Civil Engineering Reference
In-Depth Information
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if large tendons are to be anchored in blisters within a box section deck, they will
require a substantial volume of concrete and weight of reinforcement;
powerful anchors require a considerable weight of additional equilibrium steel in
the deck behind the anchor;
small anchors may need very little equilibrium steel, if any, as existing reinforcement
may prove adequate;
the webs and slabs of the concrete section must be thick enough to resist the forces
transmitted by large anchors;
large anchorage blisters may signifi cantly complicate the internal shutter,
particularly if it is mechanised for rapid construction, as in the precast segmental
system;
for tendons that are external to the concrete, the placing of ducts is likely to be a
critical activity for the construction programme, leading to the use of fewer, more
powerful tendons.
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In general, for internally prestressed decks, inexperienced designers tend to use
tendons that are too powerful, attracted by the simplicity of the profi les. The diffi culties
in housing and anchoring these tendons, with the attendant increase in thickness of
members and in the weight of reinforcement, come as a surprise later in the design
process. As a general rule, for internally prestressed decks, it is advisable to consider
tendons of modest size, with 19 No 15 mm or 27 No 13 mm strands as an upper limit.
There are of course exceptions, when the use of more powerful tendons may solve
problems and simplify construction.
For externally prestressed decks, where the tendons are anchored on pier diaphragms,
tendons up to 37 No 15 mm strands are used frequently. If the tendons are anchored
on blisters, smaller sizes will generally be chosen to limit the cost of the blisters and of
their associated reinforcement.
6.15 Calculating the prestress force
6.15.1 General
With the statically determinate beam, it was possible to calculate the prestress force
directly at any location along the beam; the only data required were the total bending
moment, the eccentricity of the prestress centroid and the geometric properties of the
cross section. For continuous beams, one cannot calculate the prestress force directly,
as the total bending moment includes the prestress parasitic moment. This parasitic
moment depends on the prestress force and on the locus of the prestress centroid
along the beam. Consequently, one has to proceed by trial and error. At each design
section there are three unknowns, the prestress force
P
, the eccentricity
e
and the
parasitic moment
M
P
. The following techniques allow the designer to control the trial
and error process.
In a continuous beam, there are several options for the arrangement of the prestress.
Two schemes will be discussed in
6.16
and
6.17
.
