284 



Introduction to the Study of Science 



where the axle supports the center in much the same way as 

 a pier supports the central part of a cantilever bridge. Exam- 

 ine Fig. 86 for the 

 distribution of forces 

 and members. One 

 of the most noted cre- 

 ations of this ty$e is 

 the railway bridge 

 across the Firth of 

 Forth, near Edin- 

 burgh, Scotland. This 

 has two main spans 

 (Fig. 87) each 1710 

 feet in length, and sup- 



FIG. 85. The principle of the cantilever ap- 

 plied to an automobile spring. Compare with its 

 use in the cantilever bridge, Fig. 87. 



ports two railway 

 tracks. Its rigidity 

 is such that express 



trains may cross it at the rate of sixty or more miles an hour. 



The large tubes, eight to twelve feet in diameter, and usually 



of circular section, are the compression members. The lattice 



work parts form the tension members. The illustration shows 



that tubular framework is 



combined with the cantilever 



design. The new Quebec 



bridge across the St. Lawrence 



River is of the cantilever 



design, with the principal span 



1800 feet long, which is prob- 

 ably the longest cantilever 



span ever built. 



121. Tubular frame. 



Several kinds of bridges have 



been worked out on the basis 



of these facts. One kind is the tubular. It resembles the 



frame of a bicycle which is made of tubes, instead of solid 



FIG. 86. Diagram of a cantilever 



bridge, showing typical truss fSrms in 



arrangement of tension and compression 



members. Compression members > < 



Tension members < ^ 



