today being the Pratt, Petit, Triangular (both simple and sub-divided and including the Warren), and K trusses.

Comparing Pratt and Petit truss-spans, for which there is no difference worth mentioning in the pound prices of the metal, the weights per foot (and therefore the costs) are alike for single-track spans of three hundred (300) feet, and for double-track spans of three hundred and fifty (350) feet; but both constructive and aesthetic reasons generally necessitate limiting the lengths of Pratt trusses to about three hundred and twenty-five (325) feet.

In respect to the comparative economics of the Pratt and Triangular trusses, there seems to be a difference of opinion amongst bridge engineers. The author has found very little variation in their total weights of metal, with occasionally a slight economy in favor of the Triangular truss. That truss has the practical advantage that changes in chord stresses occur at only every other panel point. This often makes it possible to section the chords more economically.

As explained at length in Chapter XI, for continuous trusses of very long span, the Triangular truss has quite an advantage over the Petit truss.

The K truss is applicable for long spans only, and, therefore, is in competition with the Petit truss and not with the Pratt. Its principal claim for economy lies in ease and simplicity of erection, but it also has a tendency to reduce the high secondary stresses inherent in the Petit type. It was employed to advantage in the design of the great Quebec cantilever bridge.

The statements made in this chapter apply mainly to railway bridges and heavy highway structures. For light highway bridges some of them might require a slight modification.