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Figure 20 Typical casting sequence of SLMC for SSMC In the sequence of casting (Fig 20) soffit formwork is aligned as per required geometry which is preceded by adjusting the fixing of the external form After this reinforcement cage is kept in the mould and the internal shuttering is fixed, the mould is closed with stop end form After casting the segment, it is shifted to match cast platform with help of hydraulically operated trolley Then this segment will be used as match cast segment for casting next segment After casting next segment, this segment is shifted to the stacking yard 3.2.2 SLMC for CMC SLMC for BCM (a variation of CMC) was very successfully adopted for Narmada Bridge at Zadeshwar (Fig 21) in Gujarat in 1971 for the first time in India Casting yards were setup on both the banks of River The segment length was varying between 1.5 m to m The later this technique was extensively used for 5.5 km Ganga bridge at Patna A meticulous planning of pre casting process including stacking and handling of the segments by SLMC were prepared as shown in the Figure 22 A goliath gantry was used in the casting yard to handle the segments Segments were transported using specially designed trailer Short line system demands high accuracy and exact leveling of the segment to achieve the desired deck profile and transverse camber Systems were not so sophisticated as available now, for accurate positioning during the match casting of segments The segment had a maximum weight of 90 t The pre casting beds were located at one end of the bridge and precast segments were stacked on the river bed itself At one stage for Ganga bridge at Patna, more than 1200 segments were in the stacking yard 3.3 Casting yard organisation and stacking of segments Casting yard (Fig 23) for pre casting of the segments generally consists of: – – – – – Casting area including the formworks and survey stations, Segment attacking area, Reinforcement cutting and bending area, Concrete batching plant, if concrete is produced on the yard; Additional areas for workshops, stores and so on The kern of the casting yard is the casting area The other areas are designed in such a way that the handling and transportation within the yard is optimised 686 Figure 21 First time application SLMC for CMC in 1971 in India Figure 22 Casting process design of SLMC for CMC of first time application in India The design of the lay out should consider the following: – – – – – – The area required depending upon whether LLMC or SLMC for SSMC or CMC Casting machine supporting foundations Gantry span and capacities to handle formwork, reinforcement cage and pre-cast segments Batching plant capacity for providing concrete during casting the segments The logistics of reinforcement bay for cutting steel, storage space for pre-cast segments, fabrication yard and site office The segment storage area has to be designed to cater for two major objectives which are: – The segments shall be kept on the storage area until concrete reaches the required strength for erection – Segment casting and segment erection activities have different paces and shall be synchronised to arrive at quantity of segments to be stored 687 Figure 23 Typical casting yard Figure 24 Single, double and triple decker stacking Figure 25 Lifting device privisions & low bedded trailer for transportation Generally segments are stored in casting yard itself for limiting transportation and double handling costs Typically, the span segments are usually stacked (Fig 24) on two levels However, stacking on levels or more could be considered for segment’s having higher load bearing capacity The precast segments are generally provided with lifting holes or hooks (Fig 25) to fit the lifting devices to enable handling of the segments The segments are normally transported by specially designed multi-axle trailers whose design has to cater for the limitations of transportation route 688 METHODS AND ERECTION The terminologies for methods of construction i.e whether SSMC or CMC have been derived from methods of erection adopted There have been a number of developments in the erection methods for precast segmental bridges In the early days it was common practice to erect the segments in balanced cantilever, with one segment either side of the pier stressed onto the previous segments using bars, followed by the permanent pre stress The segments were erected either using land or water based cranes or shear legs Segment delivery mechanisms are also important in achieving rapid construction methods Very seldom the permanent works layout can be adjusted to suit the preferred delivery method In balanced cantilever construction, it is necessary to ensure the stability of the balanced cantilevers during erection Typically this was done with a symmetric pair of false work towers either side of the pier However, on a number of recent projects a combination of a prop and vertical pre stress ties have been adopted which has the advantage of reducing the amount of steelwork, and minimising the foundations and work at ground level required during construction In this solution, one can use ties only on the centreline of the prop to control overturning away from the prop, or where the prop is kept close to the pier, one can also use ties on the pier centreline to control overturning towards the prop On bridges which are tightly curved in plan, it is possible to use the prop/tie solution to assist in ensuring the stability transversely as well as longitudinally As the curved balanced cantilever grows one can increase the tie forces at the pier on the outer edge of the curve thereby ensuring the transverse stability of the curved cantilever Although these methods of erection are still appropriate for many bridges, gantry erection methods are often used for larger projects Gantries usually allow more rapid construction and they can allow the segments to be delivered at deck level to minimise ground level working There are a number of types of gantry, which can erect segments either in balanced cantilever or span by span Whilst developing the design, it is necessary to consider in detail the interaction between the permanent works, the construction method and the construction programme For example there is no point in detailing the design for rapid erection, if the supply of the segments is slow Similarly the relationship between the permanent prestress design and the temporary pre stress bars used to hold the segments together during construction can be modified to suit different construction cycles Whereas it used to be common to use bars to stress the segments onto the previous segment followed by permanent prestressing, installing the permanent prestress is a relatively slow process and it is now more common to fix or segments on temporary bars before applying the permanent prestress For shorter spans it is even possible to erect all of the segments on bars and then follow up with the permanent prestress However, with such long segments one needs a lot of temporary prestress, both inside the box and outside it and this creates its own difficulties 4.1 Span by span Method of erection (SSME) In SSM of erection, an entire span (pier to pier) is erected with special equipment Depending upon the accessibility and delivery of segments, the equipment could be (Fig 26): – False work or trestles from ground – Overhead girder – Under slung girder These erected segments become self supporting only after application of posttension as such during non self supporting stage of the span, segments are to be individually supported either by: – Hangers connected between deck top slab and overhead truss for overhead girder, – Point support below the segment wings (part of deck cantilevering out from the web) for under slung girder, – Point supports below segment soffit for under slung girder and false work 689 Figure 26 Structural fit & SSME variations Figure 27 Unique combination for SSME used for Krishna bridge in 1971 There could be structural fit depending upon the determinacy of structural configuration and also provision of cast in situ stiches for SSME, which are: – Isostatic span structure which are simply supported superstructure – Continuous pan structure made of in-fill-span between the pier table with two cast in situ stitches – Continuous span made of isostatic span having cast in situ stitch right on top of the Centre line of pier 4.1.1 SSME using false work or ground supported trestles For the first time SSME (Fig 27) was adopted in India for Krshna bridge at Deodurg in 1970 The method used was unique in the sense, the segments were supported on false work and the erection of segments were done using specially designed travelling gantry moving on top of already placed segments Since the decking was designed as six span semi continuous structure, it was necessary to erect each span on temporary staging The precast segments were assembled over pipe staging erected in the span A specially designed gantry moving over already placed segments was used for erection of segments The segments were checked for proper alignment prior to gluing together Then the cables were stressed After establishing semi-continuity through gap concreting, the decking was lowered over permanent bearings The gluing material was selected by conducting field tests on various materials like epoxy, mortar etc Ordinary Max Pulls were used to impart temporary pre stress while gluing the segments together In late ninties, India embarked upon the mammoth infrastructure development especially in urban areas where elevated corridors for both roads and metro became popular Especially where ground accessibility is there, this method of erection (Fig 28) became very handy due to its simplicity and also as it could accommodate sharp curvatures 690 Figure 28 Classical SSM underslung erection using scaffolding/Trestles Figure 29 Modern erection system using Trestles The modern Erection machine (Fig 29) consists of trestle resting on screw jack assembly, knuckle bearings, main girder and sliding trolleys Footings are placed on the ground and trestles are erected on the footings supported with screw jack assembly at bottom Screw jacks enable the lowering and rising of main girder for final alignment vertically Main girder is placed on knuckle bearings provided over trestles Knuckle bearings enable main girder to rotate horizontally to facilitate as per horizontal curvature Trolleys are placed over the main girder to support the segments Two trolleys are used to support each segment Arrangements for raising, lowering the segments as well as positioning the segment in longitudinal and transverse direction are required to be provided The segments are transported (Fig 30) on special trailers having required capacity (60 to 100 ton) from casting yard to site The required capacity crane or bed gantry is used to place the segments, dry matching needs to be carried out to ensure that segments are aligned as per required geometry of the structure After dry matching the segments are to be glued with epoxy from one end to another PT bars are normally used for temporary pre-stressing Normally pressure of kg/cm2 is specified to be applied between faces of two segments After permanent pre-stressing cables are threaded through the ducts and stressed, PT bars or any other temporary external pre stressing are distressed and removed On an average, erection of spans of around 50 m can be achieved in a month with one set of under slung erection machine in spite of traffic constraint and other constraints 4.1.2 SSME using special underslung girder and camel gantry This modern SSME is in a way similar to that of erection method adopted for Krishna bridge at Deodurg except that in place tubular staging underslung gantry is used and in place of travelling 691 Figure 30 Transportation & handling of segments for SSME in underslung Figure 31 Top right: first time application of special underslung for SSME inYamuna bridge at Delhi (2000) gantry, sophisticated camel gantry/segment handler is used This sophisticated and improvised version over Krishna bridge at Deodurg was adopted first in India for SSME for Yamuna bridge at Delhi (Fig 31) in 2000 In this method of SSME (Fig 32), the precast segments are erected by means of a span long underslung type erection girder The erection girder consists of two structural steel box girders, which are supported over RCC/steel bracket one on either side of bridge decking projecting from pier These erection girders may be supported on foundations also if sufficient place is available Specially designed erection gantry (Camel Gantry) is used to feed the segments to Launching girder Sufficient capacity trailers are used which travel over already erected deck to bring precast segments below Camel Gantry Segment carriages with hydraulic jacks (for final alignment purpose) are placed on the Launching girder over which Camel Gantry places the Precast segment After placement of all the segments and aligning them to proper geometry epoxy gluing followed by temporary prestress is carried out If there are any in situ joints between pier segment and penultimate segments, the same are then to be concreted After assembly of all the segments, permanent prestressing is carried out to make the span self supporting The Launching girder is then auto launched to the next span by means of winches and Camel gantry is brought to the next feeding location The main components of the launching system are: – Launching Girder: For placement of precast segments till permanent stressing – Camel Gantry: For feeding the segment to Launching Girder – Required Capacity Hoist ( connected to Camel Gantry): For lifting the segment from Trailer and then placing it on Launching Girder – Wheel Block Assembly: For forward movement of Launching Girder 692 Figure 32 SSME using underslung girder & over head segment launcher – Outrigger: To support the Launching girder during shifting of wheel block assembly – Required capacity Hydraulic Jacks mounted in Wheel Block: Supports Launching Girder during erection of Segments – Segment Carriage – Supports individual segment during erection and also used for geometry adjustment during dry matching – Trolleys: For shifting the wheel block assembly – Winches: For pulling the launching girder during auto launching operation and for longitudinal movement of segment carriages – Front and rear boggies of camel Gantry: For forward movement of Camel Gantry – Required Capacity Hydraulic Jacks (connected to front leg of Camel Gantry) : For transferring reaction on Diaphragm segment during placement of segment on launching girder and also to transfer camel gantry load on wheel boggies before commencing forward movement of Gantry – Trailer of required capacity: For transportation of Precast segment from casting yard to erection site 4.1.3 SSME using overhead gantry Overhead gantry is a steel girder specially designed to hang the assembly of segments of the entire span and spanning over minimum consecutive piers This temporary girder has self-launching capability with mechanized launching system to be able to move longitudinally over the successive piers Overhead head gantry for SSME was perhaps first time introduced to Delhi metro viaduct constructions and there after the technology was picked up extensively even for road elevated structure and bridges later (Fig 33) The main components of Overhead gantry (Fig 34) are: – Main girder could be single or two – Carriage trolley able to pick up and handle segment Sometimes used also for launching the main girder – Launching system like hydraulic jacks, chain pully blocks, etc – Main supports like Front and Rear legs – Auxiliary supports like Rear leg, Front leg and intermediate legs if required – The bracket to support the front leg if there is no space over the pier cap – The assembly of beam and Hangers to handle the precast segment with the carriage trolley – Suspenders to support the segments 693 Figure 33 SSME using over head gantries Figure 34 Components of overhead gantry for SSME Figure 35 Typical erection of overhead launching girder Typically overhead launching girder itself) has to be erected first to enable the segments erection subsequently Typical erection sequence of launching girder could be (Fig 35): – – – – Temporary trestles placed on footings for erecting boxes of launching girder Then front support has to be erected on Front pier (Pn + 2) Middle support has to be erected on built up stool provided on Centre pier (Pn + 1) One by one box of launching girder are erected on temporary trestles and joined to each other by splice joint – After joining all the boxes slider Beam are to be erected, – Temporary trestles are to be removed Once the launching girder itself is erected, SSMC using overhead launching is done typically in stages In the 1st stage, the segment is brought on trailer and lifted with help of crab hoist provided in launching girder.Then segment is to the slider beam and shifted to its position 694 Figure 36 1st stage, the segment is brought on trailer lifted to over head girder with crab Figure 37 Segment are positioned in 2nd stage with help of slider beam.In the next stage, all the segment are positioned by procedure given in stage-1 In the 3rd stage, the following activities are carried out: – – – – – – Dry matching is carried out between segments S1 segment is positioned accurately as per required level and alignment Remaining segments are shifted back by 200 mm for applying epoxy Then epoxy is applied to the surface S1 and S2 segment Segment S1 and S2 are joined by applying force with help of attachment frame and tension bar Similarly all the segments are joined with epoxy by applying force with help of attachment frame and stressing bar – Permanent pre stressing cables are stressed – Load is transferred from slider beam to pier cap with help of span releasing jack – Segments are released from the suspenders of slider beam In stages and 5, principally auto launching of launching girder itself is implemented for erection of next span in various steps as below: – All the sliders beam are moved back at the rear end of the launching girder – Rear support is shifted and activated on S1 segment – Middle support is deactivated so that load is transferred on rear support and middle support is free to move – Middle support is shifted and positioned near front support Middle support is activated at that position 695 Figure 38 3rd stage, dry matching to release of suspenders Figure 39 Finally auto launching of the girder – Telescopic leg of front support is closed so that front support is deactivated and load is transferred on middle support – Launching girder is pushed forward with help of longitudinal jack (having stroke of 1000 mm) provided on middle support, until it reached near next pier location – Front support is activated by opening the it’s telescopic leg – Sliders are brought to the front for erecting the segments of next span There could be minor variations in SSME using overhead launching gantries, but the principle of erection is the same as explained above The minor variations could be that there could be single or two girder in the cross section, the girders could be plate girders or truss girders, the front and intermediate legs could be supported on the piercap itself or the temporary brackets attached to pier cap or on the diaphragm segment of the superstructure depending upon the availability of space on the piercap 4.2 Cantilever Method of erection (CME) CME has principally three variations VIZ Balanced cantilever method of erection (BCME), Free cantilever method of erection (FCME) and Progressive placement method of erection (PPME) Further in BCME three types of launching equipment can be used VIZ, form travelers, under slung launching girder and overhead launching girder In BCME (Fig 40), the segments are erected by pair, symmetrically on either side from the pier, each of them being cantilevered from the preceding one The erection of a pair of segments can be 696 Figure 40 BCME where cantilevers are erected in pairs progressively Figure 41 First precast segmental bridges in India by BCME Figure 42 Use of bed gantry & floating crane for BCME done either simultaneously or one after the other Equipment used for handling and delivering the segments could be mobile crawler or barge mounted cranes, Lifters sitting on each of the last pair of segments already erected, various type of launching girders, barges, bed gantries 4.2.1 Balanced Cantilever Method of erection (BCME) In India, for the first time BCME (Fig 41) was extensively adopted for Ganga bridge at Buxer and Narmada bridge at Zadeswar in 1971 and later around 1977, the same technology was adopted for Ganga bridge at Patna too The erection equipment were bed gantry on land and barge mounted crane in water (Fig 42) The segments were hoisted with the main gantry and placed over power driven trolleys, running on the completed deck The segments were picked up by floating crane in the water portion, and transported to their erection location Before its final erection, the segment was checked for its perfect matching and alignment with the previously erected segment The segment was held with the gantry and cables were threaded A specially developed epoxy formulation, successfully tested in R&D wing of the company provided an interfacial bonding surface with the match cast segments The segment was held in position with the help of a system of hydraulic jacks placed over deck slab and soffit slab, till the cables were stressed and the segment was attached with the previous segment 24/7 HT cables were used and the pre stressing was carried out after the symmetrical segments of T arm were erected The complete system for match casting, transporting and erecting the segments in position was developed in-house by a team of engineers and was applied successfully in 1971 itself In Ganga bridge at Patna the segments for land spans were erected using a bed gantry (Fig 42) where as water spans were erected using a floating crane as explained earlier Precast segments, after matching with epoxy interface, were stressed at the deck and soffit levels with high tensile bolts This is required for supporting the segment temporarily, till the main pre stressing cables terminating at the section, were stressed The entire temporary stressing system was developed to ensure uniform compression on the epoxy joint Another variation of BCME where lifting beams are used as depicted in Figure 43, has been perhaps tried out first time in India for 2nd Vivekanada extradosed bridge in Kolkotta Lifting 697 Figure 43 BCME using lifters Right: 2nd Vivekananda bridge, Kolkotta Figure 44 Use of underslung girders for BCME in 2nd Narmada bridge at Zadeswar Figure 45 Use of over head girders for BCME beams is a simple way to erect balanced cantilevers The system is composed of a steel structure sitting and fixed to the segments already erected The segments are fed under the cantilever by trucks, barge or other equipment and lifted directly to the right position The lifting operation can be carried out either by winches or strands BCME using under slung launching girder (Fig 44) was perhaps first time successfully executed for 2nd Narmada bridge at Zadeswar (1997–2000) It consisted of 13 main spans of 96.2 m with 4.63 km of approach roads and was built in a record period of 32 months The two-lane deck consists of a 7.5 m wide carriageway with 1.5 m wide footpaths on either side and is located 29 metres downstream adjacent to the existing bridge The self launching overhead gantry was used for the first time for the Viaduc d’Oléron, built in 1966 With a total length of 2862 m it was at that time the longest prestressed bridge in France The hollow box cross-section had a width of 10.62 m and a height of 2.50 to 4.50 m The balanced cantilever method and epoxy joints were used for gluing The method basically consists in installing a steel girder with two legs (one in the centre and one at the back end), the length of which is somewhat greater than the maximum bridge span The centre leg is on the first segment of the new symmetrical deck cantilever to be constructed and the back leg is on the end of the deck that has already been built (Fig 45) The girder is equipped with a trolley, which runs on the lower chords of the girder, enabling the successive segments to be installed The steel truss girder is built-in two tunnel legs which provide the opening for the segments to pass through The central leg and rear leg are supported on steel beams, which enable the necessary adjustments to be made according to the curve and inclination of the deck A temporary front leg enables the first segments of each 698 Figure 46 First bridge to be constructed by FCME in India for PSB Figure 47 Sequence of construction for FCME in Pragathi maindan bridge double cantilever to be installed BCME using overhead launching girder has to be tried out yet in India 4.2.2 Free Cantilever Method of erection (FCME) The first bridge to be built by free cantilever method of erection (FCME) using precast segments is Choisy-le-Roi Bridge in France (1962) This was constructed using a large capacity floating crane which carried and installed the segments symmetrically on either side of the piers FCME was first time adopted in India for Pragathi maidan extradosed bridge at Delhi This extradosed bridge had a total length of 196 m with the central span of 93 m having sharp radius of curvature of 300 m The lateral spans consisting of U trough cross section were erected by SSME using overhead launching system The segments for cenral free cantilevering spans were brought on top of already erected side spans and the specially designed segment launchers which has the arrangement of rotating the segments, lifting and lowering segments in addition to the facility of telescoping and holding the segment for free cantilever erection The sequence of free cantilevering and their illustrations are depicted in Figure 47 Progressive placement method of erection (PPME) is similar to FCME, the difference being that the structure is continuously built from one abutment to another by placing precast segments regularly which are transported on already erected deck using trailors The specially designed segment launcher as used in Pragathi maidan extradosed bridge above is used for free cantilever progression In this method, as the entire span has to be cantilevered during erection, a temporary staying mast (Fig 48) is used for erection purpose The temporary staying mast which lets the segments through consists of two vertical transversally braced steel legs, on which regularly spaced 699 Figure 48 PPME using temporary mast in CMC hydraulic jacks allow the tensioning of the cable-stays by moving back their anchor plates The mast is transferred from one pier to another on a trailer drawn by the multi-wheeled trailer used for segment transportation CONCLUSIONS Precast prestressed segmental is tailor made for long multi spans and fast track bridges As the entire superstructure is cast away from actual site location, minimizes the hindrance to traffic & inconvenience to public in urban environment.Prefabrication being done In factory like environment of casting yard better control on quality and dimensional tolerances can be achieved Prefabrication of superstructure segments are done independently as work on foundations progresses that reduces overall completion time Cost effectiveness is inherent in terms of total construction period, less labour and repetitive use of formwork etc Future trend could be the combined use of concrete and steel to reduce weight consisting, concrete core segment, external strut and rib of steel and Precast panels as deck slab on ribs and struts REFERENCES Precast Segmental Bridges Guide to Good Practice for structural and site engineers, Prepared by fib Task Group 10.3, Commission 10 Combault, J 2004 Precast Concrete Segments for Bridges Fabrication and Assembly Fundamental Details, fib Symposium 2004 on Segmental Construction in Concrete, November 26–29, 2004, New Delhi, India Raiss, M 2004 Developments in precast segmental and incrementally launched bridge Construction methods, fib Symposium 2004 on Segmental Construction in Concrete, November 26–29, 2004, New Delhi, India Heggade, V.N 2004 Segmental Construction-some issues, fib Symposium 2004 on Segmental Construction in Concrete, November 26–29, New Delhi, India Dharap, V.M & Joshi, G.P 2004 Evolution of pre cast segmental construction for bridges and fly overs in India, fib Symposium 2004 on Segmental Construction in Concrete, November 26–29, New Delhi, India Heggade, V.N., 2010 Bridge aesthetics-Some issues, 3rd International fib Congress 2010, Think Globally & Build Locally, May29–June02, 2010, Washington.D.C Jatkar, M.V 2011 Extra-dosed Bridges for Delhi Metro Projects, FIB days-2010, An International Conference, Jan 14–15, 2011, New Delhi Heggade, V.N 2012 Construction Technologies for concrete Bridges, fib- days 2012, International conference, 10–11 January 2012, Chennai Heggade, V.N Aesthetics & Creative aspects of bridges, Invited lecture by ICI-Chennai, IIT Chennai Heggade, V.N., 2014 A construction case study of Kolkota Metro EWE-01 Corridor, ING-IABSE Seminar on Elevated Transport Corridors, Mysore, 27–28 June 2014, pp 236–251 Heggade, V.N 2014 Bridge on river Kosi by shortline precast segments and special under slung erection technique, International seminar on recent trends in segmental construction and retrofitting of bridges & flyovers, IIBE in collaboration with BRO, 5–6 September 2014, New Delhi 700