408 DOCK ENGINEERING.
bridge, being both horizontally behind and vertically below it. At the
instant of commencing to open the bridge, the moment of the counterbalance
is slightly in excess of the
moment of the bridge, thus
assisting it to rise. The excess
continues until the centre of
expose to wind pressure. The lev
Fig. 393. — Bascule Bridge.
Sectional Elevation at outside plummer-block.
ion along centre line of Bridge.
gravity of the bridge cornes
vertically over the axis, at
which stage the line of chain
also intersects it, producing
equilibrium. As the bridge
continues its rotation the con-
trary effect is set up, the
moment of the bridge tend-
ing to increase its travel, while
the moment of the counter-
poise acts as a check. In
closing the bridge the aetion
is the same, but in reversed
order.
Bascule bridges are usually
in two leaves, meeting at the
centre of span. The under
side of each leaf is then per-
fectly curved in form, or is
provided with raking struts,
fitting into pockets or recesses
in the side walls when the
bridge is lowered. This type
of bridge forms an arch, and,
accordingly, it derives very
considérable support from the
mutual abutments at its centre
and the skewbacks at the
sides. These parts can be
adjusted to a nicety which is
not realisable in the case of
other types.
The main objections to the
employment of bascules are
their liability to corne in con-
tact with the yards and spars
of passing vessels, and also the
very large surface which they
erage exerted by the wind materially