460
DOCK ENGINEERING.
the roadway, and therefore of the bridge, is not rectangular with the axis
of the passages. This fact entails a greater length of bridge than would
othei wise be necessary. The varions details of construction will be readily
understood from the diagrams.
SECTION AT B B.
Fig. 457.—Swing Bridge at Calcutta.
Bolling Bridge at Greenock.*
A travelling bridge (fig. 458) on the same principle as a rolling
caisson, connecte the two sides of the entrance to the West Harbour
at Greenock. The bridge has this difference, however, that it is con-
structed in openwork so as to allow the tide to pass freely in and out
of the harbour.
The entrance is 103.) feet wide, and a bridge of this type was deemed
most suitable for the site, owing to the great depth (60 feet below H.W.)
to which it would have been necessary to go for a firm foundation for a
swing bridge, apart from the inconvenience attaching to the accommodation
of such a bridge upon a narrow quay. A timber gridiron, resting upon
piles driven into the hard clay, and having their heads encased in plastic
concrete, carries the rails (9 inches by 4 inches, solid section), which are laid
to a 16-foot gauge upon greenheart runners at a depth of 26 feet below H W.
The bridge structure consiste of three piers forming watertight tanks, each
18 feet by 18 feet, connected top and bottom by girders, 23 feet span.
On the underside of the lower girders, six rollers are fixed at each pier.
The bridge has a lowering deck similar to that already described (p. 360,
ante). The total cost, including the hauling machinery, was £9 700
* Kinipple on “ Greenock Harbour,” Min. Proc. Inst. C.E., vol. cxxx.