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 MIL-HDBK-1013/10
laterally by additional vertical rollers. Since wheel-supported gates are not
cantilevered, they only need to be somewhat longer than the actual size of the
gate opening, requiring one-third less straight and level storage length along
the fence line than the cantilever gate. Wheel-supported gates essentially
are not limited in the size of their opening except for the power requirements
of the gate operator. A variation of the wheel-supported gate is one using
dual pneumatic, hard rubber, or steel wheels to support the gate. These can
be identified by their lack of roadbed guides. They should only be used as a
last resort, and then only for manually opened sliding gates.
3.4.1.2.2
Overhead Gates. Overhead gate design requires either an I-beam or
an enclosed track (Figures 17 and 18), suspended over the width of the opening
and extending an equidistance on one side or the other of the gate opening to
store the gate when it is opened. Similar to wheel-supported and cantilever
gates, the gate must store parallel to the adjacent fence line. The gate
storage area must be in line with the gate opening and either be level or have
a decreasing grade to accommodate the gate when it is fully open. The gate is
suspended from the I-beam or enclosed track by a pair of rollers attached to
posts extending upward from the leading and trailing edge of the gate. The
overhead beam or track height must allow clearance for anticipated truck or
rail traffic. The gate will be suspended above the ground from the overhead
beam or track and supported laterally near the ground by vertical rollers.
The enclosed track design incorporates the best of the overhead gate designs,
and is well suited for automatic operators. The tracking system provides the
convenience of a wheel-supported or cantilever-type installation, but with a
much more efficient means to roll the gate. The amount of force to operate
gates with these designs is significantly less than that of comparable gates.
High-cycle demands, large opening sizes, or heavy gate construction may
require strengthening the overhead I-beam design. Additional upright posts
extending upward from the center of the gate will also allow the enclosed
track to carry heavier loads.
Swing Gates. Swing gates (Figures 12 and 13) should be designed
3.4.1.2.3
so that they swing inward, toward the secured area. The disadvantage of the
swing gate is the large arc of space required for operation. Swing gates can
either be designed to swing 90 degrees inward and 90 degrees outward or swing
180 degrees inward only. An important consideration in selecting a single or
double swing gate design is maintaining clearance along the bottom of the gate
as it swings through its arc from the closed to the open position. If the
grade is increasing inside of the gate, grading will be required to allow
clearance. The required 2-inch (50.8-mm) maximum clearance between the bottom
of the fence and the roadbed must be maintained when the gate is in the closed
position.
The swing gate design places considerable weight on the hinge post and
its foundation. The longer the gate, the more load (moment arm) placed on the
gate post. ASTM F 900 provides design detail for single swing gates up to 24
feet (7.3 m). Ideally, single swing gates should not exceed 14 feet (4.3 m)
and double swing gates should not exceed 28 feet (8.5 m). Since the weight of
30
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