500m Long Continuous Precast Segmental Balanced Cantilever Superstructure Construction Of Allahabad Bridge
Er. Vinay Gupta, Director & CEO, Tandon Consultants Pvt. Ltd.
The span arrangement for each of the carriage ways consists of 6 span continuous units with individual spans of 63.2m + 4x95m + 63.2m = 506.4m. There are 2 such units in each of the two carriageways thus making a total of 4 such units. Segmental construction technique was developed for bridges across large rivers to facilitate the construction in a faster and economic way. Therefore, each of the 4 units of the superstructure is to be constructed using precast segmental cantilever construction technology with epoxy glued joints. It is envisaged to do the erection with an under-slung launching girder (LG) of length of about 230m. The construction progressed by balanced cantilevering method for about half the length of the 95m span (except the central stitch).
The structural system of sub-structure consists of hollow walled piers resting on single circular well foundations. The width of the piers is equal to the width of the soffit of the box girder.The structural system of sub-structure consists of hollow walled piers resting on well foundations. The width of the piers shall be equal to the width of the soffit of the box girder. An inspection platform is also provided all-around the pier cap. The well foundations have been sunk by jack down method with individual foundation for each of the two carriageways.
POT bearings shall be used for the purpose of transfer of the vertical and lateral loads. In seismic event to distribute the longitudinal seismic force in adjoining pier the STUs (shock transmission units) have been provided. There are only 3 Expansion joints in each of the two carriageways. It is proposed to use Modular Strip/Box Seal type of joints for the high range of movements expected.
A detailed hydrological study was carried out by IIT Roorkee. As per the study length of the bridge was fixed as 1000m. Fig.1 shows the arrangement of river training and protection work, recommended by IIT, Roorkee.
To facilitate the construction in faster and environmental friendly manner and economic way, for 2 km of bridge length
(2 parallel carriageways of 1000m length each) across large river, the appropriate choice was to construct the bridge using precast segmental cantilever construction technology with epoxy jointed segments using internal prestressing. The bridge comprises of two units of 6 span continuous modules for each carriageway with individual span arrangement 63.4m + 4 x 95 m + 63.4m = 506.8 m. There are 2 such units in each of the two carriageways thus making a total of 4 such units. By providing continuity over intermediate supports, the number of expansion joints and bearings has been kept to a minimum for providing better riding quality and minimising periodic maintenance problems associated with these elements. Superstructure box girder has varying section in longitudinal direction, with a minimum depth of 2.5m in mid span, increasing parabolic ally to 5.5 over supports gives pleasing looks. Figs. 2 & 3 show general arrangement and cross-section of the bridge.
Multiple shear keys have been provided in segments at end face. The shear keys are un-reinforced small size projections with trapezoidal shape. They were limited to internal part of each web for aesthetic reasons. The keys in the deck slab help in positioning of each new segment and transferring of local shear forces ensuring continuous behaviour across the joints under concentrated traffic loads. Length of the precast segments varies between 2.5m and 4.0m, in order to control the maximum segment weight for handling and transportation. As a good practice, at least one cast site stitch of 2 x 0.1 m width has been used in each span.
All permanent pre-stressing tendons are 19 K 13. The cables were kept only in top and bottom flanges. This reduces the requirement of web thickness. Fig.4 shows the arrangement of cables. In addition to this, provision was kept for external pre-stressing equal to 15% of permanent pre-stressing in future, if required. For this purpose intermediate blister blocks and deviator block were provided and holes were left in diaphragms. The cables have been carefully positioned in such a way that space is available, along a straight line in all the segments, for lifting bars, which are required during erection and launching of segments. To avoid temporary bottom tension during cantilevering the temporary pre-stress at bottom is retained and remove only after stressing of integration cables. The pre-stressing shall also be checked for the movement of low bedded tailer.
The deck level appurtenances are also well thought out and maximum amount of pre-casting has been resorted to. This has been done to achieve good quality control and speed of construction. Precast Concrete crash barriers with cast-in-situ stitch have been provided on either side of the carriageway, a light steel railing on the edge of the footpath with cast-in-situ connection has been provided. The precast concrete facia is provided on the face of cast-in-situ connection to hide the discontinuity.
Precasting and Handling of Segments
Segments were precast using long line match casting technique. The precast segments were manufactured in a centralized casting yard located close to the site. Fig.5 shows view of match casting of segments at casting yard by long line method. External shutter was designed for movement on rails for easier construction. The minimum length of precasting bed required was equal to the part of the end span plus half the middle span. This way the length of the precasting yard bed was fixed at 110m. The stitch segment and pier segment was cast with bulkhead on both sides. Subsequently each segment was ‘match cast’ against the previously cast segment on one side and bulk head on the other. The segments were cured in curing tank using sprinklers. They were stacked in two layers, one on top of the other. Fig.6 shows stacking of segment at casting yard.
Erection and Launching
Maximum weight of segments was about 80 t and these were transported from casting yard to their respective location by means of specially designed multi-axle low-bedded tailers. The erection was carried out using an under-slung launching girder (LG) of length of about 230m (Fig. 7). The LG was supported from temporary steel brackets projecting out of the sub-structure during the erection process. Fig. 8 shows supporting arrangement for launching girder from abutment wall. The construction progressed by balanced cantilevering method for about half the length of the 95m span (except the central stitch).
The Pier head segment arriving at the site is lifted and rotated by 900 using Portal Gantry. From the portal gantry, the segment was lowered on trolleys, located underneath the segment flanges which carry the segment to be erected and roll on top of launching girder. The segment supported over the trolley was then taken to its final position. These trolleys were equipped with screw jacks, both in vertical and horizontal direction for both directional alignments. The launching girders are placed on either side of the box segments in such way that the portal gantries can travel clear of the segment flanges.
Longitudinal, transverse and vertical alignment of all the segments in its erected position by operating jacks. Permanent bearings were installed under pier head segment and temporary stabilizing arrangement was done on each side of bearing. It may be noted that no temporary jacks are provided for transferring the load during cantilevering.
Subsequently, the adjoining segments on either side of the pier head segment are launched, and dry matching carried out prior to epoxy gluing and imparting of temporary pre-stressing. Thereafter, the excess epoxy is scraped out. Subsequently, Post-tensioned cables were threaded in the assembled unit of segments and stressing was carried out.
The process was repeated till end of the cantilever arm and precast closure segment along with cars-in-situ stitch were installed and continuity cables were stressed. Launching girder was shifted forward. Fig.11 to 13 describe the above steps.
The structural system of sub-structure consists of hollow walled piers resting on single circular well foundations. The width of the piers is equal to the width of the soffit of the box girder. An inspection platform is also been provided all-round the pier cap. Spill through type of abutments have been provided in combination with the protection works. The well foundations have been sunk by jack down method, see fig.14 with individual well foundation for each of the two carriageways.
Jack Down Sinking of Well
Jack down sinking of well foundations is a specialized technique. It was first used for Nizamuddin Second Bridge, Delhi and later for DMRC Yamuna Bridge, Delhi. In this innovative technique, approximately 60m deep ground anchors are installed around each well foundation (2x4 nos. installed with a capacity of 125t each in the current case). Each pair of two ground anchors were connected to a connecting rod (vertical) which engaged by central hole screw jack at top of the well, where steining was pushed downward using a temporary steel spreader frame. This way, the steining thickness was reduced to minimum required by structural design (1.25m thick in the current case of 10m dia well), which otherwise has to be made to be made almost two and a half times thicker, in order to mobilise sufficient concrete weight for sinking of the well. The above mentioned active system of sinking also has a better control of tills and shifts of well, apart from enabling a faster sinking.
POT bearings shall be used for the purpose of transfer of the vertical and lateral loads. As a second line of defence, the concrete stoppers with elastomeric bearing have been provided to prevent the dislodgement of superstructure during seismic activity. There are only 3 Expansion joints in each of the two carriageways. The Modular 4 cell strip Seal type of joint have been provided at location of abutments and 7 cell strip seal expansion joint at intermediate location for the high range of movements expected. Fig.16 shows the bearing layout plan.
Shock Transmission Unit
In seismic event, in order to distribute the longitudinal seismic force in adjoining piers, the STUs (shock transmission units) have been provided. It is a device that is temporarily, capable of creating fixed connection when desirable and during normal operation it remains movable. The unique properties of STU are (i) allow slow movement between structures with negligible resistance during creep, temperature and shrinkage movements. (ii) For short duration load or impact it acts as rigid link.
A carefully chosen span arrangement leads to a matching construction technique and better riding quality for the users. Such a construction technique entails faster and economic structure. While the precast segmental construction technology leads to faster construction of the superstructure, jack down sinking of well foundations had its own advantages of speed and quality. Innovative materials, such as STUs made it feasible to have long continuous superstructure amidst high seismic environment.
The conception, plan, design and execution of this project were a joint effort of a team of engineers from bridge consultant “ Tandon Consultants Pvt. Ltd. “ and contractor “ Larsen & Toubro Ltd“. Acknowledgement is also due to the client “National Highway Authority of India“ and project management consultant “Scetaroute International & Frischmann Prabhu (India) Pvt. Ltd. JV.
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