Civil Engineering Reference
In-Depth Information
5.26 The introduction of prestress forces
In continuous bridge decks, it is often essential to stop off tendons at regular intervals.
The tendon anchor applies a concentrated compressive force to the concrete.
Immediately behind the anchor the elastic assumption that plane sections remain plane
is not justifi ed, and the tendon does not create a bending moment in the beam. The
anchor force disperses through the concrete section, until, at some distance from the
anchor, the normal assumption of elastic behaviour is re-established.
The angle at which the force disperses through the concrete lies between 30° and
45°. If the anchor is located at the end of a member, the anchor force can only be
transmitted forwards by compression, and the angle of dispersion will tend to be fl atter.
If the anchor is placed midway along a member, and if the section is in compression
or there is adequate following steel, some of the anchor force is carried backwards by
tension, effectively shortening the dispersion length. However, the angle of dispersion
of prestress is conventionally accepted as 35°.
Three separate effects of the prestress must be considered by the designer. First, the
prestress causes local compressive stresses. These are present before the dispersion of
the prestress is complete, and they may overstress the concrete locally if combined with
other stresses due to bending of the deck. However these local compressive stresses
may assist in cancelling out tensile bending stresses in the deck. In Figure 5.28 (a) for
instance, if the bottom slab of the box shown was subjected to tensile bending stresses
due to the bending action of the beam, the local compression due to the new anchors
would be benefi cial once it had spread over the full width of the bottom slab, well
before it had spread throughout the box section.
Second, the prestress causes defl ections of the beam even before the stresses have
been uniformly dispersed throughout the section. In a continuous structure, these
defl ections will generate prestress parasitic moments.
Finally, once the dispersion of the prestress force is complete, it causes elastic
bending and compressive stresses of
P
/
A
±
Pe
/
z
, as described in
5.8 et seq.
These three
phases are summarised in Figure 5.28 (b).
5.27 Bonded and unbonded cables
Tendons may be housed in ducts that are buried in the concrete webs and slabs of the
bridge deck. These internal tendons are protected from corrosion by adequate concrete
cover, by the lack of cracking in the surrounding concrete under the action of permanent
loads, and by fi lling the ducts with cement grout. If the grouting is well done, and if the
deck has been competently designed and built, these grouted tendons will be durable
and will need no maintenance. A correctly designed and built prestressed concrete
deck with internal tendons will be more durable than an equivalent reinforced concrete
structure, due principally to the much-reduced incidence of cracking. However, if
the above conditions are not fulfi lled, there is the possibility that the tendons will be
subject to corrosion due to cracking under permanent loads or to voids in the grout.
Such corrosion is likely to be diffi cult to repair, and may well require the demolition
and reconstruction of the bridge deck. In fact many thousands of prestressed concrete
bridge decks with internal cables have been built, and the incidence of failure due to
corrosion of the tendons is extremely small.
