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Case I.
From Plate IV find the equivalent uniform live load per lineal foot for the given span length and multiply same by the panel length, calling the product L. For single-track bridges this must be divided by two. All the live load stresses in main-truss members of single-intersection bridges can be found by substituting this value of L in Table XVII.
Just here it is proper to remark that the "Equivalent Uniform-Load Method" is not applicable to trusses of multiple intersection; but the most approved modern practice in bridge-engineering does not countenance the building of trusses or girders having more than a single system of cancellation. The "Equivalent Uniform-Load Method" does, however, apply to trusses with divided panels, such as the Petit truss; but as this style of truss nowadays involves almost invariably a polygonal top chord, its treatment herein will come under
Case II.
Where trusses have unequal panels or chords not parallel, the first step to take is the finding of all the dead-load stresses by the graphical method, starting from one end of the span and working towards the middle, where the last stress is checked by the method of moments, and the correctness of the entire graphical work is thereby proved.
The next step is to find from Pl. IV., as in Case I, the equivalent live-load per lineal foot for the span, and therefrom the value of the panel-truss live-load L. Next set a slide-rule for the ratio of dead load per lineal foot and the equivalent live load per lineal foot for the span, and, by referring to the dead-load stresses already found, read off from the rule all of the live-load stresses in chords and inclined end posts.
Next assume that there is an upward reaction at one end of the span equal to 1,000 pounds, 10,000 pounds, or 100,000 pounds (according to the size of the bridge), caused by a load placed at the first panel point from the other end of the span, then find graphically the stress in each web-member from end to end of span, caused by this assumed upward reaction.
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