136 | CONCRETO & Construções
compression in walls. Engineers also
rely on the reinforcement to further
accommodate tension. In post-ten-
sioned masonry, the structural mass
of the structure is taken into account
along with the compressive effects
of the tendons to provide the overall
compression in the masonry. Based
upon this explanation, you realize that
a non-loadbearing exterior masonry
wall will require more post-tensioning
than an exterior loadbearing masonry
wall when both have the same out-of-
plane lateral loads. The loadbearing
wall derives benefit from the added
compression of the structural mass
which therefore reduces the required
amount of post-tensioning to achieve
the same compression.
DESIGN
Internationally, theory and code
provisions for post-tensioned ma-
sonry have developed similar to those
for prestressed concrete. Today most
countries, including the United States,
utilize a limit state methodology for
their post-tensioned masonry provi-
sions. This methodology includes a
check for the ultimate limit state for
total capacity as a well as a check for
a service load (safety) limit state.
While the ultimate limit check
procedure is relatively consistent
between country codes, the service-
ability checks take several forms. For
example, Eurocode 6 (BS EN 1996-
1-1: Rules for Reinforced and Un-
reinforced masonry: 2005) requires
that walls designed with prestressed
masonry be checked for allowable
stresses using service level loads us-
ing flexural effects only, no axial ef-
fects. Combined axial and flexural
effects are evaluated under the ul-
timate limit state. Earlier standards
from Switzerland (SIA 177, now SIA
266) require a determination of the
possible tension crack size as its
serviceability check. The New Zea-
land code (NZS 4230:2004), controls
flexural strength through a stress
check, stiffness and deflections at
service loads. In the United States
code (TMS 402, Building Code Re-
quirements Masonry Buildings), ser-
viceability is checked by controlling
tension and compressive stresses
developed by service level loads.
Thus each code’s serviceability
check is similar, but different. How
can it be that each is different yet still
meets the intended purpose for their
respective codes? Could it be that
we don’t monitor our structures for
performance at service load condi-
tions and don’t really know if they are
performing appropriately? Are we not
accurate with our service loads con-
ditions? While we all check the ser-
vice limit state in some form, are the
real controlling conditions the ultimate
limit state? This topic of serviceability
could use some more research; per-
haps our standards need to change.
In 1998, a comparison using one
design example was done between
the prestressed masonry standards
for five countries (Biggs and Ganz
1
).
The variability in answers was sig-
nificant. Perhaps it’s time to update
the comparison using many examples
and all the current country codes and
standards.
CONSTRUCTION
Masonry post-tensioning is applied
through the use of tendons, usually
steel bars or strands, within the hol-
low cells of the masonry units. An-
chored top and bottom for walls, these
tendons affect the performance of the
masonry based upon how they are in-
stalled. Dependent upon the post-ten-
sioning design, the cells may be later
grouted or left hollow, but that choice
will effect the strength characteristics
and performance of the masonry.
The tendons can be categorized
as bonded or unbonded, as well as
laterally-restrained or laterally-unre-
strained. Bonded tendons are encap-
sulated in a corrugated plastic duct
much like post-tensioned concrete
tendons. The duct is later filled with
a special grout (prestressing grout)
after the tendons are post-tensioned.
The masonry space around the duct
is filled with masonry grout. The ma-
sonry grout interlocks with the duct
corrugations to create the bonded
condition; simply using masonry grout
around the tendon in the cell of the
masonry does not cause the tendon
to be sufficiently bonded to transfer
stress.
Unbonded tendons are not perma-
nently bonded to the masonry. They
can be placed in a greased sheath-
ing or a smooth duct to create the
bond separation with the masonry.
Unbonded tendons are usually used
in an ungrouted cell, but they can be
used in a grouted cell of the masonry
provided the tendon can slip.
For most walls, tendons are placed
in the center of the walls to accom-
modate out-of-plane loads in both di-
rections. When the wall deflects due
to out-of-plane loadings, laterally-re-
strained tendons are not free to move
laterally in the masonry. This restraint
is accomplished by grouting the ten-
dons in the cells of the masonry or
by utilizing tendon positioners at the
quarter points over the length of the
tendon. By keeping the tendon cen-
tered in the wall, the post-tensioning
