Civil Engineering Reference
In-Depth Information
threaded, either over their full length or just at the ends, and are locked by nuts usually
bearing on steel plates, Figure 5.9 (d).
Savings in anchorage hardware and in labour may be achieved by adopting buried
anchors for the unstressed end of tendons. These anchors generally work by exploiting
the bond of the concrete to the steel tendons, or by looping the individual wires or
strands, Figure 5.9 (e). Buried anchors make it essential to prefabricate the tendon and
to place it in the reinforcing cage before the concrete is cast, rather than threading
the tendon into the concrete after it has been cast, which is the preferred method of
construction.
The prestressing tendons must be tensioned by hydraulic jacks, Figure 5.10, and
then their force transferred from the jack to the concrete by anchors. The tendons
may be stressed from one end only, or from both ends. Clearly, single-end stressing is
preferable, as it minimises the cost of labour and reduces the number of jacks required
on site. The decision on whether to stress from one end or two depends principally on
the friction losses in the cable (
5.17.2
). When tendons are less than about 50 m long,
they are normally single-end stressed. However, much longer cables may be single-end
stressed, for instance when their alignment is very fl at and friction losses are low, or
when it is not practical to fi t a jack to one end. When the tendons are stressed they
extend, typically by 6 mm/m for wire and strand, and 3.5 mm/m for bars.
Double-end stressing generally employs two jacks acting simultaneously. However,
it is also possible to stress from one end and then to transfer the jack to the other end
to complete the process. The main limitation on this latter procedure for cables that
are anchored by wedges is that if the force at the unstressed end is too great, it may be
impossible to release the wedges.
5.16 Cable profi le
5.16.1 Arrangement of cables
Once the cable zone has been defi ned, it is necessary to arrange the individual cables,
such that their centroid lies within the zone. The simplest cable profi le would consist
of keeping all the cables in single fi le in the web. Although this is both simple to design,
and simple to build, it is clear that when compared with Figure 5.6 the eccentricity of
the cable group would be signifi cantly less than was assumed in the calculation of cable
force, and consequently a substantially greater prestress force would be necessary.
In the cable arrangement shown in Figure 5.6 some of the cables are housed in
the heel of the beam and have to pass across the plane of the links in the web. As
the cables will cross this plane of reinforcement at a shallow angle, several links will
be interrupted, and the designer has to devise a suitable reinforcement detail that
maintains the strength of the web. It is most defi nitely not satisfactory to leave this
detail to be defi ned by the contractor, or the site staff.
For a simply supported beam such as our example, a third fi xed point, other than
the arrangement of tendons at mid-span and at the quarter point, is the arrangement of
the anchors at the beam end (
5.24
). The designer's fi rst choice would be to spread the
anchorages out evenly up the full height of the web, Figure 5.11 (a). This will stress
the concrete evenly, and reduce the amount of reinforcement required. In general, the
web will have been thickened to accommodate the size of anchor chosen.
