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At each terminal where there was to be a meeting of the marine operations
to the land operations, more specifically where the carfloat was docked to
be loaded/unloaded; a special and unique piece of railroad equipment was
used.
This was the carfloat apron, also known as a floatbridge or transfer bridge.
These bridges were usually built of steel, although some were built out of
wood, as in the Howe Truss design. Floatbridges either could be supported
by a pontoon, or suspended via an overhead gantry and cable system.
Pontoon floatbridges are fairly simple in construction. Most are around 100
feet long and look like a plate girder or small pony truss bridge. The land
end is attached to the shore on a hinge or "rocker plate", mounted on either
a wood or concrete base. Extending out into the water, the moring side or
outer end of the bridge rests on the pontoon, made of a riveted steel tank
( later pontoons were of welded construction ).
The pontoon was nothing more than a large steel box with nothing in it, but
with an access hatch or two. It essentially is an airtight box, and by nature
it uses buoyancy to keep the outer end up, and thus it rises and falls with
the tides.
On the left and right sides of the floatbridge, there are a set of hand operated
winches that are used to pull the carfloat close to the bridge and secure
it. Depending on the bridge manufacturer, there could be either one or two
winches.
Depending on the manufacturer of the bridge, there could be a pump inside
the pontoon, to flood or pump out water to help with getting the correct
height to mate with the carfloat deck. Most of the time, this was not installed
and the weight of the locomotive ( which would be advanced very slowly onto
the floatbridge ) would be used to lower the floatbridge to the height of
the carfloat deck.
Also depending on manufacturer of the bridge, a structure that looks similar
to an "A" frame, was placed between the left and right set of tracks; in
which a jack was mounted to also help with setting correct height between
the carfloat and the bridge. On the deck is the standard pair of rails with
the left set having a turnout (or switch) to access the center track of the
carfloat. Aprons of this type were used in Atlantic, Bush and Fulton Terminals,
as well as 65th Street ( Bay Ridge ) and the Brooklyn Navy Yard.
The floatbridges used by BEDT at their Pidgeon Street and Kent Avenue locations,
as are the Bush Terminal and New York Dock were also of pontoon type.
Differing from the pontoon style bridges, were the overhead suspension type
of floatbridge. Instead of being supported by the floating pontoon at the
end, the suspension type are held up out of the water by an overhead gantry
system of pulleys and cables.
There are two types of overhead suspension floatbridges: the "French" type
(named after it's designer) with only one section of bridge; and the "Double
Apron", which incorporated two hinged sections.
The easiest way to discern what type of overhead suspended floatbridge you are viewing, is to count the number of gantries:
- one gantry = French Type.
-
two gantries = Double Apron.
The "Double Apron" are more a complex type, and use a pair of hinged
floatbridges. The first or primary bridge extends out from the shore
approximately 80 feet, and is supported by the first gantry. The secondary
bridge, called the apron extends out another 30 feet and is supported via
cables to another gantry on the the water end. This type of floatbridge was
mastered by the PRR.
Double Apron:
The primary bridge is raised and lowered via a set of four threaded rods
and powered worm screws in the gantry house with help of counterweights.
This entire system is powered by electricity.
The secondary apron is also supported by counterweights, but utilizes friction
brakes to support it, and was not powered. There was also a dampening piston
between the two bridges to control upwards movement.
Both the bridge as well as the apron are supported by a "Live Load" system,
which uses counterweights to keep most of the weight supported by the
counterweights.
Notes about the Greenville Floatbridges:
-The main towers that support the gantrys sit on concrete foundations, some
of which then sit on stone blocks, which then sit on a wood substructure.
-There are three control rooms, between bridge 9 1/2-10, 11-12, 13-14.
-The worm screws are powered by a pair of 100HP GE motors in the main gantry,
which can lift 4.17 inches per minute with a 170,000lb load per screw.
This graphic shows all three types of floatbridges.
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