CONCRETO & Construções | 137
force always remains concentric to
the section (Figure 1a).
Laterally-unrestrained
tendons
are free to move sideways in the wall
as the masonry deflects (Figure 1b).
The movement results in an eccentric
force on the section. This eccentric
force reduces the section capacity
and decreases the buckling capacity
of the wall. So for the same amount
of post-tensioning, a laterally-unre-
strained tendon will not be as efficient
in strengthening a wall as a laterally-
restrained tendon.
For some designs, it is beneficial
to place the tendons eccentric to the
section. For eccentric designs, the
engineer must evaluate both out-of-
plane directions. The discussion re-
garding tendon restraint still applies.
Grouting is always a major en-
deavor and expense for masonry con-
struction. It also contributes largely
to the carbon footprint of a masonry
building. So, eliminating grout when-
ever possible has a desirable environ-
mental and cost impact. Since most
post-tensioned masonry walls are
designed and constructed as partially
reinforced walls, the system naturally
has these benefits.
While using post-tensioning has
the ability to reduce the grout re-
quired, that benefit sometimes comes
at a cost to performance. Research-
ers are continuing to evaluate post-
tensioned masonry shear walls for
lateral effects due to seismic forces.
Many buildings codes, such as those
in the United States, require mild-
reinforcement grouted in place to
supplement the post-tensioning to
enhance seismic performance. That
supplemental reinforcement and grout
add cost and time. The goal of future
research should be to minimize the
need for grouted reinforcement.
TENDON LOSSES
Since the tendons are the primary
reinforcement for the masonry, engi-
neers must be able to count on their
ability to maintain the post-tensioning
force over time. Therefore, building
codes require that engineers design
post-tensioning with a reduced ten-
don capacity that accommodates
long-term losses. These losses can
be attributed to such effects as creep
of the masonry, anchorage seating
losses, thermal changes on tendons
and masonry, and moisture changes
within the masonry. Only moisture
growth of clay masonry has a com-
pensating effect on the tendon losses.
When using post-tensioning, United
States standards require the tendon
design include up to 35% losses
when used in concrete masonry and
up to 25% losses in clay masonry.
Other international codes have similar
requirements.
TENDON PROTECTION
Corrosion protection is essential
for tendons exposed to moisture wet-
ting or high humidity. In single leaf
walls, unbonded tendons are most
susceptible to corrosion due to a lack
of grout protection in the masonry unit
cells. Therefore, tendon protection is
usually supplied by coating the ten-
dons with a material that will perform
under tension, encapsulating the ten-
don in a smooth sleeve, or both.
USES OF POST-TENSIONING
IN MASONRY
While post-tensioned masonry
was first developed for the construc-
tion of new walls, the technology has
been extended to other applications.
In the following section, we will dis-
cuss various applications of post-
tensioned masonry that have created
many new opportunities for masonry
construction.
WALLS - OUT-OF-PLANE EFFECTS
Post-tensioning is an ideal system
for designing new masonry walls for
out-of-plane loadings whereby flex-
ural effects are dominant. Greater
economy is derived for designs that
are wind-dominated versus those that
are seismically-dominated. Hopefully,
greater research will find new meth-
ods for improving the performance of
seismic designs.
In new construction, it is relatively
easy to construct walls with laterally-
restrained tendons using high strength
u
Figura 1a
Laterally restrained
TENDON
DEFLECTS
WITH WALL
DEFLECTED WALL
UNDEFLECTED WALL
u
Figura 1b
Laterally unrestrained
CENTER
OF TENDON
DOESN’T
SHIFT
DEFLECTED WALL
UNDEFLECTED WALL
