SEGMENTAL BRIDGE CONSTRUCTION

SEGMENTAL BRIDGE CONSTRUCTIONPresented By:FAISAL V AKARTHIK V 1

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INTRODUCTION In contrast to classical monolithic structures a segmental bridge consists of small pieces stressed together by external tendons. Segmental construction is gaining popularity as a method of construction where the local terrain and/or traffic prohibit the erection of conventional false work .

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Why go for segmental construction?Segmental construction technique facilitates:Construction of longer span bridgesReduces intermediate piers and thus obstruction to flow and avoid system of erection of staging from river bed levelVery effective and economical technique in such situations.3

History

This method of const. was first developed in western Europe particularly in France & Germany during 1950s.

Over 70 bridges of this type where const. during 1971-75 in France.

In India during 1960-61,@ 250m long Barak bridge was constructed.

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Construction site of segmental bridges

CONSTRUCTION TECHNIQUES Delicate process & needs fine balance Any Imbalance leads to damages, so needs careful & systematic approach in casting of segments. For that proper flow chart of the activities & sequence are there for to drawn. Launching & construction sequence differ according to construction tech., & it depends on the construction site, geological studies etc. The techniques can broadly be classified as under: : Cantilever Method for Cast-in-situ Construction Incremental Launching Method. Construction with Precast Segments.

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CAST IN SITU TECHNIQUES To cast the segments in their final position on the structure Commonly used method for casting segments in place is with the use of form travellers. Form travellers are moveable forms supported by steel cantilever trusses attached to previously completed segments. When balanced cantilever erection is used, a minimum of two form travellers is required. Form travellers can be employed during cantileveredconcretegirder construction when a bridge pier can be used as a starting point.7

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Form Traveller Operation :Form travellers are powered by a hydraulic system, requiring the crews that operate them to receive specific training. The structural framework of the form traveller moves forward on rails anchored to the previously constructed span segments. The individual steel structural member of each form traveller are connected with pins to facilitate mobilization/demobilization.

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Form travellers

Alignment variations and corrections are more easily accommodated in cast-in-place construction. Increase in alignment corrections for cast-in-place construction compared to precast construction relates directly to the age of the concrete when loaded.Generally, the concrete is much younger when loaded in cast-in-place construction. The deformations due to shrinkage and creep vary logarithmically with the age of the concrete, with the values of the deformations decreasing as the age at loading increases.10

Cantilever Method for Cast-in-situ ConstructionCONSTRUCTION METHODOLOGY: Pier head is cast with detailed planning. A steel gantry is then designed & fabricated. Thereafter, soffit shuttering, shuttering for web & deck shuttering is fixed to the overhanging portion of the gantry on both sides. Reinforcement is then tied as per drawing. Then concreting is done on both sides of the gantry11

ERECTION METHODS Balanced cantilever: Balanced cantilever erection is quickly becoming the "classic" technique when considering segmental construction. The general concept is to attach the segments in an alternate manner at opposite ends of cantilevers supported by piers.The construction of loads should not increase significantly over what has been assumed in the design. 12continued

As segments are attached to the cantilever ends by single one at a time, an overturning moment is created and should be resisted

Moment might be resisted by post-tensioning the pier segment down to the pier stem, providing temporary supports on each side of the pier or stabilizing the cantilevers with the erection equipment.

A Balanced cantilever segment

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Cantilever Method for Cast-in-situ Construction

General Schedule :Construction of one unit each on both sides of pier head - 7 days . Erection of gantry - 10 days. Casting of each subsequent cantilever units (including prestressing of cables to be anchored at that units) - 7 daysSpecial Consideration during Construction :Segment need proper anchor with pier cap during whole construction

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Balanced cantilever16

The launching gantry is a machine capable of transporting a segment from a completed portion of the bridge or from below the bridge to either end of the cantilevers being erected.

Advantageous when accessibility to the area beneath the structure is restricted by environmental consideration

New bridges can be erected over existing traffic and/or buildings with minimal disturbance.

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LAUNCHING GIRDERS

PROGRESSIVE PLACING:

modification of the balanced cantilever concept.Progressive placing erects cantilevers in only one direction.The capacity of the section located at the pier is exceeded.Temporary support must be provided to prevent overstress.A system of temporary bents may be provided beneath the structure.

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Progressive placing :

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INCREMENTAL LAUNCHINGOne of the highly mechanized erection methods used in construction of bridges.Consists of manufacturing the superstructure of a bridge by sections in a prefabrication area behind one of the abutments.Each new unit is concreted directly against the preceding one .After it has hardened the resultant structure is moved forward by the length of one unit.

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PRECONDITIONS FOR THE USE OF THIS METHOD:

Used for straight bridges or where the superstructure has a spatial curve of fixed radius all over the length.The superstructure must consist of a beam of fixed section, for which the slenderness ratio ( that is the span-to-depth ratio) is not more than 17 when finished.The most suitable cross-sections are the single-cell box section or the double T-beam.A sufficient area of suitable load bearing ground must be available behind one abutment for the construction yard.

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FEATURES OF THE INCREMENTAL LAUNCHING METHOD

Construction is done completely without false work, so that there is no problem in passing over obstacles below.The fabrication yard is stationary and located behind one abutment, which makes accurate construction possible.During the construction stage the superstructure is centrally pre-stressed, to limit the tensile stresses produced by the bending moments.A lightweight nose is attached to the cantilever end of the superstructure to reduce the cantilever moment during launching.

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The bridge supports are equipped with special sliding bearings.Auxiliary supports may be incorporated between the piers for long spans and/or where the span/depth ratio is high.

Incremental launching method

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CONSTRUCTION OF BRIDGE BY INCREMENTAL LAUNCHING

Progressive placing v/s Incremental launching Progressive placing:A one-directional placement process where bridge segments are placed in sequence, extending out over supports as an ever increasing cantilever, until the next support is reached and then jacked-up into position.a temporary support must be provided to prevent overstress.

Incremental launchingInvolves the casting of a continuous chain of bridge segments on-site adjacent to the actual location of the bridge and then pushing the growing superstructure out over temporary and permanent supports at the bridge's locationTemporary support bents may or may not be required at mid spans depending on the span length.26

Span by Span construction Most economical technique for erecting segmental bridges in the medium span range.It offers a very high speed of construction.Used in conjunction with an erection truss under the bridge segments or an overhead erection gantry to guide the precast elements into position

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Span by span construction

PRE CASTING TECHNIQUES

Short line systemAlmost all the methods casting utilize the methods of match casting. Basic premise of match casting is to cast the segments so their relative erected position is identical to their relative casting position.Segments are then erected in the same sequence they were cast.Stringent control of geometry and successive correction of errors are required to ensure that the geometry of the bridge is maintained.Tolerances are in the order of a fraction of a millimeter and any deviation in excess of this will result in misalignment of the bridge.29

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Reinforcement of section

Typical precast segment

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Match casting using Short line method

Construction Methodology

Segments are casted in the casting yard.Launching girder is used for erection of precast segments. The launching girder has two parallel triangular trusses provided with hangers to hold the segments during erection.32

CONCLUSIONWith the introduction of in-situ segmental construction, the problem of constructing longer span over rivers, creeks as well as gorges was successfully overcome. Precast prestressed segmental construction is a versatile technique for construction of present day fast track jobs. Segments can be cast away from actual site, their by minimizing hindrance to traffic & public in urban environment. In casting yard better control on quality & dimensional tolerances can be achieved. Segment casting can start independently as work on foundations progresses, their by reducing overall completion time.33

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