Cantilever bridges may be divided into four types, which cover all the layouts that are used by good designers. These are shown as Types A, B, C, and D, in Fig. 12a.

Type A is the one ordinarily adopted probably as often as three times out of four. It is applicable to the case of a fairly-wide crossing, where, for some reason or other, it is not permissible or advisable to put piers in the deep water.

Type B really amounts to the doubling up of two Type-A structures by omitting the anchorages at the junction and forming the anchor arms into one continuous span. It is applicable to very wide rivers.

Type C, which is the most economic of the four, is used occasionally instead of three simple-truss spans, either for reasons connected with the navigation of the stream or because of economic motives that are sometimes based on reality but too often upon unwarranted assumption. This question is discussed at length in Chapter XII.

Type D is a combination of Types B and C, as can be seen by an inspection of the layout in Fig. 12a.

Comparing Types A and B, a glance at the two layouts of the diagram shows that there can be but little difference in the weights of metal per lineal foot of entire bridge; because, while the weight per foot for the anchor span is generally somewhat greater than that of the anchor arms, the entire weight of two anchorages is saved, whatever net difference there is constituting generally an excess for Type B. There can, however, be no real comparison between these two types for any particular case, as one is for a comparatively narrow crossing and the other for a very wide one.

Comparing Types A and C for a crossing in which the over-all length is fixed, but where the intermediate piers can be placed as desired, the ratio of the weight of Type C to that of Type A varies from about 0.8 for structures under two thousand feet in length to about 0.65 for structures three thousand feet long. The method of determining this may best be illustrated by an example.

Given Class 70 live load and a total length of structure of 2500', we have from Fig. 12a for Type A.

and for Type C,

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