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Article based on contribution by: Mr Vivek Goel , Bridge Engineer, Maharashtra
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Tendons
Tendons are a single or group of prestressing
elements and their anchorage assemblies,
which impart a compressive force to a structural member. Also included are
ducts, grouting attachments and grout. The main prestressing element is usually
a high strength steel member made up of a number of strands, wires or bars.
However, sometimes
engineers (mistakenly) use the word “Tendon” for main prestressing element (i.e
strands/wires or bars) only
Wires
A prestressing wire is a single
unit made of steel. The nominal diameters of the wires commonly used are 2.5, 3.0, 4.0, 5.0,
7.0 and 8.0 mm. The different types of wires are as follows.
1) Plain wire: No indentations on
the surface.
2) Indented wire: There are
circular or elliptical indentations on the surface.
Strands
Strand is an assembly of several
high strength steel wires  wound together (on a stranding machine). Strands
usually have six outer wires wound in long-pitch helix around a single straight
wire of a similar diameter.
Different types of strands are as
follows.
1) Two-wire strand: Two wires are
spun together to form the strand.
2) Three-wire strand: Three wires
are spun together to form the strand.
3) Seven-wire strand: In this
type of strand, six wires are spun around a central wire. The central wire is
larger than the other wires.
What
does seven ply strand mean?
Dictionary meaning of Ply is one of the strands twisted together to make yarn or rope or thread; often usedin combination;
"three-ply cord";
"four-ply yarn".
Similarly seven ply strand means that seven wires have been spun
together to form the strand.
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| Strand Pack |
Strand, Wedge & Anchor head |
Cables
With
wire tendons and strands, it may be desirable to form a cable to cope with  the
stressing requirements of large post-tensioning applications. Cables are formed
by arranging wires or strands in bundles with the wires or strands parallel to
each other. In use the cable is placed in a preformed duct in the concrete
member to be stressed and tensioned by a suitable post-tensioning method. Figure on right shows rolls of prestressing cable.
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Bars
A tendon can be made up of a
single steel bar. Post-tensioning bars are high strength steel bars. The
diameter of a bar is much larger than that of a wire. The normally available sizes are from 10mm to 44mm
(5/8 to 1-3/4in) diameter with 10, 12, 16, 20, 22, 25, 28 and 32 mm dias being
more common. The bars are usually
threaded with very coarse thread.
These
bar type tendons are used in certain types of post-tensioning systems. The
alloy steel from which these bars are made has a yield stress in the order of
620 MPa. This gives bar tendons a lower strength to weight ratio than either
wires or strands, but when employed with threaded anchorages has the advantages
of eliminating the possibility of pull-in at the anchorages and of reducing
anchorage costs.
References:
1. California Prestressing Manual
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