The Railways of America1890
THE most notable invention of latter days in bridge construction
is that of the cantilever bridge, which is a system devised to
dispense with staging, or false works, where from the great depth,
or the swift current, of the river, this would be difficult, or,
as in the case of the Niagara River, impossible to make. The word
cantilever is used in architecture to signify the lower end of
a rafter, which projects beyond the wall of a building, and supports
the roof above. It is from an Italian word, taken from the Latin
cantilabrum (used by Vitruvius), meaning the 1ip of
the rafter. If two beams were pushed out from the shores of
a stream until they met in the centre, and these two beams were
long enough to run back from the shores until their weight, aided
by a few stones, held them down, we should have a primitive form
of the cantilever, but one which in principle would not differ
from the actual cantilever bridges. This is another American invention,
although it has been developed by British engineers-Messrs. Fowler
& Baker-in their huge bridge now building across the Forth,
in Scotland, of a size which dwarfs everything hitherto done in
this country, the Brooklyn bridge not excepted.
The first design of which we have any record was that of a
bridge planned by Thomas Pope, a ship carpenter of New York, who,
in 1810, published a book giving his designs for an arched bridge
of timber across the North River at. Castle Point, of 2,400 feet
span. Mr. Pope called this an arch, but his description clearly
shows it to have been what we now call a cantilever. As was the
fashion of the day, he indulged in a poetical description:
"Like half a Rainbow rising on yon shore,
While its twin partner spans the semi o'er,
And makes a perfect whole that need not part
Till time has furnish'd us a nobler art."
The first railway cantilever
bridge in the world was built by the late C. Shaler Smith, C.E.,
one of our most accomplished bridge engineers. This was a bridge
over the deep gorge of the Kentucky River. The next was a bridge
on the Canadian Pacific, in British Columbia, designed by C. C.
Schneider, C.E. A very similar bridge is that over the Niagara
River, designed by the same engineer in conjunction with Messrs.
Field & Hayes, Civil Engineers. This bridge was the first
to receive the distinctive name of cantilever.
The new bridge at Poughkeepsie has three of these cantilevers,
connected by, two fixed spans, as shown in the illustration. The
fixed spans have horizontal lower chords, and really extend beyond
each pier and up the inclined portions, to where the bottom
chord of the cantilever is horizontal. At these points the
junctions between the spans are made, and arranged in such a way,
by means of movable links, that expansion and contraction due
to changes of temperature can take place. The fixed spans are
525 feet long. Their upper chord, where the tracks are placed,
is 212 feet above water. These spans required stagings to build
them upon. These stagings were 220 feet above water, and rested
on piles, driven through 60 feet of water and 60 feet of mud,
making the whole height of the temporary staging 332 feet, or
within 30 feet of the height of Trinity Church steeple, in New
York. The time occupied in building one of these stagings and
then erecting the steel-work upon it was about four months.
The cantilever spans were erected,
as shown in the illustration, without any stagings at all below,
and entirely from the two overhead travelling scaffolds, shown
in the engraving. These scaffolds were moved out daily from the
place of beginning over the piers, until they met in the centre.
The workmen hoisted up the different pieces of steel from a barge
in the river below and put them into place, using suspended planks
to walk upon. The time saved by this method was so great that
one of these spans of 548 feet long was erected in less than four
weeks, or one-seventh of the time which would have been required
if stagings had been used.
At the Forth Bridge, all the projecting cantilevers will be
built from overhead scaffolds, 360 feet above the water. It contains
two spans of 1,710 feet each. When spans of this length are used,
the rivets become very longseven inchesand it would
be impossible to make a good job by hand riveting. Hence a power-riveter
is used in riveting the work upon the staging. A steam engine
raises up a heavy mass of cast-iron, called "the accumulator;"
the weight of this in descending is transmitted through tubes
of water, and its power increased by contracting the area of pressure,
until some twenty tons can be applied to the head of each rivet.
One rivet per minute can be put in with this tool.
It will be seen that most of the great saving of time in modern
construction of bridges and other parts of railways is due to
improved machinery. The engineer of to-day is probably not more
skillful than his ancestor, who, in periwig and cue, breeches
and silk stockings, is represented in old prints supervising a
gang of laborers, who slowly lift the ram of a pile-driver by
hauling on one end of a rope passed over a pulley-wheel. The modern
engineer has that useful servant, steam, and the history of modern
engineering is chiefly the history of those inventions by which
steam has been able to supersede manual laborsuch as pile-drivers,
steam-shovels, steam-dredges, and other similar tools.
Poughkeepsie Bridge | Bridge
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