two layouts shown in the latter figure were next compared. It was found that, whereas, for the layouts of Fig. 30d, the bascule and the vertical lift were of equal cost for a clear height of about 170 feet, for the layouts of Fig. 30g they would. be equal for a clear height of 125 feet with

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rock foundations, and for one of 180 feet with deep-sand or pile foundations. In order to illustrate the use of the curves of Figs. 30b, 30e, and 30f, the following problems and their solutions are given:

Example No. 1

What are the quantities of superstructure materials in a bascule and in the corresponding vertical-lift span for a Class 60, double-track-railway bridge, the span-lengths being those in Fig. 30c, the clearances above high water being 20 feet with span down and 150 feet with span raised to full height, and the bascule span being permitted to encroach in one corner of the waterway clearance?