696 ON FORM AND MECHANICAL EFFICIENCY [ch. 



chief problem in the designing of a bridge is to provide due 

 resistance to the " bending-moments " which result from the load. 

 These bending-moments will vary from point to point along the 

 girder, and taking the simplest case of a uniform load supported 

 at both ends, they will be represented by points on a parabola. 

 If the girder be of uniform depth, that is to say if its two flanges, 



Fig. 340. A, Span of proposed bridge. B, Stress diagram, or diagram 

 of bending-moments*. 



respectively under tension and compression, be parallel to one 

 another, then the stress upon these flanges will vary as the bending- 

 moments, and will accordingly be very severe in the middle and 

 will dwindle towards the ends. But if we make the deph of the 

 girder everywhere proportional to the bending-moments, that is 



Fig. 341. The bridge constructed, as a parabolic girder. 



to say if we copy in the girder the outlines of the bending-moment 

 diagram, then our design will automatically meet the circum- 

 stances of the case, for the horizontal stress in each flange will 

 now be uniform throughout the length of the girder. In short, in 



* This and the following diagrams are borrowed and adapted from Professor 

 Fidler's Bridge Constrnction. 



