Page:Final Report - The Columbia River Interstate Bridge.pdf/32

 gates are manually operated and a small steel and concrete cabin is provided at each for the gate tender.

Several types of movable spans to provide the necessary channel openings for river navigation were considered, and the matter was narrowed to the selection of either a swing span or a vertical lift span. The Commissioners then desired to receive bids both upon the lift span and the swing span, and plans were prepared for the structure containing both types of movable span. The designs were made according to the same speciﬁcations and as nearly duplicate in character as possible.

Application was made to the War Department and tentative approval was received for either type of movable span, provided the swing span should have two channels, 200 ft. wide, or the lift span should provide one channel 250 ft. wide, with a vertical clearance of 150 ft. above normal high water. As has been stated, bids were received on both types of structure and it de— veloped that the lowest bid for the bridge, including a lift span, was about $70,000 less than the lowest bid for the bridge, includ— ing a swing span. The lift span was thereupon adopted; not only for the advantage of the lower ﬁrst cost, but also because, judging from the experiences of Multnomah County in operating bridges of both types, there will be continued advantages in lower cost of operation and maintenance. It is likewise more advantageous for river navigation, in that it provides a channel 250 ft. wide instead of 200 ft. wide, and the channel is not hampered by the long draw protection that would be necessary with a swing span. As the main channel is very close to the Vancouver shore, the necessary draw protection, about 500 ft. long, would have been a serious detriment to the dock frontage ad— jacent to the bridge.

The river at the site of the bridge is subject to a certain amount of scour, but inspection of the soundings taken for many years past did not disclose any indication of scour to extreme or unusual depths, although the river, both for some distance up stream and some three miles down stream, is deeper than at this site. The bases of the piers are of concrete enclosed by heavy timber cribs and extend down from 20 to 25 ft. below the river bed. The bases rest upon piles which extend to a depth about 110 ft. below low water. The piles used were about 120 ft. long in single sticks, about 10 ins. diameter at the tip and about 22 ins. at the butt. There is an average of about one pile to ten square feet area of base and in the average pier under the 265 ft. spans there are ninety piles.