Questions and Answers
Apart from functional requirements, what merits serious consideration in the construction of piers and abutments according to the content?
What is the minimum recommended proportion of cement mortar used for stone masonry?
1:4
Construction of tall reinforced concrete piers can be efficiently done with the slip form construction. Is this statement true?
True
Construction in brick and stone masonry should follow the relevant specifications prescribed by the _____________.
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What supporting arrangement can be constructed over dry beds of streams for arches?
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How many segments should the arch ring or barrel be cast in?
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It is recommended to use ferrous metal for duct material in the construction of PSC girders.
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In incremental launching method for PSC bridges, every new segment is concreted directly against the __________ one.
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What type of surface rust is acceptable on prestressing steel for PSC work?
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Tensioning of prestressing steel can begin before tests on the concrete cubes have been carried out.
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Quality control in prestressed concrete works includes proper handling, placing, and consolidation of ________.
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Match the following factors with their importance in quality control for prestressed concrete:
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When is the enveloping method suitable for replacing girders?
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Why is the enveloping method normally adopted?
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The enveloping method is a fast and straightforward process.
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What method can be used for launching multiple spans up to 18.3m with a cluster of three rails?
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What is inserted under the fully riveted 12.2m span in the dip lorry method?
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The method of launching girders with the help of a BFR requires a counterweight.
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____ method of construction involves girders being erected over temporary supports by the side of the piers and then slewing into position.
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Match the launching method with its description:
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Study Notes
Importance of Aesthetics in Construction
- Aesthetics is important in construction, considering the piers and abutments are exposed to view.
- The design of the bridge should provide a pleasing view and harmony with the surroundings.
Material of Construction
- For stone masonry, the proportion of cement mortar used should be minimum 1:4.
- For mass concrete, the mix should be minimum M.20 grade and preferably design mix using 40 mm aggregate.
- For reinforced cement concrete, the mix should be minimum M-25 grade.
- Prestressed cement concrete can be used for all piers of viaducts, with the mix adopted according to design requirements.
Piers, Abutments, Wing Walls, and Approach Slabs
- The size of piers and abutments depends on the construction materials used.
- Masonry piers are provided with a batter varying from 1 in 24 to 1 in 12.
- For masonry abutments, a front batter of 1 in 16 to 1 in 10 is used.
- Reinforced cement concrete piers can be constructed using slip form construction.
Construction Aspects
- In water construction, coffer dams or temporary sheet piles may be used to divert the water.
- Slip form construction can be used for tall reinforced concrete piers.
- Weep holes should be provided in abutments at vertical and horizontal intervals of 1 m.
Construction of Substructure and Superstructure
- As regards construction in brick and stone masonry, the relevant specification prescribed by the Chief Engineer shall be followed.
- Concreting shall conform to the requirements specified in IRS Concrete Bridge Code and IS.456.
- Weigh batching is mandatory for PSC works.
Reinforcement and Prestressing
- Reinforcement steel shall conform to one of the following specifications: Grade I mild steel, cold twisted bars, or rolled steel.
- Prestressing steel shall be one of the following: plain hard drawn steel wire, high tensile steel bar, or uncoated stress relieved strand.
- Reinforcement steel shall be free of loose mill scales, loose rust, and coats of oil, mud, or other material.
Aggregates and Water
- Aggregates conforming to IS: 383 shall only be used.
- Aggregates shall be clean, and marine aggregates shall not be used unless thoroughly washed.
- 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.
Form Work and Curing
- Form work requirement shall be as per IRS Concrete Bridge Code.
- Support shall not be removed till sufficient prestress has been imparted to the member.
- Curing of concrete is essential to ensure maximum durability and minimize cracking.
Compaction and Bearing Areas
- All concrete shall be compacted by vibration.
- Bearing areas for members shall be finished to a true plane to give uniform bearing on the entire area.
Field Laboratory
- In major works, a field laboratory should be set up at the work site, equipped with necessary equipment to carry out various tests.
Slab Bridges
- Slabs shall preferably be precast in a depot and installed at the site.
- PSC slabs may be used for spans larger than 3.05 m.
Arch Bridges
- The construction of arch bridges is done by providing stagings or temporary support underneath and putting up the arch above.
- Supporting arrangement for arches: Over dry beds of streams, stagings can be constructed from the bed itself.
RCC/PSC Bridges
- In case of slab and beam bridges, the easiest method would be to use cribs and supports from below and cast them in situ.
- The alternative method is to launch and erect a temporary girder supported on the ground or an intermediate projection from the pier.
Erection of PSC Girders
- Erection by use of launching girders: Fully cast prestressed concrete girders are not launched independently.
- Erection by cantilevering method: For very large spans, the cantilevering method may be adopted.
- Incremental launching method: The method is suitable for launching continuous girders due to site requirements.### Prestressed Concrete
- In all methods of tensioning, stress induced in tendons must be determined by measurement of elongation and direct measurement of force using a pressure gauge or other means.
- The two values must 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.
Prior to Stressing of Strands
- Bottom forms must be kept clean and accuracy of alignment ensured.
- Form surfaces to be in contact with concrete must be treated with an effective release agent.
- Special care must be exercised to prevent contamination of strands from release agents, grease, or other coatings.
Post-Tensioning Systems
- Systems must be installed in accordance with the manufacturer's directions and proven procedures.
- Manufacturer's recommendations regarding end block details and special arrangements in anchorage zones applicable to their particular system must be observed.
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.
- The trajectory of ducts must not depart from the curve of straight lines shown in the drawing by more than 1 in 240.
Prestressing Tendon
- Any slack in the prestressing tendon must first be taken up by applying a small tension.
- The procedure given in IS: 1343 must be followed for arriving at the extent of correction and the actual elongation.
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.
Grouting
- Stressed cables must be grouted immediately after the prestressing operation for the girder is completed.
- Precautions must be taken to ensure that the sheathings do not get contaminated with deleterious chemicals, salts, etc.
Protection of Prestressing Steel
- All prestressing steel must be free of deleterious materials.
- Prestressing steel must not be contaminated with form release agents used on forms or beds.
- High-strength steel must be stored under cover to prevent corrosion.
Safety
- Good safety practices must be established and each employee must comply with the same.
Quality Control
- Ensuring the required standard of quality for prestressed members is a must.
- The most important factors to be ensured are:
- Testing and inspection of materials.
- Clear and complete detailed working drawings.
- Accurate stressing procedures.
- Proper control of dimensions and tolerances.
- Proper location of anchors.
- Proper proportioning and adequate mixing of concrete.
- Proper handling, placing, and consolidation of concrete.
- Proper curing.
- Proper handling, storing, transporting, and erection of members.
- Thorough documentation.
Cracking of Concrete
- Ensure proper curing.
- Release side forms as soon as practicable.
- Use hoop steel around tendons near ends of beams.
- Handle only from designated pick-up points.
- Take adequate care during storage, transportation, and erection.
Camber
- No camber is provided in the plate girder.
Field Erection
- Plate girders and open web girders are fabricated.
- 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.
- Erection of triangulated girders: Not mentioned.
Erection by Use of Cranes
- Plate girders of spans up to 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.
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 the number of spans are few.
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.
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.
Launching by Dip Lorry Method
- This is a safer, more convenient, and quicker method for launching of multiple spans up to 18.3m.
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.
Launching of Girders with the Help of a BFR
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At locations where access to the new bridge is available by rail, the launching of girders up to 18.3 m with this method is ideal.### Launching Beam Method
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The launching beam method uses a central pivot consisting of 600 x 210mm joists.
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A tackle is attached to the balancing end of the launching beam, featuring a 3 sheaved pulley and a 2 sheaved pulley at the load point.
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No counterweight is used, as the tare weight of the BFR (Bridge Falsework) is utilized instead.
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Bolts are provided between the winch end of the launching beam and the BFR to enable the BFR to function as a counterweight.
Launching Pad Method
- The launching pad method is an alternative to the BFR method, consisting of a combination of grillage beams mounted on 4 half sets of dip lorries.
- A loading trolley in the form of an 'A' frame is provided, featuring 4 wheels (poney wheels of a steam engine).
- A suitable tie is also provided, along with kentledge of suitable quantity for additional safety.
- The launching pad is loaded with suitable wedges and other safety appliances.
Side Slewing Method
- This method involves erecting girders over temporary supports by the side of the piers, opposite to the span.
- When ready, the girders are slewed into position using full or part of the deck, with adequate bracings.
- This method can be used for new construction or replacement of girders, simultaneously with pier construction to save time.
Launching of Triangulated Girders on Trestles
- This method is used for launching spans of 30.5m and above, using trestles consisting of starred angles forming a square section.
- The trestles are fixed in the bed, and a platform is made between the pier and abutment, featuring a cluster of 90 lbs or 52 kg rails.
- The new girders are assembled on the platform, using mostly released steel, and steel cribs are made on the abutments and first pier.
Launching of Girders by Using Service Span
- A service span can be adopted for launching girders, featuring a Warren truss with verticals and cross.
- Two 5-tonne winches are fixed on top of the service span, at a distance, and a system of pulley blocks is used for lifting the girders.
- The service span has a gantry fitted with rails, over which 2 dip lorries run.
End Launching of Open Web Girders with the Help of Launching Nose
- This method is used for launching open web girders when the number of spans is more, and false work cannot be erected in the bed.
- A launching nose is connected to the main girder through a temporary connection, and the whole assembly is pulled from the far bank by winches and pulleys.
- The launching nose can be made of unit construction members, assembled on rolling arrangements.
Erection by Cantilevering Method
- This method is used for launching and erecting large span open web through girders.
- An anchor span is first erected on the approach bank adjacent to the abutment, to act as kentledge.
- The erection of the girder starts from the abutment, and the erection of members ahead is done using a derrick crane.
Enveloping Method
- This method is used for replacing 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 new girder is rolled in with the help of a set of trollies or rollers fitted on the top boom of the existing girder.
- The new girder envelops the existing girder, and the floor system of the old girder is replaced by the floor system of the new girder.
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Description
Learn about the importance of aesthetics in construction and the different materials used, including stone masonry, mass concrete, and reinforced cement concrete.
