350 PLATE GIRDER PORTAL, SHALLOW TRUSSES. Art. 175. 



175. Case No. 7. Plate Girder Portal for Shallow 

 Trusses. Case No. 7a. Posts Fixed both Top and Bottom. 

 This case is exactly similar to No. 5. 



There is a horizontal 

 shear in the girder, equal 

 to R-\-P which is trans- 

 mitted to B. This girder 

 also takes a vertical shear 

 and a bending moment. 

 The vertical shear equals V 

 between GK and G'K' and 

 equals the stress in BF be- 

 tween BE and GK. 



The moment is a max- 

 imum at GK and G'K f and 

 equals BFXf. 



With a section Im and 

 center of moments at B } 

 Horiz. Comp. of GF 

 Mi 



-11 





+ 



Fig. 225. (Case No. 7) 



BF=Vert. Comp. of GF-V. 

 II, V, M and X-L may b? found as for case No. 4a. 

 Cases No. 7b and No. 7c are to be treated in a manner exactly 

 .-similar to Cases No. ob and No. 5c. 



176. Stresses in Railway Bridges having Curved Track. 1 



The stresses in railway bridges having curved track, are different 

 from those in bridges with straight track, on account of the re- 

 sistance which must be offered by the track, in changing the 

 direction of a moving train, on account of the center line of track 

 deviating from the axis of the bridge, and on account of the dis- 

 placement of the center of gravity of the train due to the super- 

 elevation of the outer rail. 



We have then, affecting live load stresses, " centrifugal 

 force," and "eccentricity." 



The centrifugal force of a body of weight P moving in a 



PV * PV 2 

 circular path is CP= = p-^- (189) 



iFor a complete discussion of this subject see Trans. Am. Soc. 

 C. E.. Vol. XXV, p. 459. 



