220 



BRIDGE TRUSSES NOW OUT OF DATE. Art. 122. 



panel load is separately suspended by ties running to the sup- 

 ports. There is no bottom chord and the diagonal members shown 

 dotted take no calculable stress but serve to stiffen the truss. 



The Fink truss was introduced about the same time as the 

 Bollman. It was invented by Albert Fink, an assistant engineer 

 on the Baltimore & Ohio Railroad. Fig. 156 shows the principle 

 upon which this truss was built. The path of each panel load is 

 easily followed ; only the middle panel load is carried directly to 

 the supports ; parts of the other panel loads also go over the same 

 path. For long spans, the truss was usually built so that about 

 half of the posts were of the same length. The Fink truss neces- 

 sarily had an even number of panels. The stresses may be easily 

 calculated. 



BOWSTRING TRUSS 

 Fig. 157. 



In 1840 Mr. Squire Whipple built his first iron bridge and 

 it was of the bow-string type (Fig. 157). He pointed out the 

 economic advantages of this style of truss in the book referred 

 to above. Since the bending moment decreases towards the ends, 

 the chord stresses may be made constant by varying the depth 

 in a certain way. The upper and end panel points were usually 

 made to lie on a parabola, in which case, with full load, there is 

 no stress in the diagonal web members, and the stress in each 

 chord is the same throughout; the vertical members are in ten- 

 sion, each carrying a panel load to the top chord, which acts like 

 an arch whose horizontal thrust is taken by the bottom chord. In 

 this case all diagonal web members are counter ties and, for par- 

 tial loading, the verticals are in compression. 



If the upper joints lie above a parabola through the ends, 

 the counter ties will be as shown in Fig. 157; if they lie below, 

 the ties and counter ties will have the reverse directions. 



The stresses in a bowstring truss are easily calculated. Under 

 full load, and with a parabolic truss, the upper chord resists all 

 the shear in each panel. It is to be roted that the tie and counter 

 tie, in a panel, do not have the same inclination. 



