Bridge Manual 1998-118-145.docx

CHAPTER - VI ============ CONSTRUCTION OF SUBSTRUCTURE AND SUPERSTRUCTURE INCLUDING ERECTION OF GIRDERS PART A - CONSTRUCTION OF SUBSTRUCTURE ===================================== General ------- 602. Importance of aesthetics in Construction ---------------------------------------- 1....

CHAPTER - VI ============ CONSTRUCTION OF SUBSTRUCTURE AND SUPERSTRUCTURE INCLUDING ERECTION OF GIRDERS PART A - CONSTRUCTION OF SUBSTRUCTURE ===================================== General ------- 602. Importance of aesthetics in Construction ---------------------------------------- 1. Apart from functional requirements, aesthetics merits serious consideration as the piers and abutments are exposed to view. In combination with the substructure, the entire bridge should provide a pleasing view and harmony with the surroundings. Thus a design which is appropriate for viaduct may be out of place in a built up area. 2. Reinforced and prestressed concrete permit adoption of piers and abutments in various forms such as A,Y, etc. 3. Though the main dimensions and choice of spans are largely determined by functional and economic considerations, proper proportioning of the various elements of a bridge (i.e. height, width and length of piers, length of spans etc.) is also important. Variations within reasonable limits should be allowed for in design to enable proper proportioning of the bridge. 4. The final shape of a structure should also highlight the special qualities of the materials used for construction. For example, stone masonry generally goes well with an arch bridge, while the use of prestressed concrete girders with a flat decking and tall or thin piers. A bridge should not intrude into the environment and look heavy. 5. While constructing road over bridges or flyovers in heavily built up areas, the aim should be to avoid too many piers in the middle so that a road user can have a clear unobstructed view. In viaducts, it is advisable to have slender and tall piers. 603. Material of construction ------------------------ 6. For stone masonry, the proportion of cement mortar used should be minimum 1: 4. 7. When mass cement concrete is used the mix shall be minimum M.20 grade. It shall be preferably design mix, using 40 mm aggregate. 8. Reinforced cement concrete, used in the form of thin piers or as a framed structure, can be adopted for viaducts, fly overs and road over bridges. Cellular piers are suitable if the heights are considerable. For reinforced cement concrete structure, the mix concrete shall be minimum M-25 grade. 9. Prestressed cement concrete can be used for all piers of viaducts. The mix to be adopted should be according to the design requirements. Piers, abutments, wing walls and approach slabs ----------------------------------------------- 1. Piers, abutments, and wing walls : a. The size of piers and abutments depends on the construction materials used. b. Masonry piers are provided with a batter varying from 1 in 24 to 1 in 12. Their width at the top is determined keeping the minimum space required for seating of the bearings of girders as also to provide sufficient distance on the outside of the bearings to resist diagonal shearing. c. For masonry abutments, a front batter of 1 in 16 to 1 in 10 is used: a flatter slope or steppings are provided in the rear as per design requirements. d. When piers are reinforced cement concrete, typical sections used are shown in Annexure 6/1. 605. Construction aspects - General ------------------------------ 1. When the ground is dry, construction of piers and abutments will not require any special arrangement. For their construction in water either coffer dams or temporary sheet piles may be used to divert the water as indicated in Para 404. 2. Construction of tall reinforced concrete piers can be expeditiously done with the slip form construction. 3. In abutments, weep holes should be provided at vertical intervals of 1 m and horizontal intervals of 1 m in a staggered manner. Behind the abutment and wing walls boulder filling and back fill material should be provided for the full height. The boulder filling should not be less than 600 mm thickness. The back fill material should consist of granular material of GW, GP, SW groups as per IS 1498 and should be free of clay and cement (Annexure 6 / 3). 606. Important points in regard to construction of substructure and superstructure ----------------------------------------------------------------------------- 4. As regards construction in brick and stone masonry, the relevant specification prescribed by the Chief Engineer shall be followed. 5. When concrete is used in construction, the important points to be observed are given in the subsequent paras. 6. Concreting shall conform to the requirements specified in IRS Concrete Bridge Code and IS.456. 7. All works in PSC shall be done with weigh batching only. 8. Design of concrete mix shall be in accordance with any of the methods given in the recommended guidelines for concrete mix design, published by the Indian Standards Institution. 9. Ordinary Portland cement conforming to IS 269 shall be used for plain, reinforced and prestressed concrete work. Portland blast furnace cement conforming to IS 455 may also be used for plain and R.C.C. work but not for PSC work. Portland pozzolana cement (IS 1489) shall not be used for PSC and RCC works. It can be used only for foundation concrete and concrete works in Bridge substructure where reinforcement is not provided for structural strength. When Portland pozzolana cement is used it is to be ensured that proper damp curing of concrete is done at least for 14 days and supporting form work is not removed till concrete has attained at least 75% of design strength. High strength ordinary Portland cement conforming to IS 8112 (Grade 43) and IS 12269 (Grade 53) may also be used where required from consideration of mix design. 10. Reinforcement steel shall conform to one of the following specifications : i. Grade I mild steel & medium tensile steel bars conforming to IS:432 (part-I). ii. Cold twisted bars conforming to IS : 1786. iii. Rolled steel made from structural steel conforming to IS: 2062 Grade A and Grade B. 11. The prestressing steel shall be any one of the following: i. Plain hard drawn steel wire conforming to IS : 1785 (Part I) ii. High tensile steel bar conforming to IS : 2090 and iii. Uncoated stress relieved strand conforming to IS : 6006. 12. i\) Reinforcement steel shall be free of loose mill scales, loose rust and coats of oil, mud or other material ,while being used. ii. Cover and spacing of steel shall be uniform and as specified in the drawings. iii. All ends of binding wires shall be carefully turned inside so that they do not project out of concrete to induce rusting. iv. Reinforcement steel shall be adequately secured so that it maintains its position during casting and vibration of concrete. 13. Aggregates : Aggregates conforming to IS : 383 shall only be used. They shall be clean. Marine aggregates shall not be used in concrete unless they are thoroughly washed in potable water and sulphur and chlorine content are low. The tests on aggregates shall be done in accordance with IS : 2386 (Part-I) to IS : 2386 (Part VIII) 14. Water used for mixing and curing concrete shall be clean and free from injurious amounts of oil, acids, alkalis, salts, sugar, organic materials or other substances which may be deleterious to concrete or steel. Potable water is generally considered fit for use in concrete. Further details can be seen in IRS Concrete Bridge Code. 15. Form work : Form work requirement shall be as per IRS Concrete Bridge Code including stripping time. In the case of PSC works, support shall not be removed till sufficient prestress has been imparted to the member. 16. Special attention shall be given to curing of concrete in order to ensure maximum durability and minimise cracking. The method of curing shall be as per IRS Concrete Bridge Code. 17. The appropriate value of minimum cement for different exposure conditions and maximum cement content for RCC and PSC works as well as the water cement ratio shall be as per the provisions given in the IRS Concrete Bridge Code. The equipment, material and the proportions of the mix to be used shall be submitted to and approved by the engineer before the work is started. 18. i\) While transporting concrete from the mixer to the form work, no segregation shall occur nor should there be any loss of ingredients. Necessary precaution may be taken to ensure this. ii. The concrete shall be deposited as nearly as practicable in its final position without rehandling. It shall be compacted before setting commences. It shall not be subsequently disturbed. The method shall be such as to avoid segregation. There shall be no displacement of steel or form work while placing concrete. 19. Compaction of concrete: All concrete shall be compacted by vibration. Generally internal vibration shall be used on all sections that are sufficiently large to admit them. The use of mechanical vibrators complying with IS:2505, IS:2506, IS:2514 and IS:4656 for compacting concrete is recommended. i. Vibrators shall be distributed so that the concrete becomes uniformly dense and plastic mass. ii. Vibrators shall be used for compaction only and not for moving concrete horizontally along the form. iii. For horizontal and vertical operations of vibrators, the spacing of points of vibration shall be such that the zones of influence overlap. iv. For concrete deposited in layers, the vibrators shall be inserted vertically and allowed to sink due to its own weight to the bottom of the layer and be slowly withdrawn. For succeeding layer, the vibrator shall penetrate the surface of the previous layer. For further details, IRS Concrete Bridge Code may be referred to. 20. Bearing areas for members shall be finished to true plane so as to give uniform bearing on the entire area. Bearing plane shall be horizontal even for the bridges on grades. 21. In major works, a field laboratory should be set up at the work site which should be equipped with necessary equipments to carry out the various tests on coarse and fine aggregates, cement, water and concrete. PART B - CONSTRUCTION OF SUPERSTRUCTURE ======================================= Slab Bridges ------------ Slabs shall preferably be precast in a depot and installed at site; where this is not possible they may be cast in situ. From considerations of economy, PSC slabs may be used for spans larger than 3.05 m. 22. Stagnation of water or retention of water in the body of the ballast over deck bridges leads to severe damage to the decking through percolation of water and consequent corrosion of reinforcement. It is therefore, essential that on deck bridges, water is not allowed to stagnate or retained in the ballast. It is therefore, essential that the ballast is clean and the drainage arrangement of the deck is also free from any obstruction. To ensure this, deep screening of ballast as necessary should be carried out. The drainage arrangement of the deck must be cleaned annually before monsoon. 608. Arch Bridges ------------ 23. Work on a single span : The construction is done by providing stagings or temporary support underneath and putting up the arch above. Before taking up the construction of the arch, back filling of abutments must be ensured. After the material of structure completely sets and is able to take the load, the temporary structure is removed. 24. Work on multiple spans: a. Work can be done simultaneously on a number of spans using more than one set of forms. b. In this method, due care will have to be taken to see that the horizontal thrust on the pier/abutment is not such that they give way. This can be guarded against by commencing the work on the adjacent span and bridging some load to bear on the pier before the support and the framework used on the previously cast span is removed. A proper sequence of construction of multiple span arch bridge shown in Annexure 6/4 (a). c. Supporting arrangement for arches: Over dry beds of streams, stagings can be constructed from the bed itself. Due care will have to be taken in supporting the staging columns on bed by giving a suitable timber support to spread the load and to check the stagings at various stages to see that it does not settle under the load when the casting of superstructure is in progress. d. If the work has to be carried out in flowing water of the river, the staging will have to be supported over shallow thin piles driven in the sand bed for sufficient depth (say 3 to 4m into the soil). e. In case the height of the pier is considerable as in high viaducts and staging is to be put up from bed, it may be difficult and expensive and alternative methods of supporting the staging from an intermediate level have to be provided. For this purpose, intermediate ribs are provided on piers to support the temporary floor system over which the false work can be put up or props erected from bed. 25. The arch ring or barrel should be cast in segments, the minimum number being two so that the effects of shrinkage can be countered by casting shrinkage keys between them separately. These keys are cast after the major shrinkage in the segments take place. Care should be taken in the sequence of casting segments/units so as to allow for shrinkage and at the same time develop the strength at appropriate location. A suggested sequence is shown in Annexure 6/4 (b) 26. An alternative method of erection evolved after development of pre-casting techniques is by stretching a cable across the span and erecting precast units from either end and staying them with wires till the last units \"crown\" is laid and it sets. Cables will be released and removed after the arch sets and is able to act monolithically. 609. RCC/PSC bridges (Beams with slab) --------------------------------- 27. In case of slab and beam bridges, the easiest method would be to use cribs and supports from below and cast them in situ. 28. The alternative method is to launch and erect a temporary girder supported on the ground or an intermediate projections from the pier. Precast girder can be launched over this. This method can be extended for even larger spans. 610. Erection of PSC girders ----------------------- 29. Erection by use of launching girders: Fully cast prestressed concrete girders are not launched independently as the cantilevering stress developed is considerable and the design is difficult. In such cases, the method adopted is to first launch a steel or aluminium supporting frame or girder so that it spans over the gap. This is designed to take only one girder at a time. Once the launching of this temporary girder is over, the first main girder is moved over this temporary girder or frame, supported at intervals or pulled across. When the full length of the main girder has come over the launching girder, it is jacked up and temporarily held in position. The launching girder is then side slowed to take the position of the next girder over the span. The main girder launched earlier is then lowered into position with the help of jacks. A schematic diagram is shown in Annexure 6/5 (a). 30. Erection of concrete girders with cranes/derrick : If the bed is dry, the girders can be cast on the bed and erected by mobile cranes one on either end or with the help of a suitable derrick in the centre or one derrick each on either end. If the height of the pier is not much and girders are too heavy to be launched by the available crane or derrick, the girder can be jacked up from either end on temporary rails (which will also be simultaneously built up ) to pier top level and then side slewed in position. The deck slab can be cast subsequently. In the case of prestressed concrete girders transverse prestressing will also be involved. For this purpose, holes should be left in correct position to form ducting. The diaphragm with necessary ducting should be cast after all the girders are launched correctly and adjusted in position. Part prestressing is done before individual girders are lifted or launched and remaining cables are tensioned, some before and balance after or all after the deck is cast according to the design. Extreme care has to exercised in following the sequence that has been given by the designers as any deviation can cause a crack or unwanted lateral deflection in the individual girder. 31. Erection by Cantilevering Method : For very large spans, cantilevering method may be adopted. In this method, the erection starts from the abutment end and the erection of the members ahead is done by using a crane which travels by using the support on the previously erected part structure. Annexure 6 / 5 \(b) and 6 / 6 (a) 32. Incremental launching method : The method is basically a cantilever erection method for PSC bridges. By adopting this method, it is possible to effect economy in construction and ensure the quality due to adoption of factory type production and also ensure quick erection. This method is particularly suitable for launching continuous girders due to site requirements. Incremental launching is a highly mechanised bridge girder erection method. Basically, it consists of manufacturing a prestressed concrete bridge girder segment by segment in a prefabrication area behind one of the abutments. Each new segment is concreted directly against the preceding one and after it has hardened and stressed, the structure is jacked forward by the length of one segment. A steel launching nose is attached in front, to facilitate launching. Gradually the bridge unit is pushed out over the intermediate piers (Annexure 6/6 (b)). In this method the span and depth configuration is to be suitably chosen and the cross section has to be of box or a double T section. The piers should resist forces during launching in excess of those due in the permanent structure. Design has to take into consideration in advance the use of this method as the prestressing section requirements have to suitably allow for the same. The depth of the box girder in relation to the span should be able to cater for the reversal of stress and for shear in the webs without undue congestion of reinforcement and prestressing tendons. The temporary support if used for launching need to stay in place until the bridge launching process has been completed and the final prestressing force applied. Important points to be borne in mind in the construction of PSC girders ----------------------------------------------------------------------- 1. i\) The handling and erection stress. ii. Accessibility of every part of the structure for close inspection. iii. The design of the end block and bearings should permit periodical inspection and servicing of the bearings. 2. Provision shall be made to cater for an additional prestressing forces of 15% of the design prestressing force, for easy installation of prestressing steel at a later date. 3. Admixtures/plasticizers of approved type only should be used. 4. Minimum grade of concrete for PSC work shall be M/35. 5. In all methods of tensioning, the stress induced in the tendons shall be determined by measurement of elongation and also independently by direct measurement of force using a pressure gauge or other means. The two values shall be comparable to each other and the theoretical values within a tolerance of 5%. Calculations for elongations and gauge readings must include appropriate allowances for friction, strand wire slippage and other factors as applicable. Breakage of wires in any one prestressed concrete member shall not exceed 2.5% during tensioning. Wire breakage after anchorage, irrespective of percentage, shall not be condoned without special investigations. 6. Prior to stressing of strands, bottom forms should be kept clean and accuracy of alignment ensured. Form surfaces to be in contact with concrete must be treated with effective release agent. Special care must be exercised to prevent contamination of strands from release agents, grease or other coatings. 7. Cables shall not be left unstressed in ducts for long duration and hence threading of cables in ducts shall be done just prior to stressing. The initial stress due to prestressing in the cable shall not exceed 80% of UTS of the cable. 8. Post tensioning systems shall be installed in accordance with the manufacturer\'s directions and proven procedures. Manufactures\'s recommendations regarding end block details and special arrangements in anchorage zones applicable to their particular system should be observed. 9. Details and positions of ducts : Ferrous metal is recommended for duct material. Aluminium should not be used. Metal ducts must be such that destructive galvanic action on duct and tendon will not occur. 10. As the alignment and position of ducts within the member is critical, short kinks and wobbles shall be avoided. The trajectory of ducts shall not depart from the curve of straight lines shown in the drawing by more than 1 in 240. The cable position shall not deviate by more than 5 mm from the designed trajectory vertically. The area and alignment of ducts shall be such that tendons are free to move within them and there shall be sufficient area left out to permit free passage of grout. 11. Any slack in the prestressing tendon shall first be taken up by applying a small tension. For arriving at the extent of correction and the actual elongation, the procedure given in IS : 1343 shall be followed. The rate of application of load shall be in accordance with manufacturer\'s recommended procedure for post tensioning. Slip must be measured at each end and the extension for the total length. 12. Anchorage : Anchorage devices for all post tensioning systems must be aligned with the direction of the axis of tendons at the point of attachment. Concrete surface, against which the anchorage devices bear must be normal to this line of direction. Accurate measurement of anchorage losses due to slippage or other causes shall be made and compared with the assumed losses shown in the post tensioning schedule and when necessary adjustments or corrections shall be made in the operation. 13. The stressed cables shall be grouted immediately after the prestressing operation for the girder is completed. To avoid possibility of part of the sheathing getting clogged by the over laying concrete, it shall be ensured that the cables move freely inside the sheath during and also after concreting. All precautions shall be taken to ensure that the sheathings do not get contaminated with deleterious chemicals, salts, etc. during the manufacture, storage and installation of the same and they are watertight. 14. Protection to prestressing steel : All prestressing steel shall be free of deleterious materials such as grease, oil, wax, dirt, paint, loose rust, or other similar contaminants that would reduce bond between steel and concrete. Prestressing steel shall not be contaminated with form release agents used on forms or beds. High strength steel is to stored under cover to prevent \`corrosion. Prestressing steel with deeply etched or pitted surface will not be permitted for use in PSC work. However, a light surface rust strongly adhering to the steel is acceptable. Strand surface shall always be inspected prior to placement of concrete and contaminated ones shall be cleaned with an effective solvent. 15. Safety : Large tensioning forces which are necessary to all prestressing operations make such construction very hazardous. It should be ensured that good safety practices are established and that each employee complies with the same. 16. Tensioning of the prestressing steel shall not be commenced until all the necessary tests of the concrete cubes manufactured of the same concrete and cured under the same conditions have been carried out and the results found satisfactory. 612. Quality control in prestressed concrete works --------------------------------------------- 1. Quality control : Ensuring the required standard of quality for prestressed members is a must. The most important factors to be ensured in this connection are : a. Testing and inspection of the various materials selected for use. b. Clear and complete detailed working drawings. c. Accurate stressing procedures. d. Proper control of dimensions and tolerances. e. Proper location of anchors. f. Proper proportioning and adequate mixing of concrete g. Proper handling, placing and consolidation of concrete h. Proper curing i. Proper handling, storing, transporting and erection of members. j. Thorough documentation 2. Cracking of concrete : i. Ensure proper curing ii. Release side forms as soon as practicable. iii. Use hoop steel around tendons near ends of beams. iv. Handle only from designated pick up points. v. Take adequate care during storage, transportation and erection. 3. Camber : 4. Dimensional tolerances may be permitted as provided in IRS Concrete Bridge Code and any other relevant literature. PART C - FABRICATION AND ERECTION OF STEEL GIRDERS ================================================== Preparation for fabrication --------------------------- Trial Shop Erection ------------------- Preparation of surface ---------------------- 616. **Field Erection** 1. General : Plate girders and open web girders are fabricated 2. Erection of plate girders : There is no camber provided in the plate girder and they are erected on a level ground over the platform made up of compacted earth or concrete base. Over this, sleeper or timber pickings at suitable intervals are laid for laying the main members for assembly. After they are laid, levelled and aligned, splicing plates are fixed. The bracings are connected and the joints first provided with bolts. Joint holes are partially filled with drifts for bringing them into proper alignment. 40% of the holes are covered with drifts, after which the bolts are removed one by one and the rivetting done. 3. Erection of triangulated girders : 4. Upto a maximum of 40 percent of the holes of each member of the joint can be filled with drifts and balance with bolts. The holes are generally kept 1.5mm larger than the rivet shanks so that the 5. Adoption of riveted fabrication for plate/composite girders should not be done without prior approval of Board. Welded Girders -------------- Painting of New Girders ----------------------- 619. Choice of a suitable method of girder erection ---------------------------------------------- 5. Several methods are available for girder erection. The following factors generally influence the suitability of a particular erection method: a. Particulars of Bridge and spans. i. Length, width, height, & weight of girder. ii. Number and type of spans. iii. Height and width of piers and abutments iv. Skew or square span. b. Site conditions. i. Type of gap, wet, dry or partly dry. ii. Height of gap. iii. Depth of water, velocity and liability of river to spates or iv. Condition of approaches- high or low banks or cuttings. c. Access to site. d. Availability of bridging equipment and bridging materials. 6. Some of the commonly used erection and launching 620. Preliminary arrangements before girder erection ----------------------------------------------- 7. Collection of site particulars: and bank level should be plotted. Before embarking on preparation of the scheme, a through knowledge of the hydrographs of the river is essential. From the past recorded hydrographs of the river, a curve must be drawn showing the maximum levels during any year on any date; thus the Engineer will know by what time he must complete certain jobs and which of the jobs can be tackled later. When crossing rivers liable to spate, a study of weather conditions should be made so that precautions may be taken to prevent or minimise the damage in the event of sudden rise of water level in the river. Weather warning telegram from Indian Meteorological Department may also be arranged so that the precaution may be taken in case of an expected cyclone which are very frequent in summer months. Further, an anemometer may be installed at the site of work for long triangulated spans. 8. The following arrangements should be made before actual gartering work is started: a. Plan the sequence of erection work. b. Move the girder materials to the site by rail or road. c. Make yard arrangements on one bank of the river or in a nearby station. d. Arrange the plant and equipment necessary to carry out the work alongwith spares. e. Test all the equipment to be used in the erection work. f. First Aid and communication arrangement at site. g. Arrange for consumable stores. h. Arrange for necessary traffic blocks. I. Arrange for a proper organisation with Supervisors, Skilled and Unskilled Staff. site. Erection by use of cranes ------------------------- a. Plate girders of spans upto 30.5 m, built up complete with minimum decking arrangement, can be renewed/placed in position with the help of two cranes. Tentative sequence of regirdering operations is given below: i. To reduce the time of traffic block and to facilitate quick working, all rivets of the cross bracings of the span to be replaced and trough, if existing, should be cut in advance and replaced with bolts. Similarly half of the fish bolts should be removed from the rail joints and suitable speed restriction imposed. ii. The new span, assembled over the approach, is brought over the span to be changed using dip lorries. iii. Two cranes of the suitable capacity are positioned at either ends of the new span and properly supported over the approach/ adjacent span. The new girder is lifted up by both the cranes and the end brackets (temporarily attached on to both ends of girder with bolts) are lightly supported on timber supports over ends of adjacent spans. The diplorries should be removed. iv. The track of the old span and cross bracings of girders are dismantled and removed. v. The old girders are slewed out over the skid rails slowly on either side, care being taken to guard against the toppling of the girder. The bed plates should be positioned on the bed block for the new girder. vi. Temporary brackets bolted at the ends of new girder are removed and the span lowered onto the bed plates. The new span is completely assembled, track laid and connected with the track on the either end. vii. The dip lorries are brought on the new span, old girders lifted one by one with the help of cranes and placed properly on the dip lorries. viii. The cranes are released and moved on to one approach, at the same time, moving the dip lorries loaded with load girders, in the same direction. ix. Old girders are lifted from dip lorries, with the help of cranes and stacked by the side of the track keeping them clear of infringement. The dip lorries are also removed from the track and the block section is cleared. b. A crane with long jib can directly place spans upto 12.2 m. over the piers straight away. Erection with Derricks ---------------------- This method is quiet simple but cumbersome and slow. However, it is quiet useful in the cases where the height of the substructure is less and number of spans are few. The job is carried out by erecting a derrick of sufficient strength and height (out of round pipe or steel lattice structure on wooden post encased in angle frame). Sufficient number of wire rope guys are tightened from the crown plate to anchorages (natural or improvised by the way of dead men) consisting of rail pieces or sleepers buried at sufficient depth in inclined pits tied with wire ropes or chains the other ends of which have been brought out above the ground level and the pits filled up and framed firmly. To these ends of wire ropes or chains are tied the ends of the guy ropes adequately with the help of proper knots and wire rope clamps. The girders are then lifted, one at a time, after properly positioning and slinging with the help of a suitable winch and wire rope pulleys or chain pulley block or wire rope pulled by Manila rope pulley blocks. An additional wire rope will be provided just opposite to the line of action of the load so that extra load can be shared by it. The winch is located and anchored at a suitable place. Maximum precautions have to be taken to ensure rigidity of deadman or anchorages, lifting tackles, knots and fixtures, etc. If the plate girder span happens to be a semithrough one, the cross girders and rail bearers are erected with the help of same derrick after erecting the main girders. However, the main girders would have to be kept slightly wider than the exact centres to accommodate the cross girders. (Annexure 6/7). End launching methods --------------------- This method is normally adopted on new constructions. In this method, the girder is assembled on the approach bank and it is longitudinally traversed over the opening it has to span and lowered in position. For this purpose, a small temporary intermediate staging may be provided if required in the gap between piers for taking the girder across the gap or the existing piers and abutments are utilised. Different techniques are adopted in different situations. Some of them are described below. 9. Launching with rail cluster method: This method can be adopted when the number of spans to be launched are few in number and where the depth of the bed level below H.F.L. is quiet high. This method is not normally used when large number of spans are involved. a. For launching of 12.2 m spans, 2 rails of 90 lbs section are pulled across the openings on either side. After pulling the rails, wooden blocks in the form of distance pieces are inserted at intervals to prevent the tilting of the rails. The bar clamps are also fitted to the rails to prevent the rails spreading out. If the ballast walls at both ends are constructed, then the rail cluster is supported on the sleeper cribs on the abutments and piers. Suitable skids are attached to the underside of the span with the help of clamps and the rails are duly greased throughout the length to avoid excessive friction. The span is then pulled with the help of a winch on the opposite side or a pulling tackle. As soon as the span arrives at the proper position, it is jacked and the rail clusters and skid etc. removed and the span lowered on the bed plates with felt packings. H.D. bolts are inserted and grouted after aligning the span. The skidding method is not normally used where the ballast walls are high as this will involve enormous jacking. This method is used when the girders are to be launched at or near bed block level. (Annexure 6/8 a) b. Where 18.3 m. plate girders are involved and water depth and velocity of flow is not much, the work may be carried out by erecting one or two intermediate trestles in line with the piers and abutments. The top of the trestles are adjusted to the top of bed blocks of piers and abutments. Suitably designed rail clusters are laid and the spans are skidded as explained earlier. (Annexure 6/8/b). 10. Launching by dip lorry method:- This is a safer, more convenient and quicker method for launching of multiple spans upto 18.3m. a. In this method, a cluster of three rails, one on each side are pulled across the opening and temporary track with wooden sleeper is laid. Distance pieces, clamps etc. are properly fitted as suggested in the skidding process above. The fully riveted 12.2m span is then brought onto the approach and one set of dip lorry on either end is inserted under it. The span is then rolled over the temporary track laid on the rail clusters (Annexure 6/8(c)). As soon as the span comes in proper position, it is supported on jacks and the dip lorry and the rail clusters are removed in phases. Finally the span is lowered on to the bed block with the jacks. In case the ballast walls are built earlier, the clusters are laid on suitable wooden cribs. In this case, the girders would have to be lowered for a considerable height. b. This method can also be adopted for plate girders of more than 12.2m span and where it is not possible to erect intermediate trestles due to excessive height of the bed from the rail level. However instead of rail clusters, RSJs 600 x 210 mm in duplicate duly fitted with diaphragms are laid in the openings on either end by cantilever rolling method, spanning the openings. These are braced with channel diaphragms out of channels 250 x 80 x 1320mm. long at suitable intervals. Temporary track is then laid on these RSJ beams with the help of wooden sleepers to coincide with the approach track. Fully rivetted plate girder span mounted on sets of dip lorries is then rolled over this temporary track. As soon as the span comes in position, it is supported on jacks, dip lorries removed, track dismantled and the beams are shifted apart after removing diaphragms and the span lowered on the bed blocks. (Annexure 6/9 a) 11. Cantilever launching of spans by linking (coupling) and rolling: An alternative method avoiding provision of staging under the span is to assemble the spans, arrange them, one behind the other, link them up by temporary links and launch them together. In such a case, the front portion of the girder acts as a cantilever till the nose tip reaches the support at the other end. As such, it has to be designed to take the cantilever stress during launching. A further improvement on this is to provide a launching nose of lighter construction and of adequate length. The spans (12.2m or 18.3 m) to be launched on the bridge are assembled and rivetted up in one line and linked up with the help of suitable splicing. A tie for the first and second span is fitted to avoid unwanted deflection of the leading span. A suitable launching nose is attached in front to reduce the cantilevering weight of the leading span. Rolling platforms are provided on each pier and abutment under each girder of the spans. The spans are then pulled with the help of a winch making use of ordinary rounds as rollers. As soon as the spans occupy the correct position, they are jacked up and lowered on to the bed blocks after removing the tie members and the splices. (Annexure 6/9/b) 12. Launching of girders with the help of a BFR: At locations where access to the new bridge is available by rail, the launching of girders upto 18.3 m with this method is ideal. In this method, the erection tackle consists essentially of a pair of 600 x 210mm joists. The entire beam balances about a central pivot which consists of 600 x 210mm joists. The launching beam has a tackle at its balancing end, with a 3 sheaved pulley fixed on the launching beam and 2 sheaved pulley at the point where the load is taken. There is no counter weight as the advantage is taken of tare weight of the BFR itself. For the BFR to function as a counter weight, bolts are provided between the winch end of the launching beam and the BFR. With this arrangement, a completely assembled, rivetted and sleepered 12.2 m span can be lifted. (Annexure 6/10 a). 13. Erection with the help of launching pad: The BFR method described above can be suitably replaced by a launching pad since the availability of the BFR is rather difficult. The functioning of this launching pad is identical to that of the BFR and consists of combination of grillage beams mounted on 4 half sets of dip lorries, forming a kentledge. A loading trolley in the form of 'A' frame with 4 wheels (poney wheels of a steam engine) is provided in the center as shown in Annexure 6/10 b. A suitable tie is also provided as in the case of BFR. Kentledge of suitable quantity is provided on the grillage beams for additional safety. The launching pad is loaded with suitable wedges and other safety appliances. The girder is then lowered with the help of the winch on the bed blocks to rest on cross rails which are kept provisionally on the bed blocks. The sling is then removed and the launching pad pushed back to lift second girder. In the meantime already launched girder is slewed sideways to its correct position with the help of yale pull. The second girder is then lifted and brought in the opening and lowered with the help of winch. The sling is then removed and the launching pad pushed back. This girder is similarly slewed side ways to its correct position. The bracings are then fitted, the cross rails are removed and the span lowered on the bed plates with felt packings. (Annexure 6/10/b). Side slewing method ------------------- This method of construction or replacement of the superstructure is to erect girders, whether steel (trussed or plate), or precast concrete girders, over temporary supports by the side of the piers, opposite to the span and when ready, slewing the same into the position. For the ease of the movement of the girder, full or part of the deck, if any, is added after the basic girder structure with adequate bracings is slewed in. This method can be gainfully adopted in new construction by erection or casting of girders, simultaneously with the construction of the piers to save considerable time. In case of replacement of girders, similar staging will have to be erected on the other side also for the receiving the old girders and dismantling them into parts before being taken away to stores. In both cases some rolling or sliding arrangements are to be provided between the stagings and piers underneath the girder for the purpose of slewing in and slewing out of the girders. This method can be adopted when the depth of water is not more than 4 to 5m and velocity of current does not exceeds 1m/ sec. Launching of triangulated girders on trestles --------------------------------------------- The spans of 30.5m and above can be launched by making use of trestles. The trestles may consist of starred angles forming a square section. The profile of the pier should be such that a platform could easily be formed. A pair of trestles are fixed in the bed and a platform made between the pier and abutment. This platform consists of cluster of 90 lbs or 52 kg rails. In the cross-wise direction , sleepers can be provided. The new girders should then be assembled from this platform. Gantry girders built up using mostly released steel, should be kept at 3.66 m centers. Steel cribs are made on the abutments and first pier. The gantry girders should initially be brought on the one end approach. Making use of pair of gallow trollies located on the erection platform , the gantries are moved in place. With this gantries, a load can be picked up and moved along the axis of the girder as well as at right angles. Thus the components of the main girders can be brought to any point on the platform. After the first main span is assembled, riveted and the track fixed on it, the gantries could be moved forward by supporting the forward end of the gantries on the gallow trollies located on the platform of opening No. 2 and the end of the gantries on a dip lorry running on a new track of span No.1 (Annexure 6/11a). This process is repeated for all spans. Launching of girders by using service span ------------------------------------------ A service span can be adopted for launching of girders. The service span may be Warren truss with the verticals and cross. On the top of the service span, two numbers 5 tonnes winches are fixed at a distance. The arrangements for lifting the girders with winches consists of taking the load through a system of pulley blocks. This consists of a 2 sheaved pulley on the top and a 3 sheaved pulley at the bottom. The service span has a gantry fitted with rails over which 2 dip lorries run. (Annexure 6/11 b.) A girder yard is provided at the approach of the bridge where the new main girders are assembled , prestressed and rivetted. One girder at a time is brought by a special 'A' frame trolley which supports the girder at the first vertical from each end. The level of the trolley is so adjusted that when the girder end is brought into the service span, the bearing end of the girder could be supported on a dip lorry provided on the gantry. End launching of open web girders with the help of launching nose ----------------------------------------------------------------- This method can be adopted for launching of open web girders when the number of spans are more and the false work can not be erected in the bed. A launching nose fabricated with light sections is connected to the main girder through a suitably designed temporary connection. The launching nose can be made of unit construction members and is assembled on rolling arrangements. The girders are temporarily connected one after the other to act as a counter weight. The whole assembly is pulled from the far bank by winches and pulleys and wire ropes. Similar restraining winches are connected at the rear of girder assembly to control the movement of the girders. As the launching progresses and touching the first pier, the deflection can be neutralised by jacking up the nose and providing adequate packing. The launching is continued till the main girders reach their respective supports. The launching nose can then be dismantled and the girders disconnected from each other. The girders are then lowered on to the bearings, already placed on the piers. (Annexure 6/12a). Erection by cantilevering method -------------------------------- The latest method of launching and erection of large span open web through girders is the cantilevering method, which is being extensively used at present. In this method, an anchor span is first erected on the approach bank adjacent to the abutment to act as kentledge. The erection of girder starts from the abutment and the erection of members ahead is done by using a derrick crane which travels on the top boom of the previously erected part structure. The first and foremost is to design and fabricate a derrick crane with a jib of sufficient length, radius and capacity. The derrick crane is fitted with suitable double flanged wheels, one set on either side, to work on the track fitted on the top booms. A winch is fitted to the rotating platform in the front to revolve the crane for various operations. The rotating is done with the help of wheels, which are working on RSJs, in the circular beam. With the help of winch, the platform rotates either clock or anticlockwise direction as desired. The jib can be raised or lowered with the stay rope and the load is lifted by load line wire rope. (Annexure 6/12 b). Suitable arrangement can also be made to operate the crane electrically. Enveloping method ----------------- This method is suitable for replacing of girders of long spans over large rivers, where it is difficult to erect any temporary staging on the bed or the piers are very tall. The method is very cumbersome and time consuming, and as such is normally adopted only when all other methods of regirdering are not feasible. In this method, the new girder wide enough so as to completely envelop the existing girder is rolled in with the help of a set of trollies or rollers fitted on the top boom of the existing girder. After new girder reaches in correct position, the floor system of the old girder is replaced by the floor system of new girder and the load transferred to the new girder. The old girder is then taken out. In case, the new girders are not designed to be so wide, a temporary enveloping girder is made and the above process is repeated. A standard new girder can then be launched inside the temporary girder and the flooring system replaced with that of standard girder. The temporary enveloping girder is then moved on to the next span. □□□