TUBULAR GIRDER BRIDGES. 187 



flection on one side of the pier, which, acting as the fulcrum, 

 or support of the beam, has a tendency to raise, or tilt up 

 the end of the land girder to the same height exactly 

 from the abutment pier. Assuming this to be the fact, and 

 the girder to be perfectly rigid, we should then have a ten- 

 sile strain along the top side of the girder over the pier in 

 the ratio of 125 : 14, nearly as 9:1. This is one of the 

 advantages peculiar to the wrought-iron tubular girder, as, 

 in every bridge having more than one span, the girders have 

 always been made continuous ; but as repeated changes are 

 continually going forward from the passing trains, and as 

 these changes, producing a severe strain, have a tendency 

 to destroy the elasticity of the material, and the soundness 

 of the workmanship at that part, I have considered it essen- 

 tial for the public safety to neglect it in the calculation, and 

 to give in any additional strength which may arise from 

 that source. Should it, however, be determined to take 

 these advantages into account, a new formula must be de- 

 duced, and a new system of construction must be adopted 

 over the piers, in order to attain the full benefit of this new 

 element of strength. 



The excess of strength that should be given to Girder 

 Bridges, has received considerable attention not only from 

 the profession, but also from the general public. The various 

 accidents which have occurred in the failure of bridges of 

 different constructions, have created of late years consider- 

 able alarm as to the stability of those important structures; 

 and when the enormous weight of a railway train, and the 

 momentum of that train moving at fifty miles an hour, are 

 taken into consideration, it requires the utmost foresight, 

 and the greatest possible care, to have the bridge sufficiently 

 strong. These are considerations of deep importance to the 

 engineer as well as the public; and although great differ- 

 ence of opinion exists as to the exact multiplier that should 

 be given to the maximum load, to obtain the load which 

 would produce rupture, I am of opinion that it should 



