Civil Engineering Reference
In-Depth Information
some situations, when the bursting forces are particularly large, the stress in the
reinforcement carrying them back may have to be limited to a relatively low fi gure, say
150 MPa, to reduce the strain in the bars to a value that will not crack the concrete
cover.
6.24 The anchorage of tendons in blisters
Blisters for external tendons take the form shown in Figure 6.29 (a) and in Figure 15.26.
In addition to the primary and secondary effects, described in
5.24
, such anchorages
also apply very signifi cant bending moments and shearing forces to the parent concrete
to which they are attached, as well as compression immediately in front of the blister.
The thickness of the concrete member carrying the blister must be adequate for these
forces, which can be very substantial for the large anchorage units often adopted for
external tendons. When possible, the blister should be located in a corner of the box,
so that two approximately orthogonal slabs can carry these forces. If the anchorage
is remote from a free end of the concrete deck, it will also require following steel, as
described in
5.25
.
Blisters for internal tendons take the form shown in Figure 6.29 (b). Such anchorages
apply compressive forces to the slab in front of the anchorage, and require primary
and secondary bursting reinforcement, as described in
5.24
, as well as following steel.
However, although they do not apply such large moments or shearing forces to the
slabs, care must be taken to ensure that the slabs carrying the blisters are thick enough
to resist the concentrated forces and local eccentricities involved, as well as the local
bursting forces due to curvature of the tendons. Such blisters are best located in the
corners of the box if possible, Figure 6.29 (c) [13].
6.25 Checks at the ULS
For continuous prestressed concrete bridge decks, it is essential to carry out checks at
the Ultimate Limit State (ULS). The need for such checks may be simply demonstrated
by considering the two-span continuous beam illustrated in Figure 6.30 (a). In this
beam, the uniformly distributed loads are exactly balanced by parabolic cables in each
span. The cables are discontinuous over the central pier. The moment over this pier due
to the applied load, that is equal to -
wl
2
/8, is cancelled out by a parasitic moment due
to the cables, equal to +
wl
2
/8. Consequently at working load the bending moments
in the beam are zero at all sections, and no reinforcement, prestressed or passive, is
required over the supports. However, once the applied loads are factored to arrive at
the ULS, it is clear that the unreinforced pier section would fail.
More generally, the presence of parasitic moments makes it essential to check the
strength of prestressed beams at the ULS. It has become customary to check even
statically determinate beams in the same way, although for designs with internal
tendons that are familiar to the author, this is a formality as the ratio between the
working and the ultimate stress in the steel is greater than the ratio between the working
and ultimate loads. However, for statically determinate and continuous beams with
external tendons, where the force in the tendons between the working and ultimate
conditions does not increase much, the ULS is likely to be the critical case.
A second reason to check prestressed beams at the ULS is to ensure that there is
adequate ductility. In particular for twin rib type bridges, or box sections with very small
