896 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



reduce all the material to a minimum, thus guarding against dead 

 weight, at the same time avoiding all unnecessary expense. The 

 annexed diagram (fig. 1) will illustrate the general arrangement of 

 trussing in the Whipple bridge and in all its modifications. The 

 heavy lines represent cast iron, while the light ones are of wrought 

 iron. The diagonals slope at an angle of forty-five degrees. 



As will be noticed, the weights are distributed both ways to 

 the abutments, in an inverstj ratio to their lever arms, liy means 

 of a series of triangles, the rods proportionally becoming more 

 strained as we approach the ends of the truss, where they hold up 

 the whole weight of bridge aud its load. The verticals receive 

 an increasing thrust in the same way. The strains in the horizontal 

 members are the horizontal components of the diagonals, which, 

 in the lower chord are positive or tensile, and in the upper, nega- 

 tive or compressive. These horizontal components are a minimum 

 at the end panels, and increase toward the center where they are 

 a maximum. This increase is measured by the summation of the 

 horizontal components at every panel. This, in general terms, is 

 the distribution of weights in a truss of this description, with the 

 strains they engender, which strains you perceive are very readily 

 calculated. I would, if I had time, show you the arrangement of 

 parts, but as much remains to be said, I will now turn to the 

 "Fink "and "Bollman" trusses, of which the skeleton diagrams 

 below will show the general arrangement of parts. In both the 

 weight is transmitted by means of tension rods of varying length, 

 carrying it at once to the abutments, and in doing so give a uni- 

 form strain throughout the top chord. 



In the Fink truss (fig. 2) 'the tension bars of each system being 

 all of the same length, and each system of supports comprising 

 either one-half of the next larijer or embracino; two of the next 

 smallest systems, the mode of action of the truss, as also the com- 

 putation of the strains becomes a matter of great simplicity. You 

 see that each post bears a weight proportional to its covering 

 system, so that the middle post bears one-half the weight of one 

 truss, the quarter posts one-quarter, and so on. The tension rods 

 are strained proportionally to their inclination, and in very long 

 spans, unless the truss is extremely deep, they lie at a very flat 

 angle which rolls up an enormous dut}^ for them to perform. In 

 the Bollman bridge (fig. 3) the loads on each panel are equal, atid 

 are at once transmitted as I before remarked by tension rods to 

 the abutments. The spans of these bridges are limited very 



