Example No. 2

In the preceding example, what would be the comparative costs of the two superstructures, using the unit prices for materials in place which were employed in working up Fig. 30e.

Entering the lower group of curves of Fig. 30e with a 150-foot-clear height for the vertical lift, we find the cost to be $270,000; while entering it with a 150-foot channel and a clear height of "50 feet or less" for the bascule (because encroachment on one corner of the waterway clearance is permitted), we read $272,000 as the cost of the bascule. The margin in favor of the vertical lift is, therefore, given as $2,000 by Fig. 30e, and as $6,200 by the figures derived from the more accurate curves of Fig. 30b, which is a satisfactory check.

Example No. 3

What are the comparative costs of a double-track-railway, Class 60, bascule and the corresponding vertical lift for a 180-foot clear-opening flanked by fixed truss spans, the clear height required being 135 feet when the span is raised and 15 feet when the span is down, the bascule being permitted to encroach on one corner of the waterway clearance.

For the vertical-lift span, we must enter the upper group of curves in Fig. 30e with a "clear height" of 135-15+20 = 140 feet; for that diagram is plotted  for  a  clear  height of 20 feet with span down.  For a 180-foot