1 6 WIND LOADS 



where P in (8) equals the pressure at right angles to the direction of 

 the wind, Fig. 5. PI may be an uplifting, a depressing or a side pres- 

 sure. With an open shed in exposed positions the uplifting effect of 

 the wind often requires attention. In that case the wind should be 

 taken normal to the inner surface of the building on the leeward side, 

 and the uplifting force determined by using formula (8). If the gables 

 are closed a deep cup is formed, and the normal pressure should be 

 increased 30 to 70 per cent. 



That the uplifting force of the wind is often considerable in exposed 

 localities is made evident by the fact that highway bridges are occasion- 

 ally wrecked by the wind. The most interesting example known to the 

 author is that of a loo-foot span combination bridge in Northwestern 

 Montana which was picked up bodily by the wind, turned about 90 

 degrees in azimuth and dropped into the middle of the river. The end 

 bolsters were torn loose although drift-bolted to the abutments.* 



The wind pressure is not a steady pressure, but varies in intensity, 

 thus producing excessive vibrations which cause the structure to rock 

 if the bracing is not rigid. The bracing in mill buildings should be 

 designed for initial tension, so that the building will be rigid. Rigidity 

 is of more importance than strength in mill buildings. 



For further information on this subject see a very elaborate and 

 valuable monograph on " Wind Pressures in Engineering Construc- 

 tion," by Capt. W. H. Bixby, M. Am. Soc. C. E., published in Engi- 

 neering News, Vol. XXXIIL, pp. 175-184, March, 1895. 



Wind Pressure on Office Buildings. The following specifica- 

 tion for wind pressure on office buildings has been proposed by Mr. 

 C. C. Schneider, f 



Wind Pressure. The wind pressure shall be assumed at 30 Ib. 

 per sq. ft. acting in either direction horizontally: 



1. On the sides and ends of buildings and on the actually exposed 



surface, or the vertical projection of roofs; 



2. On the total exposed surfaces of all parts composing the metal 



framework. The framework shall be considered an inde- 

 pendent structure, without walls, partitions or floors. 



* For a description of the wreck by the wind of the High Bridge over 

 the Mississippi River at St. Paul, Minn., see article by C. A. P. Turner in 

 Trans. Am. Soc. C. E., Vol. 54, p. 31. 



f Trans. Am. Soc. C. E., Vol. 54, 1905. 



