84 APPLICATION TO BRIDGES. PABT II 



PART II. 



APPLICATION TO BRIDGES. 



05.- -Under the head of Parallel Forces we have already 

 given the general application of the graphical method to the 

 determination of the moments and shearing forces in beams 

 resting upon two supports only. We shall now take the sub- 

 ject up more in detail, and show the methods of determining 

 the maximum strains for all the possible conditions of loading 

 which may occur in Bridge Girders. In the following we shall 

 adhere closely to the development of the subject as given by 

 Winkler. \J)erBruckenbau, Wien, 1872.] 



G6. Forces whirl! act upon a Bridge. The forces which 

 act upon a bridge may be enumerated as follows : 



\st. The weight of the bridge itself. This, previous to the 

 calculation of the strains, is unknown, since it depends upon the 

 intensity of the strains themselves. It is customary to assume 

 the weight to begin with, by comparison with existing struc- 

 tures of similar character, and then to find the resulting strains. 

 The weight answering to these strains can then be easily ascer- 

 tained ; the strength of the materials used being known, and 

 compared with the assumed weight. According as it is less or 

 greater, the weight was then assumed too great or the reverse. 

 A second approximation to the true weight may then be made, 

 and the strains proportionally diminished or increased. As 

 rules for estimating the weight of bridge girders under 200 feet 

 Bpan, we have, for weight of girder G, 



WP 



-I2fd> 



where W = the assumed approximate total distributed load in 

 tons, including the weight of girder ; 



I = length in feet ; 

 d = depth in feet ; 



