Civil Engineering Reference
In-Depth Information
in the transverse direction for bridges that are built into their piers, for some portal
structures, and for buildings.
It is clear that if a single fl oor of the building shown in Figure 5.21 (a) were to
be prestressed, most of the force would be dissipated in the stocky columns, with
potential damage to the columns. However, if all the fl oors were to be prestressed in a
co-ordinated sequence, and if the piled foundations were fl exible enough to accept the
deformations due to the shortening of the fl oors, prestressing would be possible. The
building shown in Figure 5.21 (b), where stiff stairwells are placed at each end, and the
whole is founded on a raft, could not be successfully prestressed.
5.24 Forces applied by prestress anchorages
5.24.1 General
There are generally three areas behind an anchor that need to be reinforced. Immediately
behind the anchor are splitting forces due to the tendency of the anchor to be driven
into the concrete member by the force of the tendon. Adjacent to the anchor, on the
end face of the member, are zones of tensile stress known as spalling zones. Finally,
as the prestress force disperses into the bridge deck, further zones of tensile stress are
created.
5.24.2 Splitting forces behind anchorages
Most prestress anchorages are sized to apply a pressure on the concrete of about
37 MPa. The typical lines of force behind an anchor are shown in Figure 5.22 (a). The
change of direction of these lines of force creates transverse stresses, compression if
the lines are concave towards the centre line, and tensile when convex. The stresses
perpendicular to the line of the anchor are shown in Figure 5.22 (b). These stresses are
compressive directly behind the anchor and tensile further away. The primary splitting
or bursting reinforcement is designed to resist these transverse tensile stresses. It usually
consists of spirals or a series of mats. The design of primary bursting reinforcement is
described in references [1, 2, 3 and 4].
However, the lines of force are an elastic concept. Design methods where the sizing
of the bursting reinforcement is based on this elastic distribution of stresses produce
safe structures, although in general the amount of reinforcement required is found
to be excessive, and the design procedures generally introduce empirical methods to
reduce it.
Once the concrete reaches its limit in tension and either yields or cracks, this elastic
distribution of stresses is completely changed. At the ultimate limit state one may
see the problem as one of mechanics. The concrete immediately beneath the anchor
forms itself into a wedge, which tends to be driven into the beam member, splitting it,
Figure 5.23. The wedge that is trying to force the concrete apart may be resisted by a
tie or a series of ties placed closely behind the anchor, in the compressive zone of the
elastic force diagram.
More research on anchor zones designed on the assumption that the concrete is
cracked should allow lighter reinforcement to be used.
The design of this primary bursting reinforcement is one of the more critical activities
in the design of a prestressed member. An unthinking or over-conservative application
