MATERIAL AND ALLOWABLE STRESSES. 



173 



SHEARS AND MOMENTS IN A PLATE GIRDER BRIDGE. The maximum shears 

 and moments in an 86 ft. span deck girder railway bridge are shown in Fig. 20. In calculating the 

 maximum live load shears the girder was divided into sections about 7 ft. in length and the maxi- 

 mum shears were calculated as in a truss bridge. The maximum bending moments were also 

 r.ilrul.itrd for the same points in the girder. The make-up of the tension flange and the rivet 

 spacing is shown in Fig. 20. 



The stress diagram for a 60 ft. span single track deck plate girder bridge is shown in Fig. 21. 



3622000 



5 56 611 Max Moment 



'460 



Max Shear 167370 



Curvtoffiax 



Dead Shear-16120 



Pitch staggered 



Curve of Max 

 Moments 



Cross Frame 



I, Alignment Tangent 

 1 1 Scate o f Shears l'= 150000* 

 1 1 Scale of Moment* l"= 1800000 



'rfGIZO Dead Shear 



\ i: 

 117700 



I42}0(T^: (| 

 Effective Span 'G5'-O"- 1.-J&/67370 Max Shear 



t MI t a I * * rf . 



Curve of flax 

 Shears 



Hfe^'-tf 1 



j< - atf'-<? 



FIG. 20. SHEARS AND MOMENTS IN A RAILWAY PLATE GIRDER. 



MATERIAL. Open-hearth carbon steel complying with the specifications of the Am. Ry. 

 Eng. Assoc. as given in the last part of this chapter is commonly used for bridges up to spans 

 of 500 to 550 feet. For spans of more than 500 or 550 feet to about 650 feet carbon and nickel 

 steel are used, or nickel steel alone is used. For spans of 650 to 750 feet nickel steel alone should 

 be used. For an exhaustive discussion of the use of nickel steel in the construction of bridges see 

 article entitled "Nickel Steel for Bridges" by Mr. J. A. L. Waddell, M. Am. Soc. C. E., in Trans. 

 Am. Soc. C. E., Vol. 63, 1909. An excellent discussion of the design of large bridges is given in 

 "Design of Large Bridges with Special Reference to the Quebec Bridge" by Ralph Modjeski, 

 Consulting Engineer, in Journal Franklin Institute, September, 1913. 



ALLOWABLE STRESSES. The allowable stresses on carbon steel as adopted by the Am. 

 Ry. Eng. Assoc. are given in the specifications in the last part of this chapter. Out of 39 railroads 

 in the United States 24 were using the Am. Ry. Eng. Assoc. specifications for allowable unit 

 stresses in 1913. For additional data on unit stresses, see Table XVI. 



