 miles 

 per hour. The records of the U. S. Weather Bureau for the last ten 

 years show only one instance where the velocity of the wind as recorded 

 by the anemometer was more than 90 miles per hour. The actual pres- 

 sure of wind gusts has been found to be about 60 per cent and the 

 actual steady wind pressure only about 36 per cent of that registered by 

 ordinary small anemometers, which further reduces the intensity of the 

 observed pressures. The wind pressure has been found to increase 

 as the distance above the ground increases. 



Recent German specifications for design of tall chimneys specify 

 wind loads per square foot as follows : 26 pounds on rectangular 

 chimneys ; 67 per cent of 26 pounds on circular chimneys ; and 7 1 per 

 cent of 26 pounds on octagonal chimneys. 



The building laws of New York, Boston and Chicago require that 

 steel buildings be designed for a horizontal wind pressure of 30 pounds 

 per square foot. The Baltimore and Ohio Railroad specifies a horizon- 

 tal wind pressure of 30 pounds per square foot. 



From the above discussion it would seem that 30 pounds per square 

 foot on the sides and the normal component of a horizontal pressure of 

 30 pounds on the roof would be sufficient for all except exposed loca- 

 tions. If the building is somewhat protected a horizontal pressure of 

 20 pounds 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 sur- 

 faces is calculated. 



