CHAPTER XIV. 

 STRESSES IN BRIDGES FROM HORIZONTAL FORCES. 



158. Wind Loads. The horizontal forces acting upon a 

 bridge are those due to wind, and in the case of a railway bridge 

 with the track on a curve, those due to centrifugal force. There 

 are also stresses in the transverse bracing due to vibration and 

 swaying of the live load, which cannot be accurately deter- 

 mined, and which are taken care of by increasing the assumed 

 wind load and by stiffening the general design. 



The maximum actual pressure of the wind from observa- 

 tions covering nine years at the site of the Forth Bridge, 1 was 

 41 Ibs. per square foot on a small area of 1.5 sq. ft., and 27 Ibs. 

 per square foot on an area of 300 sq. ft. Other observations also 

 show that the wind pressures on large areas never are so great 

 as on small ones. This is probably due to the wind moving in 

 gusts and eddies of comparatively limited extent. 



Bridge specifications require the structure to be designed 

 for winds loads of from 30 to 50 Ibs. per sq. ft. of exposed sur- 

 face. Many specifications do not state the w r ind load in pounds 

 per square foot, but in pounds per linear foot of bridge. The 

 exposed surface is usually taken as double the area of one truss 

 as seen in elevation plus the vertical area of the floor system 

 considered solid. The area of a plate girder . bridge is taken 

 at 1J to \\ times the area of the elevation. 



The action of the wind on the train produces a moving 

 wind pressure which is usually taken at 10 sq. ft. X30 Ibs. =300 

 Ibs. per lin. ft. of bridge. For highway bridges, on account of 

 the smaller vibratory stresses, the specified live load wind is 

 frequently reduced to half that used for railroad bridges. On 

 account of the variable nature of the wind, some specifications 

 require that both the dead load and live load wind pressures 

 be treated as moving loads. 2 



1 " The Forth Bridge," Engineering, Feb. 28, 1890. 



2 See American Railway Engineering Association Specifications for Steel 

 Railway Bridges, Art. 10. 



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