194 



MR. WILLIAM FAIBBAIRN ON 



exact measure of strength, and what excess should be given 

 to a bridge beyond the load it is called upon to support. 



It appears to be the opinion o^i|ftie railway commis- 

 sioners, that the flexure of girl^R should never exceed 

 one-third of the ultimate deflection; and, although I 

 concur in that opinion, I would venture to afiirm that in 

 wrought-iron tubular girders, such as are now in use, the 

 effects of reiterated flexure is only one-sixth, and conse- 

 quently they present a larger margin of security than girders 

 composed of any other material. 



On the effects of impact I entertain the same views as 

 the commissioners, that the deflection produced by the 

 striking body on wrought-iron is nearly as the velocity of 

 impact, and those on cast-iron greater in proportion to the 

 velocity. These facts have, however, been strikingly ex- 

 emplified by experiments made on the first tubular girder 

 bridges constructed for the support of a railway. Two 

 bridges of this kind were erected near Blackburn, over the 

 canal and turnpike road. Both bridges were 60 feet span, 

 and before they were opened to the public they were sub- 

 jected to the following tests :— 



A train of three locomotive engines, weighing 60 tons, 

 occupied the entire span of the bridge, and, having ascer- 

 tained the deflection in their quiescent state, they were 

 started at different rates of velocity, varying from 5 to 20 

 miles an hour, which produced a deflection of i^ths of an 

 inch. Two long wedges of the height of one inch were 

 then placed upon the rails in the middle of the span, and 

 the fall of the engines from this, when moving at a speed 

 of 8 to 10 miles an hour, caused a deflection of only '42 

 inch, which was increased to '54 inch, or about half an 

 inch, when wedges 1^ inch in thickness were substituted. 



These were severe tests, such as should not again be re- 

 commended, as the enormous strength of these girders is nov/ 

 well understood, and they may safely be considered fit for 



