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9.6 Circular Prestressing
This section covers the following topics.
• Introduction
• General Analysis and Design
• Prestressed Concrete Pipes
• Liquid Storage Tanks
• Ring Beams
• Conclusion
9.6.1 Introduction
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 C-line) 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).
9.6.3 Prestressed Concrete Pipes
Prestressed concrete pipes are suitable when the internal pressure is within 0.5 to 2.0
N/mm2
. There are two types of prestressed concrete pipes: cylinder type and the noncylinder
type. A cylinder type pipe has a steel cylinder core, over which the concrete is
cast and prestressed. A non-cylinder type of pipe is made of prestressed concrete only.
IS:784 - 2001 (Prestressed Concrete Pipes (Including Specials) - Specification)
provides guidelines for the design of prestressed concrete pipes with the internal
diameter ranging from 200 mm to 2500 mm. The pipes are designed to withstand the
combined effect of internal pressure and external loads. The minimum grade of
concrete in the core should be M40 for non-cylinder type pipes.
First, the core is cast either by the centrifugal method or by the vertical casting method.
In the centrifugal method the mould is subjected to spinning till the concrete is
compacted to a uniform thickness throughout the length of the pipe. In the vertical
casting method, concrete is poured in layers up to a specified height.
After adequate curing of concrete, first the longitudinal wires are prestressed (the wires
can be pre-tensioned). Subsequently, the circumferential prestressing is done by the
wire wound around the core in a helical form. The wire is wound using a counter weight
or a die. Finally a coat of concrete or rich cement mortar is applied over the wire to
prevent from corrosion.
For cylinder type pipes, first the steel cylinder is fabricated and tested. Then the
concrete is cast around it.
The analysis and design of prestressed concrete pipes consider the stresses due to the
different actions. A horizontal layout of the pipe is considered to illustrate them.
Analysis
The stresses in the longitudinal direction are due to the following actions.
1. Longitudinal prestressing (fl1)
2. Circumferential prestressing (fl2)
3. Self weight (fl3)
4. Transport and handling (fl4)
5. Weight of fluid (fl5)
6. Weight of soil above (fl6)
Longitudinal prestressing
The longitudinal prestressing generates a uniform compression
Liquid Storage Tanks
In the construction of concrete structures for the storage of liquids, the imperviousness
of concrete is an important basic requirement. Hence, the design of such construction
is based on avoidance of cracking in the concrete. The structures are prestressed to
avoid tension in the concrete. In addition, prestressed concrete tanks require low
maintenance. The resistance to seismic forces is also satisfactory.
Prestressed concrete tanks are used in water treatment and distribution systems, waste
water collection and treatment system and storm water management. Other
applications are liquefied natural gas (LNG) containment structures, large industrial
process tanks and bulk storage tanks.
The construction of the tanks is in the following sequence. First, the concrete core is
cast and cured. The surface is prepared by sand or hydro blasting. Next, the
circumferential prestressing is applied by strand wrapping machine. Shotcrete is
applied to provide a coat of concrete over the prestressing strands.
Analysis
The analysis of liquid storage tanks can be done by IS:3370 - 1967, Part 4, or by the
finite element method. The Code provides coefficients for bending moment, shear and
hoop tension (for cylindrical tanks), which were developed from the theory of plates and
shells. In Part 4, both rectangular and cylindrical tanks are covered. Since circular
prestressing is applicable to cylindrical tanks, only this type of tank is covered in this
module.
The following types of 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.
The applicability of each boundary condition is explained next.
For base
Fixed: When the wall is built continuous with its footing, then the base can be
considered to be fixed as the first approximation.
Hinged: If the sub grade is susceptible to settlement, then a hinged base is a
conservative assumption. Since the actual rotational restraint from the footing is
somewhere in between fixed and hinged, a hinged base can be assumed.
The base can be made sliding with appropriate polyvinyl chloride (PVC) water-stops for
liquid tightness.
For top
Free: The top of the wall is considered free when there is no restraint in expansion.
Hinged: When the top is connected to the roof slab by dowels for shear transfer, the
boundary condition can be considered to be hinged.