The Republican. [volume] (Oakland, Md.) 1877-current, 1883-12-29
Cantlleyer Bridges.

The New Y DIRK Central and the Canada
Southern Railroads, will be the first
bridge of that style built in America "
"That is true" was the reply. "But
first you ask me what cantilever is.
We borrow the term from architecture;
it means a bracket used to support
balcony, etc. Now, in cantilever
bridge there are two brackets opposite
each other, each resting on a pier. The
shore ends of the brackets rest upon
and are fastened down to another pier;
the outer ends, projecting over the
water, sustain a central span, merely
resting upon them and free to respond
to the changes in the temperature. To
get rude idea of the principle of the
cantilever bridge, place two chairs two
feet apart; put foot-rule on each chair,
held down by book; the ends of the
rules will not meet across the space
separating the two chairs, but by rest
ing third rule upon their ends the
bridge is complete. This is skeleton
idea. Now, in place of the chairs sub
stitute four piers of masonry; instead of
the two rules, balance upon the piers
two beams resting upon the intermedi-
ate piers and with the shore ends of the
beams placed upon and fastened to the
two extreme piers; another span resting
upon their outer ends substituted for
the third rule, and this is the cantilevey
bridge.
"The shore arm of the cantilever or
bracket rests, as / have said, upon a
second pier, built back of the first one
at the rivers brink. If there was no
such support any weight-like that of
a railroad train-if placed upon the
shore arm, would depress it toward the
ground and correspondence elevate the
outer OF stream end 1t would be as
if, after perfectly balancing a plank on
log, you should place a weight at
one end of the plank But this is not
enough. Suppose the railroad train
had passed under the tower over the
main pier out upon the outer end of the
cantilever or balanced beam; the extra
weight would depress the outer end and
at the same time elevate the inner or
shore end. So the shore end not only
rests upon a support of masonry, but is
fastened to it, to prevent the train from
tipping the beam up into the air and
falling into the stream"
"What are the advantages claimed for
the cantilever bridge?" asked reporter.
''For bridges having long spans the
system is asserted to be the most eco
nomical. Less material need be used
to secure the same strength, rigidity and
stiffness than in a suspension bridge, for
instance. Less false work or scaffold-
ing is needed for the erection of can
tilever bridge than for any other braced
structure, thereby making it capable of
construction with more ease and less
danger of disaster and with no obstruc-
ton to navigation. The central span
resting upon the outer or stream ends
of the two brackets can freely expand
with the heat of summer and contract
with the cold weather. One end of
this central span is fastened to one of
the brackets, the other end being
placed upon rollers. The detection
from the horizontal under the full load
capacity of the bridge is not half so
great as it would be in suspension
bridge of the same length of span.
"By explaining, in general way,
the process of constructing a bracket
or cantilever bridge, you will be able to
see the economy and ease of the work.
We will start with one iron tower built
upon a pier of masonry, on the east
shore of a stream. The first panel of
the bridge is swung into place for the
shore arm of the bracket and fastened
to the iron tower. A timber support
for each panel as it is put into place is
all the false work required in building
the shore end. AS soon as two of
these panels are built, by means of q
derrick the first panel of the stream
arm can be tied to the tower. The ex-
ira weight of the two panels for the
shore end of the bracket will fully sus-
tain the single panel that is built out in
the opposite direction over the water.
With this done, that panel itself sup
plies the false work from which the
next one can be placed in position and
bolted to the first panel. In this way
panel by panel can be advanced until
the outer or stream arm of the canti-
lever is completed. Meantime, by
keeping the work on the shore arm in
advance of that on the outer arm, the
weight of the latter will not overbaf-
ance the former, which, when COM
pleted, can be placed upon the second
pier and anchored. On the west (the
opposite) shore of the stream, mean
time, the other bracket or cantilever is
being constructed in the same way.
Roughly to illustrate the idea, take a
sheet of paper and draw two rs
on the same horizontal line,
leaving little space between them.
The two perpendicular strokes repre-
sent the piers on the respective shores
of the stream. The two horizontal
strokes on the top of the perpendicular
lines represent the two cantileyers or
brackets. Make two downward per-
pendicular strokes, one from the right
hand end and the other from the left
hand end of the tops of the right-hand
and the left-hand L respectively; these
represent the anchorage of the shore
ends of the cantievers. Now your
bridge is complete, except the placing
OF thccentral span-to pursue our IL
lustration, the drawing of line to con-
meet the two free ends of the Us. This
is done by running a wooden truss into
the intervening space q certain dis
lance; by means of derrick fastening
cables to the projecting ends of the
truss from the end of the opposite can
tilever; then, by pushing and pulling,
the fruqs 10 ornftat,