. Farrow's manual of military training . e chord stress in the end panels will not exceed J4 theload on the bridge unless the length of the panel is greater thanits height, which should never be the case. Between these lower andhigher limits the chord stresses vary, but not by equal change is more rapid near the ends and less so toward themiddle. For wooden trusses, convenience in framing requires thatall chord pieces have one dimension the same, and it is not cus- BRIDGES AND PONTOONS 641 tomary to make more than one change in the aggregate chordsections. This is done by boltin
. Farrow's manual of military training . e chord stress in the end panels will not exceed J4 theload on the bridge unless the length of the panel is greater thanits height, which should never be the case. Between these lower andhigher limits the chord stresses vary, but not by equal change is more rapid near the ends and less so toward themiddle. For wooden trusses, convenience in framing requires thatall chord pieces have one dimension the same, and it is not cus- BRIDGES AND PONTOONS 641 tomary to make more than one change in the aggregate chordsections. This is done by bolting extra timbers on each side of thelower chord over its middle third. The Pratt Truss.—The form and the distribution of stressesare the same as the Howe truss. The disposition of materials inweb members is reversed, the verticals being of wood and incompression and the diagonals of metal and in tension. Thearrangement at panel points is modified accordingly. The chordsare the same as for a Howe truss of the same span, height, PRATT TRUSS. In both Howe and Pratt trusses care must be taken not tointroduce unnecessary initial strain by setting up the rods tootight. The upper chord should be V/2 inches longer than the lowerone for each 100 feet of span; the excess to be divided equallyamong the panels. This prevents the upper chord becoming short-er than the lower when it is compressed and the lower onestretched by the load. The effect when the bridge is light is togive the truss a slight crown or camber. The Lattice Truss.—This truss may be constructed entirely}f 2 or 3 inch planks and wooden pins, avoiding the latter if bolts:an be had. The disposition of material is clearly shown in thedrawing. If there are three sets of planks, the pairs must be in the 642 MILITARY TRAINING position of braces and the single planks of counter braces. Theplanks are 2 or 3 inches thick and 9 to 12 inches wide, accordingto the span. They are placed about 2Yi feet apart, measured along
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