Paper No.524 DESIGN AND CONSTRUCTION OF PRE-TENSIONED SUTLEJ BRIDGE IN PUNJAB† V.N. HEGGADE*, R.K. MEHTA** & R. PRAKASH*** SYNOPSIS Currently in vogue fast track construction has encouraged the adoption of pre-tension technology for urban flyovers. After having successfully experimented pretensioned spans for River Bridge upto span of 30 m in Hadakiya Bridge in Gujarat, the technology was first time extended upto 35m spans for Beas and Sutlej river bridges in Punjab. The inherent peculiarities such as single stage prestressing, transfer of prestress through bond between concrete and cables by obviation of grouting and sheathing ducts, tensioning of tendons before the concrete is cast and transfer of prestress after the attainment of required strength in concrete derive certain advantages in favour of pretensioning in terms of durability, quantity reduction, construction speed, design and construction expediency. However, in the Indian scenario there are no codal guidelines accounting for these peculiarities for bridges. The enumeration with illustration is intended to provide basis for formulating guidelines for pretensioning in bridge building. The Paper also deliberates on optimization of beam cross section in relation to lateral stability during transfer of prestress accounting for casting imperfections, handling and erection of beams before the beams are transversly stiffened by deck slab which may help the code makers to have fresh look on the guidelines for lateral stability of the prestressed beams. 1. DESCRIPTION OF THE PROJECT The project consisted of design and construction of highlevel bridge across the river Sutlej including approaches and guide-bunds connecting Nakodar and Jagraon. The construction of the bridge facilitates in reduction of the distance between the towns by 50 km, reduction in traffic of NH-1 due to traffic from Rajkot, Maler Kotla and Jalandar and reduction of traffic in the city of Ludhiana. The bridge proper, 810 m long between the inner faces of dirt walls is made up of 23 spans of 35.20 m, while the approaches of lengths 1369 m and 1115 m on Nakodar side and Jagraon side respectively flanked the bridge proper. The main flow is confined and guided through the bridge linear waterway without causing damage to the bridge and its approaches by provision of divergent guide-bunds along the river flow, upstream and downstream on both the banks. The superstructure of 35.20 m span bridge consisted of 6nos. precast pretensioned concrete beams spaced at 2.15 m centres and cast-in-situ RCC deck slab. The width of the carriageway has been kept 7.50 m flanked either side by 2 m wide cycle track making the total width of the bridge deck to be 12.95 m including crash barriers and steel railings. The vehicular way is separated from cycle ways by crash barriers while cyclists are protected by steel railings from being toppled over. The beams were simply supported on POT-cum-PTFE bearings having slab steel expansion joints between the spans. The abutments were solid non spill-through types to go with same family of plate type piers flaring towards pier cap in the direction * ** *** † of river flow (transverse) to minimise the size of the RCC cap. The piers were founded on 6 m dia well foundations while the abutments were resting on 7 m dia wells. The detail of the general features of the bridge is given in Fig. 1. 2. DETAILS OF THE CONTRACT Punjab Infrastructure Development Board, on behalf of Punjab PWD provided developmental outline proposal with the condition that the contractor should submit his own proposal with the approximate dimensions of various components of the bridge structure to fairly establish that the technical requirement were met with. The tender proposal of the contractors were to include certain obligatory conditions such as length of the bridge, approaches and guide-bunds, carriageway and cycle track width requirements, linear water way and vertical clearance and type of foundations, and formation levels. The departmental outline proposal had the span of 40.50 m and the variation in span length was permitted up to ±20 per cent. The contractor had to give detailed design calculations and drawings in support of his proposal after the award of the work to comply with the design requirements stipulated in tender documents. Qualified engineers supplemented by independent quality control consultant in line with ISO requirements were supervising the execution of the job. Some of the salient design parameters specified in the tender documents are as below: River hydraulics Head of Technical Mgt. Gammon India Ltd., Mumbai Dy. Manager (Tech) E-mail : [email protected] Project Manager Written comments on this Paper are invited and will be received upto 30th Sept., 2006. 8 t/m3 • φ (angle of internal friction) : 300 • δ (Friction between soil and face): 200 • SBC for well foundation : 75 t/m2 gross at founding level. Material • For condition of exposure : Moderate • Concrete grades ⇒ For pretensioned beams : M40 ⇒ For well foundation : M30 ⇒ Reinforcements : HYSD bars conforming to IS:1786 Loading • Live load • Footpath live load • Cycle track loading Miscellaneous • Type of bearings • Wearing coat :IRC 70R single lane or Class-A 2 lanes : As per IRC: 6 : As per IRC: 6 : POT and POT cum PTFE : 25 mm thick mastic asphalt over 40 mm thick bituminous concrete : As per departmental drawing : STAAD III Release 22.868 Depth of water at lowest : 3. 1. recently in vogue fast track .255 Seismicity • Seismic zone : IV • Seismic coefficient : 0075G • Permissible increase in SBC : 25 per cent • Permissible increase in stress : As per IRC: 6 Soil parameter • λ (dry) : 1. However.87 m/sec High flood level : RL 227.0m water level • Scour level : RL 204. MEHTA & PRAKASH ON Fig.144 HEGGADE. the slab girder system with cast insitu post-tensioned beams are successfully adopted. upto even 65 m cast insitu box girders are adopted.0 • Cycle track and parapet • Software package 3. General arrangement of Sutlej bridge • • • • Design discharge : 18912 Cumecs Maximum mean velocity : 4. CONCEPTUALISATION In many of the river bridges in Punjab upto 45 m spans. Beyond 45 m. which withstood the otherwise catastrophic earthquake in Gujarath on 26th January 2001. transportation and erection.07 while for Zone-3 the same was ranging between 0. below the top of the well cap. The inherent peculiarities such as single-stage prestressing. 4.62 to 1. This helps in reducing concrete quantities rendering in lighter beams. From the design angle.50 kg/cm2 (Cu). the quantities for pre-tensioned girders are substantially lesser than post-tensioned beams. it was but natural for Sutlej and Beas Bridges in Punjab to extend the span length up to 35 m as a part of evolutionary process. These peculiarities derive certain advantages in favour of pretensioned beams in terms of durability. FOUNDATIONS Before the award of the job. design and construction expediency. transfer of prestress through bond between concrete and cables by obviation of grouting. In case of Sutlej Bridge. soil bearing capacity and founding levels along the bridge alignment (Fig.40 m and number of beams of six on the cross section (Fig. 2. consisting of 2. the bearing pressure at founding level in sandy strata after passing through the clayey strata was specified as 75T/m2 on conservative side. sheathing ducts. as a part of tender documents a thorough soil investigation was carried out by the department to arrive at soil characteristics. 4). The silt factor for Zone-1 varied from 0. consequently reducing productive working period in a season. The absence of the cables in the web and the elimination of end blocks and blisters to house the anchorages.5 m deep well cap. reduces initial mass on substructure and foundation due to seismic and reduces steel congestion in end blocks and anchorage zones. tensioning of tendons before the concrete is cast and transfer of prestress after the attainment of required strength in concrete. pre-tensioning uses the prestress efficiently on smaller sections with higher eccentricities. the subsoil strata were divided into 3 distinct zones. The various options in segmental and non-segmental technology is exploited in precast construction using post-tensioning or pre-tensioning. On the basis of 75 mm maximum settlement criteria. designated as Zone-2 was sandwiched between sandy strata designated as Zone-1 and Zone-3. As the pretensioned girders are manufactured in factory like environment where the bonding between concrete and tendons is direct due to the absence of grouting inside the sheathing duct. Construction-wise. Average value of sandy strata was around 340 while clayey strata had undrained shear strength of around 1. The design scour depths near the piers and abutments were evaluated on the basis of maximum discharge.01. 2). On the basis of soil investigation. The conventional cast insitu construction and its expediencies like staging/trestle support for superstructure in the riverbed is not only time-consuming. wobble and slip.05 m is tapered to 0. finally the competition was between post-tensioned versus pre-tensioned beams. Fig. In this particular context.DESIGN AND CONSTRUCTION OF PRE-TENSIONED SUTLEJ BRIDGE IN PUNJAB 145 construction conceptualisation facilitates expeditious construction. grouting operation and number of prestressing operations is eliminated. Pre-tensioning vis-a-vis post-tensioning VE for Sutlej bridge The thickness of the steining was decided by using the relationship given in IRC:78 to facilitate smooth sinking by gravity without excessive Kent ledge and damage to steining . construction speed. Overall seven numbers of boreholes were drilled for depths up to 40 m and standard penetration tests were performed as per IS:2131 to arrive at ‘N’ values. facilitating in attenuation in cost of handling. warrants specially designed casting bed which should be capable of imparting required quantum of prestressing force.75 m at scour depth of around 18 m. The value engineering carried out during conceptualisation stage for Sutlej Bridge revealed that for the same span of 35. 3). Around 12 m below the ground level along the alignment a silty clay strata of average band depth of around 12 m. Silt factors were calculated on the basis of particle size distribution following the principles of Lacey’s silt factor. In all 22 numbers of piers were supported on 32 m deep and 6 m dia well foundations (Fig. The steining thickness of 1. the flyovers in urban areas especially in metropolitan cities are realised by precast construction. river regime and velocity of the river. pretensioned beams had certain advantages and also peculiarities vis-à-vis its counterpart.1 m deep kerb. Though the pretensioned technology has been used extensively for flyovers and ROBs for the span range of 18 to 22 m. In the recent years. also susceptible to flood damages. among the various options available. allows the section optimisation from strength criteria alone. 1. the activities associated with post-tensioning such as threading of cable inside the sheathing. quantity reduction. on the basis of experience gained through the fast track flyovers and Surajbari Bridge. reduces the immediate losses like friction. the better durability and corrosion resistance is achieved.35 to 1. Perhaps. for the first time for bridge across river Surajbari in Gujarat the technology was adopted with 26 m spans in India. 146 HEGGADE. 3. MEHTA & PRAKASH ON Fig. Typical well foundation for Sutlej . 4. Bore hole details along bridge alignment Fig. etc. which warranted sand islands. Construction of well cap below GL To raise the steining to the required level. Since the bed profile was having large variations.2 m depth at P3 location. the grip for side earth resistance below the scour level was ensured to be of the maximum depth of scour below the design scour. protection with wire crated boulders. Most of the well foundations were constructed conventionally on land. Finally after 4 months. the service road was made up to A/R and wells up to P10 were started simultaneously. At P16 location. As the sinking proceeded. the cutting edge fabricated of mild steel was laid on the ground level and curb with required reinforcements was concreted. However. well cannot be sunk to the precise verticality.) and 3.) below bed level. As the cofferdam was quite thin compared to steining. To circumvent the creation of sump to sink last 1. In the process the well jumped and sunk by 3. the extensive shoring.DESIGN AND CONSTRUCTION OF PRE-TENSIONED SUTLEJ BRIDGE IN PUNJAB 147 due to differential earth pressure. This called for the creation of the sump below the founding level to facilitate gradual sinking. In the conventional construction (Photo 1. the other measures such as air jetting. The material inside was gradually scooped out with grabs to facilitate sinking under its own weight. almost all wells were required to be sunk up to 5 m below the ground level (Photo 2. the service road was made on A/L side and with the help of the site made temporary bridge between P8 and P10. etc. Initially.5 m to further the sinking due to increase in weight. The river bed level varied between RL 222 m to 225.6 for normal and seismic conditions respectively.723 m. The circular cofferdam except a small flare to accommodate piers was cast up to water level. the wells were tackled in the channel. water jetting and Kent ledge on top of false wall were attempted. Photo 2. P10.0 m below the water level. The side earth resistance was calculated by Bombay Committee Method with the passive resistance factor of safeties of 2. had to be resorted to apart from stabilising the 6 m deep false walls by adequate structural bracings. the non-availability of required weight hampered the sinking.). Curb reinforcements & cutting edge . barring a couple in deep channels.50 m having deep channel between the pies P3 and P10 with the low water level being at RL 223. on recession of floods. As normally. Cofferdam with flare to accomodate pier Had the well cap level been fixed at ground level or LWL. the job could have been completed three to four months earlier and substantial additional expenditure as a consequence of taking well cap below ground level could have been saved. After the monsoon was over.0 and 1. Photo 1. Photo 3.e. As the well foundations were to be plugged on soil.70 m at one go rendering the steining top almost 8 m (Photo 3. the steining was built up in lifts. as the sinking through the same was consuming almost 7 to 15 days per metre depth. 5 m excavations outside the well and dewatering yielded the well to the required depth. normally of around 2. continuous dewatering. the well sinking was planned with four cranes. the sump required was 3 m to enable last 1. the sandwiched clayey strata necessitated overall eight cranes. the design catered for the cumulative moment effect of 1 in 80 tilts and 150 mm shift apart from accounting for other severe load combination. The well cap was designed and detailed as uniformly thick plate for the external reactions and reaction components at the bottom of the pier with boundary condition as partially fixed at supports on well steining all around. it was ensured that the stresses at different levels of steining during service conditions and construction stage were within permissible limits. sand blow and sudden drop.80 m sinking. the combined effect of 450 t Kent ledge. Up to the deep channel i. warranting the CG of the prestressing force at a distance equivalent to moment divided by prestressing force. The present post-tensioned Code IRC:18 prescribes working stress method of design and permissible stresses seem to be on highly conservative side. the mix had to be made little richer with higher workability in order to enable smooth placement of concrete for the thin sections.40 m long precast pretensioned beams. whereas nearly zero-stressed bottom fibre Photo 4. 5. the depth and width of web are required to be optimised on the basis of above constructability issues. apart from theoretical aspects of prestressed concrete. After having done the recuperation test for soundness of plug after 14 days. Shuttering Arrangement for Pier The height of the first lift was 3.e. Universally. As there are no design criteria laid down in IRC standards for pretensioning. The pressure line (resultant of stresses) in the prestressed concrete flexural member shifts its location within the section upon the application of external loads. A system of formwork consisting of steel channels and shuttering was used in piers.148 HEGGADE. which was concreted in two stages (Photo 4). though generally there are no separate codes for post-tensioning and pre-tensioning. the good engineering practice of fixing the well cap at LWL/bed level. permissible stresses and transmission length for pre-tensioning. etc. is light for handling and transportation. To worsen the matter further. Thus to cater for the compressive force by virtue of pressure line above the CG of section at the midspan. 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. the standard that is also based on allowable stress method (ASD) design. The grout leakage through the joints of shuttering was totally avoided by judicious planning during the fabrication of shuttering such as overlapping of plates. Bottom plugging was carried out by shifting the concrete from batching plants through buckets and placing by tremie pipes. The sizes of bottom and top flanges. The concreting for each lift was carried out in continuous operation without the cold joint. each well cap was completed within five to six days including reinforcement fixing and concreting. It is expected that the optimum concrete section that is materially influenced by prestressing force and loading. to be made up of 35. transversely held by 200 mm thick RCC deck slab. 3 times 1000 h relaxation losses. where prestress transfer is feasible in stages. At the every alternative well location. the surface texture of the concrete pier has been of excellent quality. the balance second lift shuttering was fixed immediately in 3 to 5 hours. Selection of the beam cross section for long span pretensioned girders warrants experience in field supervision. etc. 6. allows at least 33 per cent higher flexural stresses during transfer and 25 per cent higher flexural stresses during service condition. the soil investigation was carried out up to 9 m depths to deduce ‘C’ and φ values to confirm the soil bearing capacities. those perhaps are rationalised for post-tensioned construction on the basis of past experience. with the concrete characteristic strength of 35 N/mm2. the intermediate diaphragms could have been avoided. efforts and its financial ramification quite considerably. PIERS The RCC piers were of wall type flaring from well cap to accommodate the pier cap. would have reduced the sinking duration. the stress at the bottom-fibre is zero. In simply supported beams at the midspan for service condition. At the midspan pressure line is above the CG of the section. MEHTA & PRAKASH ON Though the aggregate sinking of 854 m was accomplished in a short period of 620 days. The comparable AASHTO-94. the prestressing code itself give separate design parameters such as time dependent loss parameters. invariably IRC:18 meant for post-tensioned construction is adopted and insisted upon. no tension allowed as per codal provision. the sand filling and intermediate plugs were expedited. after reaching the founding level. the top flange requirement is high. By virtue of large shuttering and minimum number of lifts in concreting. for pretension construction also. which is yet to be taken into cognisance.1 Choice of Cross Section Due to the obvious advantages enumerated in Conceptualisation Para. Though the design-wise and from aesthetical considerations.25 m and after concreting the first lift. the an isotropic deck was considered . With the help of irrecoverable shuttering supported on precast beams and cofferdam. minimum 80 per cent of characteristic strength at full transfer of prestress. the ‘IRC’ stipulates 20 per cent additional time dependent losses. i. prestressing operation and concreting friendly. the same has been provided to satisfy contractual requirement which are in fact structurally redundant. SUPERSTRUCTURE 6. Though the grade of the concrete is same as that of used for well cap. 5 m depth was provided with 0. 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. 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. the same could be successfully executed by adhering to the specified casting imperfections. However. 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. 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. it has to be ensured that the stresses are within the limits in overhang portions. and when the same is related to depth to width ratio of 4. 5). apart from accommodating cables.. the beam starts rotating and becomes totally unstable even without the initial imperfection and without improper location of lifting hook. where the span to width ratio 50 followed the depth to width ratio of 3 (safer than Sutlej Bridge).57 satisfying both the considerations given in Indian Codes for slenderness. in case of Sutlej Bridge as the vertical clearance was not a constraint. The stretching the slenderness to codal limits to keep the weight minimum. 35 m long beams with 2. Thus the chosen beam section for Sutlej called for thorough investigation and justification vis-à-vis lateral stability during transfer of prestress. in case of pre-tensioning as the strands are bonded during the transfer of prestress. during execution.40 of the depth for symmetrical I-beams. though the same was not required by design and constructability angles. Mr. the thickness of the flanges should never be less than 0. transportation and handling. If one has to account for imperfections causing the initial angle q1 and limiting the maximum lift to θmax.6. After the tilting is initiated by the initial angle θ1 near the support locations.35 mm during casting. the section has to store large prestressing force at bottom fibre.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. the CG of the prestressing force can be judiciously located to be concentric to avoid any requirement of flanges. 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. Mait in PCI Journal Jan-Feb 1989. However. 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. In Sutlej and Beas bridges. 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. the web thickness of 150 mm is considered to be adequate for normal ‘I’ shaped beam for honeycomb-free concreting. 5. 6”. Because of the transfer of prestress. etc. Unlike in post-tensioning. The above tilting of beam induces the lateral deflection about weak axis of the beam. Towards the supports as the moments are gradually reduced to zero. The above calls for widening of the bottom flange and in fact decides the width.1 of the depth and width of the flanges should not be less than 0. the span to depth ratio of 15 is considered to be optimum. the span to width ratio works out to be 60.DESIGN AND CONSTRUCTION OF PRE-TENSIONED SUTLEJ BRIDGE IN PUNJAB 149 does not require flange. Thus an ‘I’ section where the pressure line can move larger distance without the tensile stresses is chosen for pretensioned girders. Though the very slender cross sections from lateral stability considerations was chosen in Sutlej Bridge. the factor of safety reduces to . while deliberating on Guyon’s contention that for the beams depths of 5 to 8 ft. Thus the safety against the lateral buckling is a measure of Yr vis-à-vis Zo and is called gross factor of safety (FOS = Yr/ Zo) for a perfect beam without imperfection. for the simply supported beams. While the span to depth ratio ranges between 16 to 22. The details of the same are given in Fig. 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. the . However. at the intervening stage before the application of imposed loads including live load. which would be neutralised on application of external intermittent loads latter. was considered to be very bold especially after the classical beam collapses of Roop Narayan Bridge on National Highway No. it is more logical to deduce the factor of safety against lateral stability by dividing maximum possible tilt θmax with that of equilibrium rotation θ at midspan. bringing the bottom fibre stress to zero. lifting hook location tolerance. Normally the casting imperfections considered 1:1920 in Sutlej gets manifested itself by way of curvature in plan of prestressed beam after detensioning. In his paper “A study of the failures during launching of precast prestressed concrete beams of the Roop Narayan Bridge on National Highway No. Normally. Lifting hook placement tolerance was allowed to be 6. In the figure as Zo approaches Yr. 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. However. handling and erection of the beams before the beams are transversely rigidised by deck slab. 15 m centre to centre with 250 mm thick end diaphragms to support 200 mm thick cast-in-site RCC deck cantilevering by 1. The RCC deck slab was cast on formwork supported on girders and the same was removed after the sufficient attainment of strength in the deck. the structure was idealised as a grid of longitudinal and transverse members. For finding all the longitudinal beam reaction components. the aspect of prestressing is a new variable and as such the same guidelines may not be applicable for prestressed beams. The effect of differential shrinkage and temperature variation were also considered in the design. The precast girders were transported to site and placed on bearings followed by casting of the end diaphragm. The grillage analysis results especially for superimposed . the structure was assured to be a composite section. for the further loads such as weight of crash barrier. the dummy longitudinal members with negligible section properties were provided at the edges and transverse grillage members were continued to connect them. apart from providing means for load sharing between longitudinal beams. 5. etc. The composite girders consisting of precast beam and deck slab was descretised to be placed along the axis of the girder while deck slab and deck slab with diaphragm was placed as transverse grillage members along the line of each of end diaphragm in the structure. in the case of prestressed beams. wearing coat.150 HEGGADE. The flaring properties of precast beams at the end for the distance of 2. 6) was used for superimposed dead loads and live loads. Design of Superstructure The superstructure consists of six numbers of precast pretensioned girders spaced at 2. Thereafter. However. MEHTA & PRAKASH ON Fig. The pretensioned girders in the casting yard were specified to be prestressed after 24 hours when the strength of the concrete was 31 MPa. the grillage analysis (Fig.65 m from 200 mm to 300 mm thick has been accounted for in the descretisation.2. Laterial stability of long prestressed concrete beams with factor of safeties beams cannot buckle which was also taken into cognisance during lateral stability investigations. transportation and erection of prestressed. Therefore transverse members having slab properties were provided to reflect the load sharing characteristics of the deck.10 m from the centre line of external girders on either side. In view of this the factor of safeties enumerated above may be included for ensuring the lateral stability of beams during shifting. The lateral stability guidelines for precast beams given in Indian Codes for prestressed concrete members are similar to that of for RCC and steel beams in terms of span to depth ratio and depth to top flange width ratio. For the application of the loads due to railing. precast beams in the prestressed concrete codes. while the concrete grade for the beams were M 40. railings and live load. The slab acts to transmit applied loads to beams by spanning transversely between them. 6. which were found to be in agreement to a large extent as illustrated in Table 1. with net strand cross sectional area of 140 mm2. Each strand of 15. around 500 mm was considered as transmission length in the said bridge. However. Normally. In fact this property is responsible for bonding pretensioned wires to concrete. prestressed beams with the depth of 2. 30 times the diameter of the tendon for strands i.2 mm dia was made up of 7 wires with 6 wires surrounding the centre wire configuration resulting in enhanced bond characteristics due to Hoyer’s effect. 68 tonne weighing. 80 per cent of the maximum prestress is developed over half the transmission length. 35. Fig. and as such half of the transmission length was projected beyond bearing supports for simply supported girders. . The cost benefit The stresses in bottom and top fibres of the beam before and after the composite action were ensured to be within the permissible limits as specified in IRC:18 at various temporary and service stages as tabulated in Table 2. of strands (tendons) conforming to class-2 of IS:14268 with UTS of 1900 N/mm2. which is popularly referred as “Hoyer’s effect”. The key factors in the choice and capacity of pretensioning bed was the availability of time for precasting girders and the economical considerations. The length of bond prevention has to be deduced after catering for “transmission length” required to develop full tension in the tendons. The same is achieved by preventing the portion of the tendons from bonding. thereby preventing from stressing the concrete at the ends. The Hoyer was the first German Engineer who developed the theory of “transmission length” due to the formation of wedge shape in prestressing tendon where the stress gradually decreases from maximum to zero with the increase in diameter of tendon. Each beam consisted of 34 nos. the size of the tendon. After the detensioning.e. The transmission length depends upon number of variables. PRE-TENSIONING The bridge of 23 spans consisted of 138 nos. SUMMARY OF STRESSES AT MID SPAN In case of the pretensioned girders with straight tendons.50 m. which is called “transmission length”. It is interesting to note that the stress variation over transmission length being parabolic. the stress in the wire at the end is zero and maximum after certain length. 7. the diameter of the tendon is reduced due to poison’s effect and the original diameter is regained after the release of prestress. friction between the tendon and concrete and initial and effective stresses in steel. 7). the most important being the strength of the concrete at the time of transfer. TABLE 1.2 m long.DESIGN AND CONSTRUCTION OF PRE-TENSIONED SUTLEJ BRIDGE IN PUNJAB 151 the prestressing moments near the simply supported ends need to be reduced as the moments induced by self weight and external loads gradually diminishes towards the supports from midspan. the bond prevention is achieved by provision of tight-fitting split plastic tube or heavy paper or cloth tape. TABLE 2. GRILLAGE ANALYSIS METHOD VIA-A-VIS LITTLE & MORRICE When the pretensioning tendon is stressed. As per the guidelines of IS:1343. Grillage idealisation for deck slab with girders dead loads and live loads were compared with Classical Little and Morrice method for verification. 6. for the accurate placement of tubes after the pre-tensioning a 20 mm dia ‘PVC’ tubes were used in Sutlej Bridge as bond prevention media at the ends (Fig. Due to the shrinkage of concrete clubbed with reduction in temperature. the bed for casting three beams was chosen.80 UTS. for stressing 115 m long strands (Fig. Each strand was to be stressed to 21 tonnes. TABLE. shrinks the concrete along with the strands in the bonded length thereby elongating and inducing further stress in the unbonded length. the strands can be stressed up to 0. for stressing 15 m long strands . COST BENEFIT ANALYSIS OF PRETENSIONING BENCH The largest long line prestressing bed had its own share of problems.5 m. due to shrinkage and temperature variation between the duration of casting of concrete and detensioning. 7. It is essential to design the pretensioning bed to additional 20 per cent capacity as the prestressing force on reaction abutment (Fig. Pre tensioning bench of 122. Debonding arrangement for stands at the ends analysis for various capacities of long line pre tensioning bed was carried out as per the Table 3. MEHTA & PRAKASH ON Fig.152 HEGGADE. warranting the capacity of the pretensioning bench to (2x34) = 714 tonnes. making perhaps the longest pretensioning bench in the country with the length of 122. 8). Since the expenditure on three beams casting was found to be economically optimum. Prior to detensioning in the casting yard. 9) will increase due to “long line bench effect” after casting of concrete. 3. 8. saving almost 15 months.5 m. If the increase in stress in unbonded length before Fig. Cross section of Bulkhead detensioning is beyond UTS. After the erection of one face of shuttering the alignment to the precision could be carried out by adjustment of turnbuckles.5 m from supply level a mechanical mode was devised for placement. which were removed once placed in position in casting bed. 9. This is also affected by curing time and is more severe when the ambient temperature during stripping is low. the strands in some cases may even start snapping. which can be tightly sealed to avoid leakage and bleeding. DESIGN MIX DETAILS The transfer of prestress was induced by cutting strand by acetylene torch in a pre-decided sequence after concrete achieved the strength of 31 MPa. (c) Cleaning. TABLE 4. Fig. After erection of one face of shuttering the alignments to the precision could be carried out by adjustments of turnbuckles.50 m3 at a time. The shuttering panels of 3 m length were erected using 8-ton capacity hydro crane. The debonding pipes were positioned and sealed with epoxy and tapes as per the drawing after the stressed cables were anchored. carry the concrete through conveyor for 3. (g) The minimum joints. The concrete compaction was achieved by poker and shutter vibrators. After the erection of one The concrete produced by batching plant of capacity 30 m3/hr as per the design mix (Table 4. the cables were prestressed from stressing end in predetermined sequence. The increase in the stress of the unbonded tendon is directly proportionate to the ratio of the length of the embedded strands to that of total strand length. As the concrete was to be placed at height of 3. setting. The best cycle time achieved in the beam casting was 66 hours though on a average time .DESIGN AND CONSTRUCTION OF PRE-TENSIONED SUTLEJ BRIDGE IN PUNJAB 153 shuttering face.) transported through a lead of 100 m by tractor trolley. and reduced the concreting cycle to 2 hours from manual concreting cycle of 5 hours. The gaps were filled with foams for preventing leakage and one end of the shuttering was provided with 50 mm wooden packing and thermo coal to facilitate easy removal of shuttering after concreting. (f) Rigid structural soffit form to secure and hold the side form without movement during concreting. The cages were suitably stiffened by diagonal bars during transportation.50 m height and deliver to tremie for placement through funnel. adjusting and handling ease. The cables were threaded manually inserting through 20 mm dia ‘PVC’ pipes of required length meant for debonding. which was supported on ground anchors by turnbuckles. While opening the coil. (e) The ability to withstand the form and other vibrations. After fixing up the anchorages. receive concrete from trolleys up to 0. The key decisive factors in the choice of formwork for pretensioned girders were: (a) High resistance to damage due to rough handling. (d) The ability of erecting one side independent of other. as happened in Sutlej Bridge. The other face of the shuttering was then lifted up and connected to already erect face by 16 mm through bolts. HT strands were cut to 115 m length and stacked over raised platform along the line of casting bed. (b) The precise dimension of the panels to fit together to form a large unit with ease. 10) at site in such way that it could move on a track line parallel to pretensioning bed. HT strands were passed through water tank to remove protective coating. the reinforcement cages fabricated in three pieces of 11 m each were shifted to casting bed by hydro crane. The device could be electrically operated by operator seated on it. The device consisted of an automatic conveyor designed and fabricated (Fig. The motorised trolleys were moved up to the span where beam was to be erected. However. the individual stressing of strands was resorted to. The side shifting was done using the crab assembly and winches set on top of gantries. the bearings were fixed at the soffit of beams with sleeves already embedded during concreting. maintenance of bed alignment. The side shifting trolleys were used to shift the beams from casting bed to stacking yard and from there to longitudinally motorised trolleys with the help of jacks. The surface irregularities were dealt by application of 2 mm thick epoxy over the bearings. not to constrict the water in a too narrow passage. The lifting was done with the aid of 750 mm stroke hydraulic jacks and 16 m long suspenders. As the stressing individually called for monostrand jack of 25 T capacity with a stroke 1000 mm. 10. Till the time the recess was grouted and end diaphragms were cast. 11) by 3 pairs of side shifting trolleys. a pair of motorised longitudinal trolleys and a pair of 35-toon capacity bed gantries. side shift and lowering of beam on pedestal as depicted in the Fig. CYCLE TIME FOR BEAM ERECTION Fig. the casting of 138 nos. Mechanical device for concrete placement cycle was 72 hours with the individual activity break-up as shown in Table 5. MEHTA & PRAKASH ON longitudinal trolleys. . repairs of shuttering panels. On an average 5 hours cycle was comfortably achieved as shown in the Table 6. the bed gantries and longitudinal trolleys were moved on railway track over specially constructed embankments with the provision of hume pipes at suitable intervals for passing the water from u/s to d/s. TABLE 5. The pretensioning can be done either by stressing each tendon individually or all together at a time. BEAM ERECTION The transportation and erection of beams were accomplished (Fig. The lowered beams were rested at about 20 mm above the pedestal and the recess was grouted using non-shrink cement grout. between A/R to P8 and P10 to A/L. 8. TABLE 6. Prior to the lifting of the beams. with the progress of 3 beams per day on a regular basis. This temporary bridge had single-line of piling on d/ s side to cater for the movement of one leg of the gantry where as on upstream side two lines of piles were provided to move longitudinal trolleys and transport other materials. the beams brought by side shifting trolleys were lowered to In the water spans. The longitudinal trolleys being designed at lower levels. CYCLE TIME FOR BEAM CASTING Despite the unforeseen impediments like non-availability of stacking facilities. a temporary service bridge was made on both sides of the piers to move the bed gantries between P8 and P10. The gantries were used at the location to lift. To facilitate the movement of bed gantries and longitudinal trolleys. the track line is laid on wooden sleepers at 0. and rain. etc. 12 & 13.70 m c/c as per railway specification over well-prepared compacted embankments.154 HEGGADE. the beams were placed on wooden sleepers and held by temporary bracings. of beams were completed in 320 days. This method was adopted for shifting both the gantries across P8 & P10 in seven days time without any risk and just taking care by dropping plumbs at four locations on both sides of the gantry to check the evenness of the movement. Casting Yard Layout for Sutlej trolley on top of the deck slab and supporting half of the gentry (Fig. This arrangement for bridges with small deck width can Fig. Among the alternatives considered. side shifting & placement The single line of piles on downstream side was collapsed during floods when the erection was in progress between P7 & A/L on Nakodar side. The typical bearing layout adopted in the bridge is shown in the Fig. BEARINGS POT and POT-cum-PTFE bearings were used in the Sutlej Bridge. 13. Transportation of gantry on deck 9. providing a . 12. Erection. 14) on it proved to be safe.DESIGN AND CONSTRUCTION OF PRE-TENSIONED SUTLEJ BRIDGE IN PUNJAB 155 Fig. However. Earlier. Fig. on Jagraon side the gantry was to be brought back to erect the beams on unfinished span A/R-P22. economical and fastest solution. 14. fixed bearings (rocker) with a small play provision on one end and free bearings (roller) in the longitudinal direction having fixity in transverse direction has been successfully used for straight superstructure like Sutlej bridge. normally for the simply supported bridges up to two lanes. 15. 11. Showing the Erection scheme of Beam Fig. during the period deflection readings were taken at one-hour interval for the sustained loading. one of the spans was to be validated by load testing to the designed IRC loading. At each pier location for a span. As could be seen from the layout. This semi classical layout helped in reducing the types of bearings to suit the precast construction. In the precast construction like Sutlej Bridge. a typical semi classical bearing arrangement as shown in the layout was adopted in Sutlej Bridge. depending upon the forces.156 HEGGADE. As shown in the Fig. For each stage of loading and unloading. Bearing configuration for Sutlej bridge be still successfully adopted as expansion/contraction taking place in pier caps and diaphragms connecting the superstructure are same. LOAD TESTING OF SUPERSTRUCTURE In line with the contract agreement. Then the unloading was simultaneously carried out in 25 per cent decrements with the readings taken during each decrement. Normally. where as two extreme girders on either side were transversely guided while on other side left free. The deflection readings of unloaded structure continued at one-hour interval for further 48 hours. The maximum load was sustained for 24 hours. which was well synchronised in this project. rotations and movements. 15. the linearity of the load deflection curves or any other abnormalities in the . the grooves have to be left in the beams at the bolting locations and as such the manufacture of the bearings have to be approved prior to the precasting of beams. two central bearings were fixed which were guided longitudinally on the other side. there were two types of bearings in span and these two types of bearings might have different plate sizes and bolting locations. the observations were made about the likely appearance of cracks. 16 the IRC loadings were simulated for the maximum moments in the midspan including cycle track loadings. Nevertheless. including impact factor. to avoid the transverse restraint likely to be caused by thermal effects and wind force. the approval of bearing manufacture is a very high lead-time item. 10. All the pedestals were progressively and simultaneously loaded by progressive increments of 25 per cent of the test load and the deflections were recorded at the midspan of all girders and and ¼th span of the third girder from left side. MEHTA & PRAKASH ON Fig. 95. The spindles of the dial gauges were connected by a pair of adapters in plumb line with GI wire. 11. Design aspect (a) Separate guidelines applicable to pretensioned concrete in IRC:18. the deflection readings were taken at fixed timings for all the operation of loadings and unloading. etc. there are no separate codes for post tensioning and pretensioning as the majority of the aspects of prestressing are applicable to both pretensioned and post tensioned concrete. Load Testing of Super Structure load deflection behaviour. To eliminate the effects of temperature. such as time dependent loss criteria. transmission length for pretensioning. the guidelines for ensuring the lateral stability given in the codes are based on steel and RCC beams. The following design and construction aspects deserve special mention in the context of Sutlej Bridge. precast beams. to be applicable to both types of prestressing by giving separate design parameters. with the percentage recovery of 94. In this method stools were embedded in firm ground and dial gauges of least count of 0. (c) ‘Long line bench effect’ for pretensioning and ‘Hoyer’s effect’ for transmission length. SUMMARYAND CONCLUSION Precast pretensioning technology up to the span of 50 m for river bridge decks can be economically exploited due to its material efficiency. The maximum observed deflection for G3 girder at midspan was 5. Providing well cap below the ground level by 3 to 5 m will only enhance construction difficulties and does not serve any aspect of good engineering practice. It was ensured that the bearings are functional by measurement of rotation.01mm were clamped to them. the three independent factor of safeties i. The deflection measurements were done by suspension wire method at the required locations using dial gauges. there is an urgent need to modify the IRC: 18 which is meant only for post tensioned concrete bridges. (i) the factor of safety with respect to rotation against tidal instability for the nearly perfect beams. wherever relevant and applicable. calculated as per IRC: SP-51. 16.3 mm.35 mm vis-à-vis maximum theoretical deflection of 7. with due recognition of prestressing effects. (b) Lateral stability for long span prestressed. permissible stresses. Apart from span to depth and depth to top flange width ratios. However.DESIGN AND CONSTRUCTION OF PRE-TENSIONED SUTLEJ BRIDGE IN PUNJAB 157 Fig.e. (d) Well cap at the level of LWL/GL to facilitate quality construction. . (ii) the factor of safety with respect to rotation with casting imperfection (iii) the factor of safety vis-à-vis the maximum rotation permissible has to be established. 1. Universally. but the effect of which is transferred to concrete at the time of pre stress transfer due to which relaxation losses calculated as per IRC:18 shall be divided by 2 for assessment of actually available pre stressing force at transfer. 3.36(Fck)0. (e) Bearing configuration. In case of pre tensioned girders relaxation losses start as soon as pretensioning is carried out. Proposal for codal guidelines for pretensioning On the basis of designing and executing long span pretensioned bridge spans. (c) Mechanisation of concreting for pretensioned beam. Chief Engineer and his colleagues of Punjab Infrastructure Development Board (PIDB) without whose cooperation.O. the tension shall be permitted at the top fibre of the girder which may be restricted to 0. .1 (c) Provided adequate un-tensioned reinforcements are designed for pre cast girder. and site of Gammon India Limited for their monumental efforts to construct such a large bridge within a very short period. ACKNOWLEDGEMENTS The authors would like to place on record the sincere thanks to honourable Managing Director.5 (iii) FOS with Actual angle of tilt > 1. fixing of bearings and stabilisation of beams after the erection till the casting of diaphragms and deck slab. (d) Use of large panel formwork for piers and beams. MEHTA & PRAKASH ON DESIGN AND CONSTRUCTION OF PRE-TENSIONED SUTLEJ BRIDGE IN PUNJAB 2. the authors propose the following: (a) The minimum dimensions of the cross section shall be (i) Thickness of top flange: 100 mm (ii) Thickness of bottom flange: 150 mm (iii) Thickness of web: 150 mm (b) The span to depth and depth to width ratios for optimum beam cross section from lateral stability considerations shall satisfy following Factor Of Safeties (i) FOS with out casting imperfections > 2 (ii) FOS with initial imperfections > 1. Joint Managing Director. (f) Pre tensioning bench for long line casting shall be designed for 20 per cent additional capacity to overcome ‘Long Line Bench Effect’. The authors would also like to thank all the engineers at H. the successful completion of this bridge would not have been possible. (b) Provision of temporary Service Bridge across the river. Construction aspect (a) Provision of track-line for gantries as proper embankment in the river bed. Many of the above design and construction aspects warrant in depth knowledge and meticulous micro planning to suit the adoption of particular type of technology.158 HEGGADE. guidance and encouragement.5. It is not necessary to consider 20 per cent additional time dependent losses for pre-tensioned girders. The elastic shortening loss shall be considered for a condition that all strands are stressed at the same time and pre stress transfer to concrete is simultaneous. (e) Transmission length due to ‘Hoyer’s effect’ shall be considered to be 30 times the diameter of tendon. (d) The losses due to seating and friction are not applicable to pre tensioning.
Report "Design and Construction of Pre-Tensioned Sutlej Bridge in Punjab"