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gantry cap.  The column may have the access ladders either on the inside or the
outside.  The lower portion of the gantry is in the form of two tapered boxes
flaring out from the gantry cap and with their longitudinal axes positioned in
line with the rails.  The box section construction makes these portal bases rigid
and provisions must be made in the travel truck system to maintain wheel contact
on uneven rails.  When required for crane stability, ballast in the form of
concrete or steel may be placed near the bottom of the portal base structure.
The width of the portal base opening is made as wide as the rail gauge
permits, and the height is not less than 22 feet.  Portal cranes are required to
traverse sharp curves, which impose large spreading or squeezing side loads and
bending moments on the portal base.  The older cranes with open riveted or bolted
construction require extensive leg-to-gantry cap reinforcement to withstand the
bending moments.
Gantries and Semi-Gantries.  Gantries and semi-gantries of cranes with
overrunning trolleys, depending on the size and vintage, may be constructed of
open structural sections or box section weldments bolted together.  The smaller
and older gantries are likely to be of the open structural section design; the
larger and newer, of the box section design.  Gantries of container cranes are
assembled from large box-section weldments, which have accessible interiors with
built-in ladders and platforms for inspection and repairs.
Floating Crane Tub Structures.  The upper portions of the tubs are
similar to those of portal crane bases.  They normally protrude 7 to 8 feet above
the barge deck to provide walking clearance and access to the roller path assembly
or the rotate bearing.  The tubs are built into the barge structure with extensive
gusseting to distribute the loads uniformly into the barge structure.
Portal Base, Gantry, and Tub Penetrations.  Openings, such as doors and
hatches, in these structures must be reinforced around their perimeter with
reinforcing rings (collars), structural frames, or doublers to
maintain the section modulus (and strength) of the cross section.  Any door or
hatch into a closed space must include positive means of opening from either side.
All exterior openings in horizontal surfaces must have coaming or raised
structural reinforcement to prevent entry of rainwater into the structure
Machinery Houses and Outdoor Operator's Cabs.  These structures are
designed with sheet metal panels over a structural frame.  Glazing is provided as
appropriate for entry of natural light and operator visibility.  Localized
structural reinforcements are added as required for specific purposes; such as
walkways, electrical resistor enclosures, diesel engine exhaust system, window
cleaning platforms, etc.  Machinery house walls of older cranes may incorporate
"soft patches" sections which can be easily cut out for equipment replacement
and then reinstalled.  Machinery house roofs may have access hatches above items
of equipment, which are expected to require periodic maintenance in the shop.
Machinery house roofs also require slots for the routing of hoists' wire ropes to
the A-frame.  All roof openings are provided with coaming to minimize the entry of
rainwater.  Additionally, wire rope slots are covered with rubber flaps, which are
pried open by the wire ropes as they move across them.


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