In applying either equation (43) or equation (44) care must be taken to give each stress and its corresponding strain (lengthening or shortening) the proper sign. As the formulæ have been written and used, a tensile stress and its resulting stretch must each be written positive, while a compressive stress must be written negative. This holds true for both the stresses _Z_ and _S_ (or _z_ and _s_). The magnitude of the assumed load _P_ is a matter of indifference, since the stress _Z_ will always be proportional to it and the ratio _P ÷ Z_ will therefore be constant. _P_ is frequently taken as unity; or, as in the case just given, it may have any value that the conditions of the problem make most convenient. =112. Application of Method for Deflection to Truss.=—In making application of the deflection formulæ to any steel railroad truss similar to that shown in Fig. 29, it will first be necessary to determine the stresses in all its members due to the dead and moving loads, since the deflection under the moving load is sought. These loads will be considered uniform, and that is sufficiently accurate for any railroad bridge. The moving train-load will be taken as covering the entire span, assumed, for a single-track railroad, 240 feet in length between centres of end pins. There are eight panels of 30 feet each, and the depth of truss at centre is 40 feet. Other truss dimensions are as shown in Fig. 29. The dead loads, or own weight, are taken at 400 pounds per linear foot of span for the rails and other pieces that constitute the track; at 400 pounds per linear foot for the steel floor-beams and stringers, and 1600 pounds per linear foot for the weight of trusses and bracing. The moving train-load will be taken at 4000 pounds per linear foot. This will make the panel-loads for each truss as follows: Lower-chord dead load, 30 × 800 = 24,000 pounds per panel. Lower-chord moving load, 30 × 2000 = 60,000 ” ” ” ------ Total load on lower chord = 84,000 ” ” ” Upper-chord dead load, 30 × 400 = 12,000 ” ” ” The structure is a “through” bridge, hence all moving loads rest on the lower chord. [Illustration: FIG. 30.] The stresses in the truss members due to the combined uniform dead and moving load are best found by the graphical method. One diagram only is needed to determine all the stresses, and it is shown in Fig. 30. This diagram is drawn accurately to scale, and the stresses measured from it are shown in the table on page 136.