INCREMENTAL PUSH LAUNCHING METHOD .
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 into section of 10m to 30m length. depending upon the length of the span and time available for construction.These steel frames which remain in same place for concreting all sections. . Each section is concreted immediately behind the abutment of the bridge in steel framed forms.
• 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. .
.• 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 and Teflon-coated reinforced elastometic pads.
Pre-stressing tendons are spliced so that the two segments are joined as one longer pre-stressed unit and then jacked forward another length. After the stripping of the forms. is affixed to stainless steel plates on top of the piers. . in turn. • The section is then jacked forward. tendons are inserted and stressed. sliding on the Teflon.• A section is manufactured in formwork in lengths corresponding to onethird to one-half of the normal span length. which. • A new section is then constructed immediately adjoining the rear end of the first.
• The pushing is achieved by hydraulic jack which acts against the abutment. . a steel nose consisting of a light 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.• At the head of the first section.
the superelevation must be constant. the bearing seats may be jacked up to remove the Teflon.• Low capacity hydraulic jack would be enough to move the bridge decks. but this must be constant for each launching point. and the permanent bearings are installed. • After the launched bridge is in place. Similarly. different curvatures may be used. • When launching from both ends of the bridge. • 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. Jacking points should have the required supplemental reinforcing to permit this application of force at a location offset from the permanent bearings. .
• The superstructure is made up of units of 15 to 25 m length. • It is a speedy construction technique. • To achieve quality work with reduced amount of labour. since each unit is concreted directly against the preceding one. • It is a economic construction technique. rivers. each completed in one week. railways. so that there is no problem in passing over obstacles below.FEATURES OF THE INCREMENTAL LAUNCHING METHOD • Construction is carried out completely without falsework. . such as roads. buildings areas. there are no joints.
CONCEPT OF CANTILEVER CONSTRUCTION METHOD .
and is considered suitable for the span range of 50 to 200m. • The segmental cantilever construction may adopt cast-in-situ place segments or precast segments.SEGMENTAL CANTILEVER CONSTRUCTION • The Economy of continuous construction method is enhanced by the adoption of the segmental cantilever method by M/s. • This method is particularly economical where construction of superstructure using staging from the river bed is not feasible. .Dyckehof & Widman in Germany. though a few bridges have been built beyond 200m Span.
• The Two webs may be vertical or sloping. in view of the torsional rigidity and the availability of the top and bottom slab portion for accommodating the tendons.• The box section is the best suited cross section for cantilever construction. • The single cell box is used in many bridge construction in India for deck width less than 13m. Box sections with two or three cells are used for wider decks. .
• Types • Cast-in-Place Segments • Precast Segments .
• Other prestressing tendons are carried through unstressed at a length corresponding to the static requirements of the construction. so that at every stage the construction is safe against rupture. • This process is continued till mid span . When the concrete attains specified strength.the unbalanced moment kept minimum. • Each section is cast in the cantilever form traveler(CFT) which is attached to the end o constructed portion.5 m length. At each section. The construction should proceed on both sides of piers such that at any stage . the segment is pressed against the previous one by means of prestressing tendons ending at the front of the section. the tendons ending there may be bonded by grouting the duct.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.
lowered or rotated so that wide section can pass between the supporting legs of launching truss and can be lowered into the proper position. • The traveling frame has a spreader beam which can be raised. . from where picked up a traveling frame hoist suspended from the bottom chord of the truss. 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.PRECAST SEGMENTS • Long span prestressed concrete bridge can also be built using precast segments.
• The section is then fastened in place with epoxy and prestressing tendons. . • One problem with the use of precast segments is in joining segments prior to stressing. each segment is cast directly against the face of previous segment. • This process is called as Match-Casting. In order to ensure precise matching of the mating faces.
typically at 1 m/day/CFT for cast-in-place construction and possibly 3m/day with the use of prefabricated segments. • Creep in concrete is less due to age at time of initial loading. enabling construction of structures with tall piers and over deep valleys.ADVANTAGES • Centering and false work are avoided. • 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. . • The speed of construction is enhanced.