each comparison the total length of bridge considered is the same. The substructure was designed for several cases piers on rock at various depths, piers on deep-sand foundations, and piers on piles loaded to the limit of 30 tons each. Calculations were made for clear-channel widths of 100 feet, 150 feet, 200 feet, and 250 feet.

For the layouts shown on Fig. 30c, in which there is the same number of piers in the two cases, it was found that ordinarily the substructure costs nearly the same for the two types. In some instances the vertical-lift substructure was cheaper, while in others that of the bascule was a trifle more economic. The comparison for these layouts, consequently, is almost entirely a question of superstructure costs. For the layouts shown in Fig. 30d, the bascule substructure is always the more expensive, so that both substructure and superstructure must be considered.

Fig. 30e gives comparative costs for double-track-railway-bridges designed for Class 60 loading. The following unit prices were used:

The lower group of curves, for layouts such as shown in Fig. 30c, gives superstructure costs only; while the upper group, for layouts such as indicated in Fig. 30d, records the total cost of superstructure and substructure, with the piers resting on piles. The comparison for the latter substructure condition gives average results, and was therefore adopted. For other types of substructure the relative costs differ but slightly, excepting that the bascule is considerably more costly with deep or expensive foundations. The full lines for the bascule costs, noted "Clear Height 50 ft. or less," apply for greater heights when an encroachment on the corner of the clearance diagram is permitted.

Fig. 30f gives, for various clear-channel widths, the vertical clearances at which the vertical-lift bridge will just equal the bascule in cost. This is shown for layouts both with and without long flanking-spans. The two full lines apply for clear heights up to 50 feet, and for greater heights when an encroachment on the corner of the clearance diagram is allowable; while the two dotted lines are for greater heights with no encroachment permitted.

In working up the curves of Figs. 30d and 30e, it was assumed that the distance from center to center of piers exceeds the clear channel by from 20 to 30 feet, and that the clearance above the water line is 20 feet when the moving span is down. With a smaller down-clearance than this, the clear heights at which the types are equal will be reduced; and with a greater down-clearance it will be increased.