While superficially similar, Single girder beam launcher should not
be confused with movable scaffolding systems, which also are used
in segmental bridge construction. Both feature long girders
spanning multiple bridge spans which move with the work, but
launching gantry machines are used to lift and support precast
bridge segments and bridge girders, while movable scaffolding
systems are used for cast-in-place construction of bridge
segments.Typically, precast segmental bridges and precast girders
are placed using ground-based cranes to lift each segment or
girder. However, ground access to the spans may be challenged by
the presence of existing infrastructure or bodies of water or the
height to which the segments must be raised can exceed the reach of
ground-based cranes. A launching gantry can be used to solve these
issues.The most visible feature of a launching gantry are the twin
parallel girders which can either be above (upper-beam) or below
(lower-beam or underslung) the bridge deck. However, a single beam
can also be used, typically in upper-beam configuration. The
launching gantry machine usually is sized to the construction
project, with the length of the twin main girders approximately 2.3
times the distance between spans. This length enables the launching
gantry to span the gap between two adjacent bridge piers while
providing allowances for the distance required for launching to the
next span and flexibility of movement to accommodate curved paths
between piers. In some cases, hinges have been inserted into the
gantry girders to allow tighter curves. The launching gantry
girders are supported at each pier by braced frames which have a
limited range of movement to facilitate placement of bridge
segments or bridge girders; the launching gantry does not generally
contact the bridge deck.Two gantry trolleys can run the full length
of the launching gantry girders. Each trolley is equipped with two
winches: a main winch to suspend the load, and a translation winch
to move the trolley along the girders. When bridge segments (or
bridge girders) are delivered at the ground level, the launching
gantry is used to pick them up and raise them to deck or pier
height. If the segments (or girders) are delivered instead at the
bridge deck level, the launching gantry moves back to allow the
forward trolley to pick up the front end of the next segment (or
girder), while the back end of the segment (or girder) is supported
by the transportation vehicle; as the forward trolley moves
forward, the rear trolley takes over supporting the back end from
the vehicle.Bridge segments (or bridge girders) are set in place by
the launching gantry until the span between adjacent piers is
completed. For segmental bridges, typically a span-by-span or
balanced-cantilever approach is adopted to place segments. To free
up the gantry trolley(s), temporary hangers are used to support
each segment after it has been placed. In the span-by-span
approach, all the segments for a span are placed before bridge
tendons are tensioned; in this fashion, work progresses from one
pier towards an adjacent pier. In the balanced-cantilever approach,
segments are placed simultaneously on each side and work progresses
from a central pier towards the two nearest piers instead. In
either case, the launching gantry girders and hangers essentially
serve as false work prior to tensioning.Once the bridge span
between adjacent piers is completed, the winches on the trolleys
are used to lift the gantry girders and "launch" them ahead to the
next span. The process of lifting and placing bridge segments (or
girders) followed by launching the gantry girders ahead is repeated
until the bridge is complete.Launching gantries are often
distinguished by the design of the main girders. Honeycomb
Girder:The honeycomb girder launching gantry has been proven
suitable to a lifting range of 5 to 300t.The main girder of a
honeycomb girder is fabricated from welded plates, forming an
isosceles triangle cross-section. Regular hexagonal holes are cut
in the inclined web plates to reduce wind resistance. Because the
honeycomb girder is formed by relatively long seam welds joining
plates, the welds will not lose integrity easily due to small
welding defects.Truss Girder:The main girder of a truss girder is
fabricated from welded steel poles. The welds in a truss girder are
point-to-point (joining steel poles to other poles), which requires
more stringent welding technology and technique. With small
self-weight and large deflection, the truss girder launching gantry
is suitable for highway bridge construction projects.Box Girder:The
main girder of a box girder also is fabricated from welded plates.
This type of girder is usually used in high-speed railway bridge
construction
projects.------------------------------------------------------------Design
and manufacturing
features---------------------------------------------------------------The
Beam Launcher has many advantages, suc
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