47 8 



THE POPULAR SCIENCE MONTHLY. 



in the construction of the "bridge we have an open, clear span 

 in the center three times the width that would be possible with 

 the ordinary framed truss, and we have what is known and has 

 lately become famous as the cantilever bridge. The spans C A 

 and B D are called the cantilever spans, and A B the suspended 

 span. 



The variations in the cantilever bridge are almost infinite, 

 although the principles in all of them are the same. In the place 

 of using the upright truss, for example, this truss can be turned 

 over, and the ties then become braces, the floor comes upon the 



^J^iMxi 



CANTILEVER. 



Fig. 32 



top, the shore ends being firmly anchored in place, and, the sus- 

 pended span held in place, we have a cantilever of the type that 

 has just been erected across the Hudson at Poughkeepsie (Fig. 

 32), while the first example given you is the type of cantilever 

 that crosses the St. John River at St. John, New Brunswick 

 (Fig. 31). 



Then, when a greater length is desired and increased strength, 

 as in the case of the bridge across the Frith of Forth, in Scot- 

 land, we simply combine the two, putting the two king-post 

 trusses base to base, and hanging the suspended truss between 

 (Fig. 33). 



In that way we have the strongest form for the cantilever 

 bridge, and there is hardly any limit to the length of span that 

 can be made by this method. 



CANTILEVER. 

 Fig. 33 



The advantages gained by the use of the cantilever for long- 

 span bridges are as follow : 



As a substitute for the suspension bridge it is, up to a certain 

 length of span, less expensive, and it can be given great rigidity 

 and stability which are impossible in the suspension. 



As a substitute for the ordinary framed truss it has the ad- 

 vantages of not requiring any false works for its erection. In the 

 erection of the ordinary bridge there must be built first a timber 

 frame or staging between the piers to sustain the weight of the 



