Civil Engineering Reference
In-Depth Information
of prestressing steel, this solution is not often adopted in modern precast beam decks.
In order to fi t a stressing jack onto the anchor, it is necessary to provide a wide and
deep pocket, Figure 5.14 (a), which would require a local thickening of the web/slab
junction, complicating the fabrication of the form and the design and placement of
the reinforcement, and affecting the appearance of the beam. Also, the pocket would
interrupt the transverse reinforcement of the top slab over a considerable length,
requiring relatively heavy trimming steel.
In fact, as the tendons are short, they may be tensioned from one end only. At the
unstressed end, where no stressing jack has to be fi tted to the anchor, a dead anchor
may be used, needing a much smaller pocket, Figure 5.14 (b).
An alternative tendon arrangement would have been to design cables 1 and 2 as an
anti-symmetrical pair, with each being anchored at the end of the web at one end, and
swept up to the top fl ange at the other, Figure 5.11 (c). This would allow the cables to
be stressed from the beam end, with a dead anchor in the top fl ange. In fact, within a
220 mm web this arrangement is not possible, as cable 1 must pass cable 2. However,
in bridge decks with thicker webs, such anti-symmetrical tendon arrangements are
frequently used in designs for both isostatic and continuous prestressed structures. It is
always necessary to check that there is room for the cables to pass each other.
Although the pocket for a dead anchor is much smaller than that for a live anchor,
it is still necessary to provide an adequate thickness of concrete beneath the anchor
plate. This is unlikely to be available in the node created by the junction of the 220 mm
thick web and the 200 mm thick slab of this example, leading to the need for a local
thickening of the web.
In order to avoid this complication, the designer may use, for the cables that are
swept up, less powerful tendons whose anchorages are compatible with the thickness
of concrete available. The disadvantage of this solution would be imposing on the
contractor the use of two different sizes of stressing jack. If the project includes a
large number of beams, which would in any case require several sets of prestressing
equipment, this may not be a problem.
Tendon anchorages housed in top pockets as described have been out of favour
as there is a risk that water may infi ltrate into cracks between the parent concrete
of the beam and the second-stage concrete used to fi ll the pocket after stressing the
tendons. In particular, if this water is contaminated with de-icing salts, there would
be a risk of corrosion of the tendons. During construction, before the pockets have
been concreted, the ducts may fi ll with rainwater, corroding the prestressing tendons.
Furthermore, in some climates the water in the ducts may freeze, splitting the beams.
All of these risks are real. However, it is quite possible to overcome them, in the
temporary state by good specifi cation and site procedures, and in the permanent works
by covering the pocket with an additional layer of waterproofi ng, such as a fi lm of
epoxy resin.
To economise on the cost of a dead anchor and to avoid the need for a top pocket
and a web thickening, the designer could opt for a buried anchor, Figure 5.9 (e), with
the disadvantages described in
5.15
.
The tendons that have been swept up apply forces to the concrete right up to the
anchorage itself. For the example shown in Figure 5.11 (b), the calculation of the effect
of the prestress at the quarter point needs to include the
P
and the
e
of the swept up
tendons. While they are within the central kern, they add compression to the concrete
