Design Requirements for Prestressed Concrete

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Questions and Answers

What percentage of design prestressing force should be catered for easy installation of prestressing steel at a later date?

20%

25%

15% (correct)

10%

What is the minimum grade of concrete required for PSC work?

M/30

M/25

M/40

M/35 (correct)

How should the stress induced in the tendons be determined?

By calculations only

Only by measurement of elongation

Only by direct measurement of force using a pressure gauge

By both measurement of elongation and direct measurement of force using a pressure gauge (correct)

What is the maximum percentage of wire breakage allowed during tensioning in any one prestressed concrete member?

2.5% Signup and view all the answers

Why should bottom forms be kept clean and accuracy of alignment ensured prior to stressing of strands?

To ensure proper bonding between concrete and strands Signup and view all the answers

What is the maximum initial stress allowed in the cable due to prestressing?

80% of UTS Signup and view all the answers

How should post tensioning systems be installed?

According to both the manufacturer's directions and proven procedures Signup and view all the answers

Why should cables not be left unstressed in ducts for long durations?

To prevent loss of prestressing force Signup and view all the answers

What is the primary concern while removing support in PSC works?

Imparting sufficient prestress to the member Signup and view all the answers

What is the recommended method for compacting concrete?

Using mechanical vibrators complying with IS codes Signup and view all the answers

What is the significance of curing concrete in bridge construction?

To ensure maximum durability and minimize cracking Signup and view all the answers

What is the purpose of submitting equipment, material, and mix proportions to the engineer before starting the work?

To obtain approval from the engineer Signup and view all the answers

What should be avoided while depositing concrete in its final position?

Rehandling of concrete Signup and view all the answers

What is the recommended practice for distributing vibrators during compaction?

Distributing them uniformly to achieve a dense and plastic mass Signup and view all the answers

What is the purpose of vibrating concrete during compaction?

To achieve a uniform, dense, and plastic mass Signup and view all the answers

What is the significance of following the IRS Concrete Bridge Code in concrete bridge construction?

To ensure compliance with industry standards Signup and view all the answers

What is crucial to prevent cracking of concrete?

Ensure proper curing and handle with care during storage, transportation and erection Signup and view all the answers

What is the purpose of using sleeper or timber pickings in plate girder erection?

To support the main members for assembly Signup and view all the answers

How should joint holes be filled during plate girder erection?

Partially filled with drifts for alignment and then filled with bolts Signup and view all the answers

What is the maximum percentage of holes that can be filled with drifts in each member of the joint?

40% Signup and view all the answers

What is the required tolerance for holes in riveted fabrication?

1.5mm larger than the rivet shanks Signup and view all the answers

What is necessary for riveted fabrication for plate/composite girders?

Prior approval of the Board Signup and view all the answers

What is essential for proper proportioning and adequate mixing of concrete?

Proper handling, placing and consolidation of concrete Signup and view all the answers

What is necessary for proper handling, placing and consolidation of concrete?

Proper proportioning and adequate mixing of concrete Signup and view all the answers

What material is recommended for ducts in post-tensioning systems?

Ferrous metal Signup and view all the answers

What is the maximum deviation allowed for the trajectory of ducts from the curve of straight lines?

1 in 240 Signup and view all the answers

How much can the cable position deviate from the designed trajectory vertically?

5 mm Signup and view all the answers

What is the purpose of applying a small tension to the prestressing tendon?

To take up any slack in the tendon Signup and view all the answers

What is the importance of grouting the stressed cables immediately after the prestressing operation?

To ensure the cables move freely inside the sheath Signup and view all the answers

How should anchorage devices for post-tensioning systems be aligned?

Aligned with the direction of the axis of tendons Signup and view all the answers

What should be done if the anchorage losses due to slippage or other causes are more than the assumed losses?

Make adjustments or corrections in the operation Signup and view all the answers

Why is it important to ensure the cables move freely inside the sheath during and after concreting?

To avoid the possibility of part of the sheathing getting clogged by the overlying concrete Signup and view all the answers

What is essential to ensure when manufacturing, storing, and installing sheathings?

They are not contaminated with chemicals and salts Signup and view all the answers

What should be avoided when storing high-strength prestressing steel?

Storing it in open areas Signup and view all the answers

What is an acceptable surface condition for prestressing steel?

Light surface rust strongly adhering to the steel Signup and view all the answers

When can tensioning of the prestressing steel commence?

After the necessary tests of the concrete cubes have been carried out and found satisfactory Signup and view all the answers

What is a crucial aspect of ensuring quality control in prestressed concrete works?

Testing and inspection of materials Signup and view all the answers

Why is it important to establish good safety practices in prestressed concrete construction?

To prevent injuries due to large tensioning forces Signup and view all the answers

What should be inspected prior to placement of concrete?

The prestressing steel strands Signup and view all the answers

Why should prestressing steel be free of deleterious materials?

To ensure proper bond between steel and concrete Signup and view all the answers

What is the primary focus of the IRS Concrete Bridge Code while curing concrete in order to ensure maximum durability?

To prevent cracking and ensure maximum durability Signup and view all the answers

Why is it necessary to submit the equipment, material, and mix proportions to the engineer before starting the work?

To ensure the quality of the materials used Signup and view all the answers

What is the recommended method for transporting concrete from the mixer to the form work?

By taking necessary precautions to prevent segregation and loss of ingredients Signup and view all the answers

What is the primary purpose of compacting concrete using vibration?

To remove air pockets and achieve uniform density Signup and view all the answers

Why is it necessary to ensure that the concrete is deposited as nearly as practicable in its final position?

To prevent rehandling and disturbance of the concrete Signup and view all the answers

What is the recommended practice for distributing vibrators during compaction?

To distribute them uniformly to achieve a dense and plastic mass Signup and view all the answers

What is the purpose of ensuring that the support is not removed till sufficient prestress has been imparted to the member in PSC works?

To prevent disturbance of the concrete during setting Signup and view all the answers

What is the significance of following the IRS Concrete Bridge Code in concrete bridge construction?

To ensure the quality and durability of the concrete bridge Signup and view all the answers

What is the primary purpose of providing an additional wire rope opposite to the line of action of the load?

To share the extra load and ensure rigidity Signup and view all the answers

What is the typical scenario where the rail cluster method is adopted for launching girders?

When the depth of the bed level below H.F.L is quiet high Signup and view all the answers

What is the purpose of inserting wooden blocks in the form of distance pieces during the rail cluster method?

To prevent the tilting of the rails Signup and view all the answers

What is the advantage of using intermediate staging during the end launching method?

It enables the girder to be launched over a larger gap Signup and view all the answers

What is the primary consideration when constructing multiple span arch bridges?

Ensuring the horizontal thrust on the pier/abutment is not excessive Signup and view all the answers

What is the purpose of commencing the work on the adjacent span in multiple span arch bridge construction?

To bridge some load to bear on the pier before the support and the framework used on the previously cast span is removed Signup and view all the answers

Why are the main girders kept slightly wider than the exact centres during plate girder erection?

To accommodate the cross girders Signup and view all the answers

What is the primary consideration for ensuring the rigidity of deadman or anchorages?

Maximum precautions to ensure rigidity Signup and view all the answers

What is the recommended method for supporting stagings over dry beds of streams?

Constructing stagings from the bed itself Signup and view all the answers

What is the typical location of the winch during plate girder erection?

At a suitable place, anchored securely Signup and view all the answers

Why is it important to check the stagings at various stages during the construction of an arch bridge?

To prevent the stagings from settling under the load when the casting of the superstructure is in progress Signup and view all the answers

What is the purpose of providing intermediate ribs on piers in high viaducts?

To support the temporary floor system over which the false work can be put up Signup and view all the answers

What is the purpose of bar clamps in the rail cluster method?

To prevent the rails from spreading out Signup and view all the answers

What is the recommended method for supporting stagings in flowing water?

Using shallow thin piles driven into the sand bed Signup and view all the answers

What is the primary consideration when constructing arch bridges over streams?

Taking due care to support the staging columns on the bed by giving a suitable timber support Signup and view all the answers

What is the purpose of providing a suitable timber support to spread the load when constructing arch bridges over streams?

To prevent the stagings from settling under the load Signup and view all the answers

What is the primary function of the launching beam in the BFR method?

To balance the entire beam about a central pivot Signup and view all the answers

What is the purpose of providing bolts between the winch end of the launching beam and the BFR?

To take advantage of the tare weight of the BFR as a counterweight Signup and view all the answers

What is the main advantage of using a launching pad over the BFR method?

Easier availability of the launching pad compared to the BFR Signup and view all the answers

What is the purpose of providing a loading trolley in the form of an 'A' frame in the launching pad method?

To provide additional safety features Signup and view all the answers

What is the purpose of providing kentledge on the grillage beams in the launching pad method?

To provide additional safety features Signup and view all the answers

What is the primary function of the rolling platforms in the launching of girders?

To pull the spans with the help of a winch Signup and view all the answers

What is the purpose of jacking up and lowering the spans onto the bed blocks after removing the tie members and the splices?

To occupy the correct position of the spans Signup and view all the answers

What is the maximum length of girder that can be launched with the help of a BFR?

18.3 m Signup and view all the answers

What is the primary purpose of the platform made between the pier and abutment?

To assemble the new girders Signup and view all the answers

What is the weight of the rails used in the platform, in kilograms?

52 kg Signup and view all the answers

How are the gantry girders initially brought on the one end approach?

Using a pair of gallow trollies Signup and view all the answers

What is the purpose of the dip lorry running on a new track of span No.1?

To support the forward end of the gantries Signup and view all the answers

What is the type of truss used in the service span?

Warren truss Signup and view all the answers

What is the purpose of the 2 sheaved pulley on the top and the 3 sheaved pulley at the bottom?

To take the load through a system of pulley blocks Signup and view all the answers

Where are the new main girders assembled, prestressed, and riveted?

In the girder yard Signup and view all the answers

What is the purpose of the dip lorries running on a new track of span No.1?

To support the forward end of the gantries Signup and view all the answers

Study Notes

Prestressed Concrete (PSC) Works

The design of the end block and bearings should permit periodic inspection and servicing of the bearings.
Provision should be made to cater for an additional prestressing force of 15% of the design prestressing force, for easy installation of prestressing steel at a later date.
Admixtures/plasticizers of approved type only should be used, and the minimum grade of concrete for PSC work shall be M/35.

Tensioning of Prestressing Steel

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.

Preparation and Installation of Cables

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 an effective release agent.
Special care must be exercised to prevent contamination of strands from release agents, grease, or other coatings.
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.

Post-Tensioning Systems

Post-tensioning systems shall be installed in accordance with the manufacturer's directions and proven procedures.
Manufacturers' recommendations regarding end block details and special arrangements in anchorage zones applicable to their particular system should be observed.

Ducts and Anchorage

Ferrous metal is recommended for duct material, and Aluminium should not be used.
Metal ducts must be such that destructive galvanic action on duct and tendon will not occur.
As the alignment and position of ducts within the member are 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.

Grouting and Anchorage

The stressed cables shall be grouted immediately after the prestressing operation for the girder is completed.
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.

Concrete and Curing

The method of curing shall be as per IRS Concrete Bridge Code.
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.
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.

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.
Vibrators shall be distributed so that the concrete becomes uniformly dense and plastic mass.
Vibrators shall be used for compaction only and not for moving concrete horizontally along the form.

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 be stored under cover to prevent corrosion.

Safety and Quality Control

Good safety practices should be established and each employee complies with the same.
Quality control is essential to ensure the required standard of quality for prestressed members.
The most important factors to be ensured in this connection 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

Form Work Requirements

Form work requirements 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.

Curing of Concrete

Special attention shall be given to curing of concrete to ensure maximum durability and minimize cracking.
The method of curing shall be as per IRS Concrete Bridge Code.

Concrete Mixing and Transportation

The equipment, material, and proportions of the mix to be used shall be submitted to and approved by the engineer before the work is started.
No segregation shall occur while transporting concrete from the mixer to the form work, and necessary precautions shall be taken to ensure this.
Concrete shall be deposited as nearly as practicable in its final position without rehandling, and compacted before setting commences.

Compaction of Concrete

All concrete shall be compacted by vibration, generally using internal vibration on all sections that are sufficiently large to admit them.
Mechanical vibrators complying with IS:2505, IS:2506, IS:2514, and IS:4656 shall be used for compacting concrete.

Work on Single Span

The construction of a single span arch bridge involves providing stagings or temporary support underneath and putting up the arch above.
Back filling of abutments must be ensured before taking up the construction of the arch.

Work on Multiple Spans

Work can be done simultaneously on a number of spans using more than one set of forms.
Due care shall be taken to ensure that the horizontal thrust on the pier/abutment is not such that they give way.

Supporting Arrangement for Arches

Over dry beds of streams, stagings can be constructed from the bed itself.
Supporting the staging columns on the bed by giving a suitable timber support to spread the load and to check the stagings at various stages.
If the work has to be carried out in flowing water, the staging will have to be supported over shallow thin piles driven into the sand bed.

Launching Methods

End launching methods involve assembling the girder on the approach bank and traversing it over the opening it has to span.
Different techniques are adopted in different situations, such as launching with rail cluster method, launching with the help of a BFR, and launching with the help of a launching pad.

Launching with Rail Cluster Method

This method involves pulling two rails across the openings on either side and inserting wooden blocks in the form of distance pieces at intervals to prevent tilting.

Launching with the Help of a BFR

This method involves using a pair of 600 x 210mm joists to balance the beam about a central pivot.
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.

Launching with the Help of a Launching Pad

The launching pad consists of a combination of grillage beams mounted on 4 half sets of dip lorries, forming a kentledge.
A loading trolley in the form of an 'A' frame with 4 wheels is provided in the center.

Erection with the Help of Gantry Girders

Gantry girders are built up using mostly released steel, and kept at 3.66 m centers.
Steel cribs are made on the abutments and first pier, and the gantry girders are moved in place using a pair of gallow trollies.

Launching of Girders by Using Service Span

A service span can be adopted for launching of girders, and consists of a Warren truss with verticals and cross.
The service span has a gantry fitted with rails over which 2 dip lorries run.

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Description

This quiz covers the essential design requirements for Prestressed Concrete (PSC) work, including provisions for bearing inspection, additional prestressing forces, and concrete grade. It also touches on tensioning methods and stress induction in tendons.

Design Requirements for Prestressed Concrete

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Questions and Answers

What percentage of design prestressing force should be catered for easy installation of prestressing steel at a later date?

What is the minimum grade of concrete required for PSC work?

How should the stress induced in the tendons be determined?

What is the maximum percentage of wire breakage allowed during tensioning in any one prestressed concrete member?

Why should bottom forms be kept clean and accuracy of alignment ensured prior to stressing of strands?

What is the maximum initial stress allowed in the cable due to prestressing?

How should post tensioning systems be installed?

Why should cables not be left unstressed in ducts for long durations?

What is the primary concern while removing support in PSC works?

What is the recommended method for compacting concrete?

What is the significance of curing concrete in bridge construction?

What is the purpose of submitting equipment, material, and mix proportions to the engineer before starting the work?

What should be avoided while depositing concrete in its final position?

What is the recommended practice for distributing vibrators during compaction?

What is the purpose of vibrating concrete during compaction?

What is the significance of following the IRS Concrete Bridge Code in concrete bridge construction?

What is crucial to prevent cracking of concrete?

What is the purpose of using sleeper or timber pickings in plate girder erection?

How should joint holes be filled during plate girder erection?

What is the maximum percentage of holes that can be filled with drifts in each member of the joint?

What is the required tolerance for holes in riveted fabrication?

What is necessary for riveted fabrication for plate/composite girders?

What is essential for proper proportioning and adequate mixing of concrete?

What is necessary for proper handling, placing and consolidation of concrete?

What material is recommended for ducts in post-tensioning systems?

What is the maximum deviation allowed for the trajectory of ducts from the curve of straight lines?

How much can the cable position deviate from the designed trajectory vertically?

What is the purpose of applying a small tension to the prestressing tendon?

What is the importance of grouting the stressed cables immediately after the prestressing operation?

How should anchorage devices for post-tensioning systems be aligned?

What should be done if the anchorage losses due to slippage or other causes are more than the assumed losses?

Why is it important to ensure the cables move freely inside the sheath during and after concreting?

What is essential to ensure when manufacturing, storing, and installing sheathings?

What should be avoided when storing high-strength prestressing steel?

What is an acceptable surface condition for prestressing steel?

When can tensioning of the prestressing steel commence?

What is a crucial aspect of ensuring quality control in prestressed concrete works?

Why is it important to establish good safety practices in prestressed concrete construction?

What should be inspected prior to placement of concrete?

Why should prestressing steel be free of deleterious materials?

What is the primary focus of the IRS Concrete Bridge Code while curing concrete in order to ensure maximum durability?

Why is it necessary to submit the equipment, material, and mix proportions to the engineer before starting the work?

What is the recommended method for transporting concrete from the mixer to the form work?

What is the primary purpose of compacting concrete using vibration?

Why is it necessary to ensure that the concrete is deposited as nearly as practicable in its final position?

What is the recommended practice for distributing vibrators during compaction?

What is the purpose of ensuring that the support is not removed till sufficient prestress has been imparted to the member in PSC works?

What is the significance of following the IRS Concrete Bridge Code in concrete bridge construction?

What is the primary purpose of providing an additional wire rope opposite to the line of action of the load?

What is the typical scenario where the rail cluster method is adopted for launching girders?

What is the purpose of inserting wooden blocks in the form of distance pieces during the rail cluster method?

What is the advantage of using intermediate staging during the end launching method?

What is the primary consideration when constructing multiple span arch bridges?

What is the purpose of commencing the work on the adjacent span in multiple span arch bridge construction?

Why are the main girders kept slightly wider than the exact centres during plate girder erection?

What is the primary consideration for ensuring the rigidity of deadman or anchorages?

What is the recommended method for supporting stagings over dry beds of streams?

What is the typical location of the winch during plate girder erection?

Why is it important to check the stagings at various stages during the construction of an arch bridge?

What is the purpose of providing intermediate ribs on piers in high viaducts?

What is the purpose of bar clamps in the rail cluster method?

What is the recommended method for supporting stagings in flowing water?

What is the primary consideration when constructing arch bridges over streams?

What is the purpose of providing a suitable timber support to spread the load when constructing arch bridges over streams?

What is the primary function of the launching beam in the BFR method?

What is the purpose of providing bolts between the winch end of the launching beam and the BFR?

What is the main advantage of using a launching pad over the BFR method?

What is the purpose of providing a loading trolley in the form of an 'A' frame in the launching pad method?

What is the purpose of providing kentledge on the grillage beams in the launching pad method?

What is the primary function of the rolling platforms in the launching of girders?

What is the purpose of jacking up and lowering the spans onto the bed blocks after removing the tie members and the splices?

What is the maximum length of girder that can be launched with the help of a BFR?

What is the primary purpose of the platform made between the pier and abutment?

What is the weight of the rails used in the platform, in kilograms?

How are the gantry girders initially brought on the one end approach?

What is the purpose of the dip lorry running on a new track of span No.1?