4.2 - VSL

Figure 41: Possible tendon arrangements
Under this loading case, the slab is stressed
only by centric compressive stress. In regard
to punching shear, it may be advantageous
to position more than 50 % of the tendons
over the columns.
In the most commonly encountered
cases, the tendon arrangement illustrated
in Fig. 41 (d), with half the tendons in each
direction uniformly distributed in the span
and half concentrated over the columns,
provides the optimum solution in respect
of both design and economy.
5.1.2. Spacings
The spacing of the tendons in the span
should not exceed 6h, to ensure
transmission of point loads. Over the column,
the clear spacing between tendons or strand
bundles should be large enough to ensure
proper compaction of the concrete and allow
sufficient room for the top ordinary
reinforcement. Directly above the column,
the spacing of the tendons should be
adapted to the distribution of the
reinforcement.
In the region of the anchorages, the spacing
between tendons or strand bundles must be
chosen in accordance with the dimensions of
the anchorages. For this reason also, the
strand bundles themselves are splayed out,
and the monostrands individually anchored.
14
5.1.3. Radii of curvature
For the load-relieving effect of the vertical
component of the prestressing forces over
the column to be fully utilized, the point of
inflection of the tendons or bundles should
be at a distance d s /2 from the column edge
(see Fig. 30). This may require that the
minimum admissible radius of curvature be
used in the column region. The extreme fibre
stresses in the prestressing steel must
remain below the yield strength under these
conditions. By considering the natural
stiffness of the strands and the admissible
extreme fibre stresses, this gives a minimum
radius of curvature for practical use of
r = 2.50 m. This value is valid for strands of
nominal diameter 13 mm (0.5") and 15 mm
(0.6").
Table III - Required cover of prestressing
steel by concrete (in mm) as a function of
conditions of exposure and concrete grade
1) for example, completely protected against
weather, or aggressive conditions, except for
brief period of exposure to normal weather
conditions during construction.
2) for example, sheltered from severe rain or
against freezing while saturated with water,
buried concrete and concrete continuously under water.
3) for example, exposed to driving rain, alternate
wetting and drying and to freezing while wet,
subject to heavy condensation or corrosive fumes.
5.1.4. Concrete cover
To ensure long-term performance, the
prestressing steel must have adequate
concrete cover. Appropriate values are
usually laid down by the relevant national
standards. For those cases where such
information does not exist, the requirements
of the CEB/FI P model code [39] are given in
Table I I I.
The minimum concrete cover can also be
influenced by the requirements of fire
resistance. Knowledge obtained from
investigations of fire resistance has led to
recommendations on minimum concrete
cover for the post-tensioning steel, as can be
seen from Table IV. The values stated should
be regarded as guidelines, which can vary
according to the standards of the various
countries.
For grouted tendons with round ducts the
cover can be calculated to the lowest or
highest strand respectively.
5.2. Joints
The use of post-tensioned concrete and, in
particular, of concrete with unbonded
tendons necessitates a rethinking of some
long accepted design principles. A question
that very often arises in building design is the
arrangement of joints in the slabs, in the
walls and between slabs and walls.
Unfortunately, no general answer can be
given to this question since there are certain
factors in favour of and certain factors
against joints. Two aspects have to be
considered here:
Table IV - Minimum concrete cover for the post-tensioning steel (in mm) in respect of the fire
resistance period required