Design and Construction of Pre-Tensioned Sutlej Bridge in Punjab

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148 Though the aggregate sinking of 854 m was accomplished in a short period of 620 days, the good engineering practice of fixing the well cap at LWL/bed level, would have reduced the sinking duration, efforts and its financial ramification quite considerably. At the every alternative well location, after reaching the founding level, the soil investigation was carried out up to 9 m depths to deduce ‘C’ and φ values to confirm the soil bearing capacities. Bottom plugging was carried out by shifting the concrete from batching plants through buckets and placing by tremie pipes. After having done the recuperation test for soundness of plug after 14 days, the sand filling and intermediate plugs were expedited. With the help of irrecoverable shuttering supported on precast beams and cofferdam, each well cap was completed within five to six days including reinforcement fixing and concreting. 5. PIERS The RCC piers were of wall type flaring from well cap to accommodate the pier cap, with the concrete characteristic strength of 35 N/mm 2 . Though the grade of the concrete is same as that of used for well cap, the mix had to be made little richer with higher workability in order to enable smooth placement of concrete for the thin sections. A system of formwork consisting of steel channels and shuttering was used in piers, which was concreted in two stages (Photo 4). Photo 4. Shuttering Arrangement for Pier The height of the first lift was 3.25 m and after concreting the first lift, the balance second lift shuttering was fixed immediately in 3 to 5 hours. The grout leakage through the joints of shuttering was totally avoided by judicious planning during the fabrication of shuttering such as overlapping of plates, etc. The concreting for each lift was carried out in continuous operation without the cold joint. By virtue of large shuttering and minimum number of lifts in concreting, the surface texture of the concrete pier has been of excellent quality. 6. SUPERSTRUCTURE 6.1 Choice of Cross Section Due to the obvious advantages enumerated in Conceptualisation Para, the an isotropic deck was considered HEGGADE, MEHTA & PRAKASH ON to be made up of 35.40 m long precast pretensioned beams, transversely held by 200 mm thick RCC deck slab. Though the design-wise and from aesthetical considerations, the intermediate diaphragms could have been avoided, the same has been provided to satisfy contractual requirement which are in fact structurally redundant. As there are no design criteria laid down in IRC standards for pretensioning, invariably IRC:18 meant for post-tensioned construction is adopted and insisted upon, for pretension construction also. The present post-tensioned Code IRC:18 prescribes working stress method of design and permissible stresses seem to be on highly conservative side. The comparable AASHTO-94, the standard that is also based on allowable stress method (ASD) design, allows at least 33 per cent higher flexural stresses during transfer and 25 per cent higher flexural stresses during service condition. To worsen the matter further, the ‘IRC’ stipulates 20 per cent additional time dependent losses, 3 times 1000 h relaxation losses, minimum 80 per cent of characteristic strength at full transfer of prestress, those perhaps are rationalised for post-tensioned construction on the basis of past experience, where prestress transfer is feasible in stages. Universally, though generally there are no separate codes for post-tensioning and pre-tensioning, the prestressing code itself give separate design parameters such as time dependent loss parameters, permissible stresses and transmission length for pre-tensioning, etc. In view of this the author had suggested to IRC Code Making Committee to make IRC: 18 a common code for prestressed concrete road bridges common for both pretensioned and post tensioned concrete with the separate design parameters wherever relevant and applicable, which is yet to be taken into cognisance. Selection of the beam cross section for long span pretensioned girders warrants experience in field supervision, apart from theoretical aspects of prestressed concrete. It is expected that the optimum concrete section that is materially influenced by prestressing force and loading, is light for handling and transportation, prestressing operation and concreting friendly. The sizes of bottom and top flanges, the depth and width of web are required to be optimised on the basis of above constructability issues. The pressure line (resultant of stresses) in the prestressed concrete flexural member shifts its location within the section upon the application of external loads. In simply supported beams at the midspan for service condition, the stress at the bottom-fibre is zero, i.e. no tension allowed as per codal provision. At the midspan pressure line is above the CG of the section, warranting the CG of the prestressing force at a distance equivalent to moment divided by prestressing force. Thus to cater for the compressive force by virtue of pressure line above the CG of section at the midspan, the top flange requirement is high, whereas nearly zero-stressed bottom fibre
does not require flange, apart from accommodating cables. Towards the supports as the moments are gradually reduced to zero, the CG of the prestressing force can be judiciously located to be concentric to avoid any requirement of flanges. However, at the intervening stage before the application of imposed loads including live load, the section has to store large prestressing force at bottom fibre, which would be neutralised on application of external intermittent loads latter, bringing the bottom fibre stress to zero. The above calls for widening of the bottom flange and in fact decides the width. Thus an ‘I’ section where the pressure line can move larger distance without the tensile stresses is chosen for pretensioned girders. While the span to depth ratio ranges between 16 to 22, the web thickness of 150 mm is considered to be adequate for normal ‘I’ shaped beam for honeycomb-free concreting. However, in case of Sutlej Bridge as the vertical clearance was not a constraint, the lavish span to depth ratio of 14 was adopted to reduce the prestressing strands with the web thickness of 200 mm as constrained by IRC:18, though the same was not required by design and constructability angles. The decision on the width of the top flange is very crucial as the extremely narrow top flanges may buckle the precast beams during side shifting, transportation and handling. The Indian Codes categorise the beams as slender beams when the span to top flange width ratio exceeds 60 or depth to flange width ratio exceeds 4 and specifies reduction in permissible stresses and adequate temporary restraints during handling and erection from lateral stability considerations. Normally, for the simply supported beams, the span to depth ratio of 15 is considered to be optimum, and when the same is related to depth to width ratio of 4, the span to width ratio works out to be 60. In Sutlej and Beas bridges, 35 m long beams with 2.5 m depth was provided with 0.70 m top flange to keep the weight of the girder to minimum with span to flange width ratio of 50 and depth to width ratio of 3.57 satisfying both the considerations given in Indian Codes for slenderness. The stretching the slenderness to codal limits to keep the weight minimum, was considered to be very bold especially after the classical beam collapses of Roop Narayan Bridge on National Highway No.6, where the span to width ratio 50 followed the depth to width ratio of 3 (safer than Sutlej Bridge). In his paper “A study of the failures during launching of precast prestressed concrete beams of the Roop Narayan Bridge on National Highway No. 6”, while deliberating on Guyon’s contention that for the beams depths of 5 to 8 ft., the thickness of the flanges should never be less than 0.1 of the depth and width of the flanges should not be less than 0.40 of the depth for symmetrical I-beams, Mr. Seetharaman through his investigation concludes that the span to depth ratio should be 15 and depth to width ratio should be less than 3 for transverse rigidity of precast beams. Thus the chosen beam section for Sutlej called for thorough investigation and justification vis-à-vis lateral stability during transfer of DESIGN AND CONSTRUCTION OF PRE-TENSIONED SUTLEJ BRIDGE IN PUNJAB 149 prestress, handling and erection of the beams before the beams are transversely rigidised by deck slab. The lateral stability of Sutlej beams during handling and erection was ensured by extensive investigations on the basis of special report “lateral stability of long prestressed concrete beams” by Robert F. Mait in PCI Journal Jan-Feb 1989. The improper lifting hook placement and casting imperfection cause the beam to be tilted at an initial angle θ1 near the lifting hook location about the roll axis (Fig. 5). Normally the casting imperfections considered 1:1920 in Sutlej gets manifested itself by way of curvature in plan of prestressed beam after detensioning. Lifting hook placement tolerance was allowed to be 6.35 mm during casting. The above tilting of beam induces the lateral deflection about weak axis of the beam. Because of the transfer of prestress, there is already tension at the top fibre of the beam for which the tensile stress caused about the weak axis by the component of the self weight due to tilt gets added which needs to be within the permissible limits and in fact decides the maximum tilt (θ max) to which the beam can be subjected to. After the tilting is initiated by the initial angle θ near the 1 support locations, the beam achieves its equilibrium with a uniform lift angle θ (shown at midspan) with CG of the mass of the deflected beam right under the roll axis. In the figure as Z approaches Y , the beam starts rotating o r and becomes totally unstable even without the initial imperfection and without improper location of lifting hook. Thus the safety against the lateral buckling is a measure of Y vis-à-vis Zo and is called gross factor of safety (FOS = Y / r r Z ) for a perfect beam without imperfection. o If one has to account for imperfections causing the initial angle q and limiting the maximum lift to θ , the factor of 1 max safety reduces to . However, it is more logical to deduce the factor of safety against lateral stability by dividing maximum possible tilt θ max with that of equilibrium rotation θ at midspan. Moving the lifting position inwards improves the factor of safety against lateral stability by virtue of reduced deflections caused by rotations about the weak axis. However, it has to be ensured that the stresses are within the limits in overhang portions. Though the very slender cross sections from lateral stability considerations was chosen in Sutlej Bridge, the same could be successfully executed by adhering to the specified casting imperfections, lifting hook location tolerance, etc. during execution. The details of the same are given in Fig. 5. Unlike in post-tensioning, in case of pre-tensioning as the strands are bonded during the transfer of prestress, the
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Page 3 and 4: construction conceptualisation faci
Page 5: due to differential earth pressure,
Page 9 and 10: Fig. 6. Grillage idealisation for d
Page 11 and 12: Fig. 9. Cross section of Bulkhead d
Page 13 and 14: Fig. 12. Showing the Erection schem
Page 15 and 16: load deflection behaviour. It was e

Design and Construction of Pre-Tensioned Sutlej Bridge in Punjab

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