i
456
DOCK ENGINEERING.
drawbridge. It is in two leaves and has two decks. The upper carries the
electric railway, the lower carries a double line of rails provided for the
dock traffic. The lower level is arranged with bascule leaves, so that barges
and small craft can use the
passage without the neces-
sity of swinging the whole
structure and interrupting
the railway service which
is a very frequent one.
The bridge is, in its normal
condition, a fixed struc-
ture, resting upon bearing
blocks at the tail end and
l^igs. 447 and 448. —Swing Bridge at Liverpool.
upon two legs at the front
of each abutment. To
enable the bridge to be
completely opened the fol-
lowing movements have
to be made. The tail end
of each leaf is slightly
lifted to allow the bearing
blocks to be withdrawn,
and then it is lowered
until it rests upon the
roller path. In accom-
plishing this, the pivot of
the bridge cornes in con-
tact with its socket, the
girders are canted upward
at the nose end, the inter-
mediate supporting legs
are lifted off their bear-
has a diameter of 12 feet, and is turned by a
of the combined structure is 600 tons.
ings, and the bridge is
ready for swinging. The
load on the pivot is 270
tons. The length of the
bridge between pivot cen-
tres is 80 feet 6 inches,
and 114 feet 6 inches is
the extreme length. The
width of way between the
longitudinal girders is 21
feet. The slewing drum
lf-inch chain. The weight