282 



Introduction to the Study of Science 



COMPRESSION STRESS 



TENSILE STRESS 



A. Single girder. 



B. Triangular truss. 



into two equal parts by the support at the center. The work- 

 ing lengths are thus from the center b to the ends A and C. 

 Suppose a load is resting upon the center of the bridge. Where 



_^ does the weight actually 



L . r-L rest ? How is it transmitted 



to the supports ? Trace out 

 the members which transmit 

 it to the supports. Which 

 are the crushing or com- 

 pression members? Which 

 are the pulling or tension 

 members ? 



Howe truss. It is evi- 

 dent from this that the tri- 

 angle type not only gives 

 greater strength than a sim- 

 ple girder, but also makes 

 possible much longer spans 

 without intermediate piers 

 or supports. By a modifi- 

 cation of the triangle, still 

 longer spans are possible and 

 with an increase rather than 

 a decrease in strength for 

 sustaining maximum loads. 

 A modified triangle is shown 

 in diagram C. This is 

 known as the Howe Truss. 

 Can you find the complete 

 triangle? By imagining it 

 traced out in continuous 



C. Howe truss. 



D. Pratt truss. 



FIG. 84. Typical forms of- the truss 

 bridge. The simple girder and triangle 

 make possible bridges of great length and 

 strength, and relatively small weight. 



form, you realize that it would reach, if it were not thus divided, 

 an undesirable height for long spans. Hence it is cut into two 

 parts, and the top m', ri, is lowered to rest at points d and e. The 

 apex reaches the top bar which connects the sides of the triangle. 



