Civil Engineering Reference
In-Depth Information
A similar example occurred on one of the earlier projects in which the author was
involved (but did not design!). A large number of 32 m long post-tensioned Tee beams
(Chapter 10) were being prefabricated for a viaduct. In the casting yard, they were
temporarily supported some distance in-board of their fi nal bearing position. The
tendons that were anchored in the end of the beams gave a very signifi cant relieving
shear force that, when the beams were on their fi nal bearings, would partially counteract
the working shear. However, in the temporary condition, in the absence of dead load
shear in the length beyond the temporary bearing, they created a principal tensile
stress that was orientated approximately perpendicular to the prestressing ducts, as
described above. When the tendons were stressed, the beams cracked along the line of
the ducts, and successive increases in shear links failed to cure the problem. In some
beams, the experimental installation of vertical prestress did eliminate the cracking.
The designer must be wary of any cumulation of tensile stresses, which may be
due, for example, to a temporary bending condition, to restrained heat of hydration
shortening, to the presence of holes or other stress raisers, to temperature gradient
effects, as well as to prestress dispersion.
The forces due to the dispersion of prestress, and the reinforcement required to
resist them, are assessed at the SLS. In the example of the web of the deck shown in
Figure 5.26, it is important that the concrete does not crack when these forces are
combined with the working load shear forces. In making this assessment, it should be
noted that when the tendons are stressed, the force applied may be up to 80 per cent
of the strength of the tendon, but by the time the live load shear forces are applied, this
will have fallen to about 60 per cent. The reinforcement to resist the dispersion forces
should not be added to the shear reinforcement calculated at the ULS.
5.25 Following steel
If a prestress anchor were to be placed in an elastic medium, far from any free edge,
the force of the anchor would be carried half in compression in front of the anchor,
and half in tension behind. If the elastic medium was to be replaced by unreinforced
concrete, as it is weak in tension, the concrete behind the anchor would crack and all
the force would be carried in compression. Consequently, it is necessary to provide
local reinforcement that controls this cracking, and which carries a proportion of the
force in tension. Generally, it is adequate to provide following steel that can carry a
maximum of one-third of the prestress force with the reinforcement working at a
stress of 250 MPa, Figure 5.27.
If the concrete in which the anchor is situated is compressed due to prestress and
to the overall bending of the deck, this compression will have to be overcome before
the concrete behind the anchor may crack, and consequently the following steel may
be reduced. Also, if there is a free concrete edge close behind the anchor, less of the
anchor force will be carried in tension (to the limiting case when the anchor is at the
end of the member and all the force is carried in compression), and less following steel
is required.
A special case exists in precast segmental decks, which have frequent unreinforced
transverse joints. If prestress anchors are placed closely in front of such joints, there
is a possibility of the joint being opened locally by the following tensile strains. It is
good practice to place anchors at least one metre in front of such joints, when some
following steel needs to be provided.
