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INCREMENTAL
PUSH LAUNCHINGMETHOD
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Concept
The basic concept of INCREIMENTAL PUSH
LAUNCHING METHOD (ILM) is to prefabricate
approximately 10m to 30m long units of the bridge under
factory conditions behind an abutment and launch the units by
sliding them on Teflon bearings into final position without the
aid of scaffolding.
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The total deck slab of the bridge is subdividedlongitudinally into section of 10m to 30m length,depending upon the length of the span and time
available for construction.
Each section is concreted immediately behind theabutment of the bridge in steel framed forms.
These steel frames which remain in same placefor concreting all sections.
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The forms are designed in such a way that it can be moved
transversely or rotated on hinges to facilitate easy stripping
after sufficient hardening of the concrete.
The forms are hydraulically or mechanically activated to
ensure rapid stripping.
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This is the launching
equipment, which consists
of a combination of
hydraulic jacks acting
vertically and horizontally
on sliding bearings.
The sliding bearing consists
of concrete blocks covered
with stainless steel andTeflon-coated reinforced
elastometic pads.
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A section is manufactured in formwork in lengths
corresponding to one-third to one-half of the normal spanlength. After the stripping of the forms, tendons are inserted
and stressed.
The section is then jacked forward, sliding on the Teflon,
which, in turn, is affixed to stainless steel plates on top of the
piers.
A new section is then constructed immediately adjoining the
rear end of the first. Pre-stressing tendons are spliced so that
the two segments are joined as one longer pre-stressed unit and
then jacked forward another length.
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At the head of the first section, a steel nose consisting of alight truss is attached to facilitate reaching of the first and
subsequent piers without including a too large cantilever
moment during construction.
The second and following sections are concreted directly on
the face of the hardened portion and the longitudinal
reinforcement can continue across the construction joint.
The pushing is achieved by hydraulic jack which acts against
the abutment.
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Launching Process
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Low capacity hydraulic jack would be enough to movethe bridge decks.
Intermediate pier supports or towers may be used to
give temporary support across longer spans since the
practical limit of a span length for this type of
construction is about 60 meters.
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When launching from both ends of the bridge, differentcurvatures may be used, but this must be constant for each
launching point. Similarly, the super-elevation must be
constant.
After the launched bridge is in place, the bearing seats may be
jacked up to remove the Teflon, and the permanent bearings
are installed. Jacking points should have the required
supplemental reinforcing to permit this application of force ata location offset from the permanent bearings.
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Features of the incremental
launching method
Construction is carried out completely without falsework, so
that there is no problem in passing over obstacles below, such
as roads, railways, rivers, buildings areas.
The superstructure is made up of units of 15 to 25 m length,each completed in one week; there are no joints, since each
unit is concreted directly against the preceding one.
It is a economic construction technique.
To achieve quality work with reduced amount of labour.
It is a speedy construction technique.
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