In respect to the relations which should exist between length of main opening, perpendicular distance between central planes of trusses, and the various truss depths, the author's practice is to make the least distance between parallel trusses one twenty-seventh of the main opening; the least distance between axes of vertical posts over main piers, when the trusses converge towards the suspended span, one twenty-fifth of the said opening; the truss depth for the suspended span, when the chords are parallel, from one fifth to one sixth, or for very long spans even one seventh, of the span; and the height of the vertical posts over main piers not to exceed four, or preferably three and a half, times the perpendicular distance between their axes. For through cantilever bridges the author generally makes the height of these posts about fifteen per cent of the length of the main opening.

For the sake of appearance the centres of the top-chord pins in cantilever-arms are placed on arcs of parabolas, the vertices of which are located at the hips of the suspended span; and the anchor-arms are laid out to the same curve, beginning at the tops of the posts over the main piers.

In concluding this chapter, a check on the correctness of percentage curves for weights of cantilevers will be given by applying the curves to the published estimated weights of metal in the various members of the longest cantilever bridge that has ever yet been designed in detail, viz., the proposed 2300-ft. span (measured between centres of main piers) for the North River Bridge at New York City. This proposed structure was designed by the Union Bridge Company.

The total weight of metal in trusses and laterals of the 720 ft. suspended span is 10,400,000 lbs. The trusses, which are of the Petit type, are divided into six main panels of 120 ft. each; consequently the panel weight is 10,400,000 ÷ 6 = 1,733,000 lbs. In the cantilever-arm there are six and five eighths main panels; consequently the weight of trusses and laterals therefor will be

1.20W + 1.40W + 1.65W + 2.00W + 2.40W + 3.00W

+ 5/8 X 3.60W = 13.90W = 24,090,000 lbs.