From Fig. 55ddd on page 1274 of "Bridge Engineering" we find, for Class 70 and L = 1538', a weight of metal per foot of 28,000 lbs.; and from Fig. 55jjj on page 1282 thereof, for that loading and L = 1,250', a weight of 21,000 lbs. The ratio of these weights is 21/28 = 0.75.

Comparing Type D with the other types in Fig. 12a, it is evident that, for the same value of the hypothetical opening, L, its weight of metal is intermediate in amount; but, as before, there is really no necessity for contrasting this type with the others, because, for any crossing where it would be suitable, the other types would be wholly unsuitable.

Recapitulating, there is never any necessity for a discussion as to which of the four types should be adopted for any proposed crossing, because the profile thereof with its governing conditions will indicate clearly which is the only type applicable; but there is occasionally an economic question to determine as to whether a simple-truss layout or a cantilever layout should be adopted. This question is treated at length in Chapter XII.

In respect to the economic division of span-lengths for any proposed layout, the author determined this question for Type A nearly a quarter of a century ago when writing "De Pontibus," his findings being as follows:

First. The economic length of the suspended span is about three-eighths (3/8) of the length of the main opening, but a considerable increase or decrease of this proportion does not greatly change the total weight of metal.

Second. The most economic length of anchor arms, where the total length between centers of anchorages is given, and when the main piers can be placed wherever desired, is one-fifth (3) of the said total length, or one-third (1/3) of the main opening. By keeping the anchor arms short, the top chords may be built of eye-bars, provided that, with the usual allowance for impact, there is no reversion of chord stress; and this effects quite an economy of metal. But it is conceivable that cases might arise where, from danger of washout of falsework, eye-bar top chords would be objectionable; hence this method of economizing must be used with caution.

It must not be forgotten that for every dollar saved by reducing the total weight of metal through the shortening of the anchor arm, it will be necessary to spend about twenty cents for extra concrete in the anchorages. On that account, for the conditions assumed, the truly economic length of each anchor arm of a three-span, Type A, cantilever bridge may be a trifle greater than twenty per cent of the total distance between centers of anchorages.

Dr. Steinman in his "Suspension Bridges and Cantilevers," by a theoretical investigation and by using certain constants determined from computed structures, shows that for this case the length of anchor arm for economy should be four-tenths of the main opening, or four-eighteenths (0.22) of the total length of structure. This checks quite closely with the author's long-previous determination of "two-tenths or slightly more."