12 Facts About Circular Prestressing for Civil Engineers

Circular Prestressing is employed to denote the prestressing of circular structures such as pipes and tanks where the prestressing wires are wound in circles.

In contrast to this term, “linear prestressing” is used to include all other types of prestressing, where the cables may be either straight or curved, but not wound in circles around a circular structure.

In most prestressed circular structures, prestress is applied both circumferentially and longitudinally, the circumferential prestress being circular and the longitudinal prestress actually linear.

Circular Prestressing

Define Circular Prestressing

When the prestressed members are curved, in the direction of prestressing, the prestressing is called circular prestressing. For example, circumferential prestressing in pipes, tanks, silos, containment structures and similar structures is a type of circular prestressing. In these structures, there can be prestressing in the longitudinal direction (which is parallel to axis) as well. Circular prestressing is also applied in domes, shells and folded plates.

What are the types of boundary conditions are considered in the analysis of the cylindrical wall?

The following boundary conditions are considered in the analysis of the cylindrical wall:

  • a) For base: fixed or hinged
  • b) For top: free or hinged or framed.

What are the uses of prestressed concrete tanks?

Prestressed concrete tanks have been widely used for the storage of fluids, such as water, oil, gas, sewage, granular materials like cement, process and liquids chemicals, slurries and more recently, cryogens.

By connecting the walls and the base slab what are the developments in the main walls?

The ring tension and bending moments developed in the walls of the tank are mainly influenced by the type of connection between the walls and the base slab.

What are the uses of prestressed concrete poles?

Prestressed concrete poles are currently mass produced and are widely used in most countries for railway power and signal lines, lighting poles, antenna masts, telephone transmission, low and high voltage electric power transmission and substation towers.

What are the advantages of prestressed concrete poles?

The advantages of prestressed concrete poles are:

  • a) Resistance to corrosion in humid and temperate climates and to erosion in desert areas.
  • b) Freeze – thaw resistance in cold regions.
  • c) Easy handling due to less weight than other poles.
  • d) Clean and neat in appearance and requiring negligible maintenance.

What are the advantages of Prestressed concrete Sleeper?

The main advantage of prestressed concrete sleeper is that it has a technical necessity for high-speed and heavy-density tracks from the point of maintainability of track geometry and riding comfort requirements.

What are the types of prestressed concrete sleeper?

The types of prestressed concrete sleeper are:

  • a) Two block sleepers connected by a pipe filled with concrete and containing high tensile bars for compressing the concrete in the blocks.
  • b) Longitudinal sleepers located continuously under the rails and connected by flexible tie bars for gauge retention.
  • c) Beam type single piece prestressed concrete sleepers, which are quite similar to the conventional wooden-type sleeper in shape, length and supporting area.

Mention the importance of shrinkage in composite construction?

The time dependent behavior of composite prestressed concrete beams depends upon the presence of differential shrinkage and creep of the concretes of web and deck, in addition to other parameters, such as relaxation of steel, presence of untensionedsteel, and compression steel.

Write any two general failures of prestressed concrete tanks.

General failures of prestressed concrete tanks are:

  • a) Deformation of the pre-cast concrete units during construction.
  • b) Manufacturing inaccuracies led to out of tolerance units being delivered to the siteunder investigation and may have affected the ability to achieve a good seal.

 

The circumferential prestressing resists the hoop tension generated due to the internal pressure. The prestressing is done by wires or tendons placed spirally, or over sectors of the circumference of the member. The wires or tendons lay outside the concrete core.

Hence, the centre of the prestressing steel (CGS) is outside the core concrete section.

When the prestressed members are curved, in the direction of prestressing, the prestressing is called circular prestressing. For example, circumferential prestressing in pipes, tanks, silos, containment structures and similar structures is a type of circular prestressing.

In these structures, there can be prestressing in the longitudinal direction (parallel to axis) as well. Circular prestressing is also applied in domes and shells.

The circumferential prestressing resists the hoop tension generated due to the internal pressure. The prestressing is done by wires or tendons placed spirally, or over sectors of the circumference of the member. The wires or tendons lay outside the concrete core.

Hence, the centre of the prestressing steel (CGS) is outside the core concrete section. The hoop compression generated is considered to be uniform across the thickness of a thin shell.

Hence, the pressure line (or Cline) lies at the centre of the core concrete section (CGC). The following sketch shows the internal forces under service conditions. The analysis is done for a slice of unit length along the longitudinal direction (parallel to axis).

Liquid retaining structures, such as circular pipes, tanks and pressure vessels are admirably suited for circular prestressing.

The circumferential hoop compression induced in concrete by prestressing counterbalances the hoop tension developed due to the internal fluid pressure.

A reinforced concrete pressure pipe requires a large amount of reinforcement to ensure low-tensile stresses resulting in a crack-free structure.

However, circular prestressing eliminates cracks and provides for an economical use of
materials. In addition, prestressing safeguards against shrinkage cracks in liquid retaining structures.

To reduce the loss of prestress due to friction, the prestressing can be done over sectors of the circumference. Buttresses are used for the anchorage of the tendons. The following sketch shows the buttresses along the circumference.

IS Codes Related to Prestressed Concrete

The codes related to prestressed concrete that are published by the Bureau of Indian Standards are as follows. These are taken from NPTEL.

  1. IS: 784 – 2001 Prestressed Concrete Pipes (including fittings) ‒ Specifications
  2. IS: 1343 – 1980 Code of Practice for Prestressed Concrete.
  3. IS: 1678 – 1998 Specification for Prestressed Concrete Poles for Overhead Power, Traction and Telecommunication Lines
  4. IS: 1785 – 1983 Specification for Plain Hard Drawn Steel Wire for Prestressed Concrete, Part-1: Cold-drawn Stress-relieved Wire, Part-2: As-drawn Wire
  5. IS: 2090 – 1983 Specification for High Tensile Steel Bars used in Prestressed Concrete
  6. IS: 2193 – 1986 Specification for Precast Prestressed Concrete Steel Lighting Poles
  7. IS: 3370 – 1967 Code of Practice for Concrete Structures for Storage of Liquids, Part-3: Prestressed Concrete Structures
  8. IS: 6003 – 1983 Specification for Intended Wire for Prestressed Concrete
  9. IS: 6006 ‒ 1983 Specification for Uncoated Stress Relieved Strand for Prestressed Concrete
  10. IS: 6461 – 1973 Glossary of Terms Relating to Cement Concrete, Part-11: Prestressed Concrete
  11. IS: 10790 – 1984 Methods of Sampling of Steel for Prestressed and Reinforced Concrete, Part-1: Prestressing Steel, Part-2: Reinforcing Steel
  12. IS: 13158 – 1991 Specification for Prestressed Concrete Circular Spun Poles for Overhead Power, Traction and Telecommunication Lines
  13. IS: 14268 – 1995 Specification for Uncoated Stress Relieved Low Relaxation Seven Ply Strand for Prestressed Concrete

The following code related with prestressed concrete is published by the Indian Roads Congress.

IRC: 18 ‒ 2000 Design Criteria for Prestressed Concrete Bridges (Post-tensioned Concrete)

There are other codes published by the Indian Road Congress which are related to the analysis and design of road bridges. When you are moving into a professional career, you have to learn the use of these codes.

Thank You.

12 Facts About Circular Prestressing for Civil Engineers 1
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