Incremental & Cantilever Method(24.03.10)

7/27/2019 Incremental & Cantilever Method(24.03.10)

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I NCREMENAL PUSH LAUNCHING &

CANTI LEVER CONSTRUCTION METHOD


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INCREMENTAL PUSH LAUNCHING METHOD


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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.

The total deck slab of the bridge is subdivided longitudinally intosection of 10m to 30m length, depending upon the length of the spanand time available for construction.

Each section is concreted immediately behind the abutment ofthe bridge in steel framed forms. These steel frames which remain insame place for concreting all sections.


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http://localhost/var/www/apps/conversion/tmp/scratch_12/Holland-Flyover.html


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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 onsliding bearings.

The sliding bearing consists of

concrete blocks covered with

stainless steel and Teflon-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 span length. 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 thefirst. 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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Launching Process


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Low capacity hydraulic jack would be enough to move the 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.

When launching from both ends of the bridge, different curvatures 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 at a 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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CANTILEVER CONSTRUCTION METHOD


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CONCEPT OF CANTILEVER

CONSTRUCTION METHOD


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SEGMENTAL CANTILEVER CONSTRUCTION

The Economy of continuous construction method is enhanced by the

adoption of the segmental cantilever method by M/s.Dyckehof &

Widman in Germany.

This method is particularly economical where construction of

superstructure using staging from the river bed is not feasible.

The segmental cantilever construction may adopt cast-in-situ place

segments or precast segments, and is considered suitable for the span

range of 50 to 200m, though a few bridges have been built beyond

200m Span.


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The box section is the best suited cross section for cantilever

construction, in view of the torsional rigidity and the availability of the

top and bottom slab portion for accommodating the tendons.

The single cell box is used in many bridge construction in India for

deck width less than 13m.

The Two webs may be vertical or sloping. Box sections with two or

three cells are used for wider decks.


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Types Cast-in-Place Segments

Precast Segments


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CAST-IN-PLACE SEGMENTS

The Structure is built from a pier or support across the open span

without the temporary support in sections of about 3.5 m length. The

construction should proceed on both sides of piers such that at any

stage ,the unbalanced moment kept minimum.

Each section is cast in the cantilever form traveler(CFT) which is

attached to the end o constructed portion. When the concrete attains

specified strength, the segment is pressed against the previous one by

means of prestressing tendons ending at the front of the section.

Other prestressing tendons are carried through unstressed at a lengthcorresponding to the static requirements of the construction. At each

section, the tendons ending there may be bonded by grouting the duct,

so that at every stage the construction is safe against rupture.

This process is continued till mid span


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PRECAST SEGMENTS

Long span prestressed concrete bridge can also be built using precast

segments. The segments, precast in a nearby casting yard are about

3.5m long covering the full width of bridge.

Each segment is transported by a trolley system from the plant to the

launching truss, from where picked up a traveling frame hoist

suspended from the bottom chord of the truss.

The traveling frame has a spreader beam which can be raised, loweredor rotated so that wide section can pass between the supporting legs of

launching truss and can be lowered into the proper position.


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The section is then fastened in place with epoxy and prestressing

tendons.

One problem with the use of precast segments is in joining segmentsprior to stressing. In order to ensure precise matching of the mating

faces, each segment is cast directly against the face of previous

segment.

This process is called as Match-Casting.


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ADVANTAGES

Centering and false work are avoided, enabling construction of

structures with tall piers and over deep valleys.

The speed of construction is enhanced, typically at 1 m/day/CFT for

cast-in-place construction and possibly 3m/day with the use ofprefabricated segments.

Enhanced levels of quality and workmanship or facilitated due to

mechanization repetitive tasks.

The cost of construction permits competition with alternative design of

a steel structure of long span.

Creep in concrete is less due to age at time of initial loading.


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Incremental & Cantilever Method(24.03.10)