CONCRETO & Construções | 135
u
industrialização da construção
New opportunities with
post-tensioned masonry
INTRODUCTION
M
asonry construction has
been around for millen-
nia. But, it continues
to evolve and offer new opportuni-
ties for architects and engineers to
create magnificent masonry struc-
tures. Post-tensioned masonry is
a relative newcomer to the world of
masonry. While early uses of post-
tensioning masonry extend back to
the late 1800s, the methods we use
now have been developed since the
1960s. The theoretical background
for modern post-tensioned masonry
has as its basis the development of
post-tensioned concrete that resulted
from research by Frenchman Eugene
Freyssinet in the 1930s and was ad-
vanced by German engineers during
the 1940s.
Using masonry criteria developed
in Great Britain and Switzerland, the
United States building code has its
own post-tensioned masonry pro-
visions that currently address only
post-tensioned masonry walls. These
provisions first made it into our coun-
try’s standards in 1999. Since then,
creative engineers have found vari-
ous opportunities to use this tech-
nology. Now there are nearly 35,000
structures in the United States that
have used post-tensioned masonry,
the majority being residences in the
southwest part of the country.
BACKGROUND
Most engineers are familiar with
the concepts of prestressed con-
crete (pre-tensioned versus post-ten-
sioned). For pre-tensioning, tendons
composed of wire strands or bars are
stretched lengthwise between the
ends of the concrete formwork. Next,
the tendons are stressed and the con-
crete is cast around the tendons. As
the concrete cures, the tendons bond
to the concrete. When the formwork
is released, the prestressed force is
released and transferred into the con-
crete member.
For post-tensioning concrete, two
types of tendons are used. For one
type, the tendons are in greased sleeves.
They are placed, aligned and then en-
cased in concrete. Once stressed, the
end anchorages impart the force through
bearing into the end of the element. The
greased sleeve keeps the tendon un-
bonded from the concrete and provides
corrosion protection.
In the second, tendons of strand
or bars are installed loosely inside
corrugated sleeves or ducts that are
aligned and positioned within the con-
crete formwork. The ducts provide
long-term corrosion protection for the
tendons. Once the concrete is placed
and cured, the ducts are fully bonded
to the concrete. Next, the tendons
are anchored to the ends of the mem-
ber and post-tensioned. The final step
is to fill the interstitial space within
the ducts with a special grout that
bonds the tendon to the duct and pro-
vides further corrosion protection to
the tendons.
So, both pre-tensioned and post-
tensioned concrete members are
compressed from the force of the
tendons. However, each method has
distinct ways it affects the design and
performance of the member.
Often,
masonry
conferences
and literature will refer to either pre-
stressed masonry or post-tensioned
masonry. Unlike with concrete where
there is a distinction between pre-
tensioned and post-tensioned, the
terminology for masonry (prestressed
and post-tensioned masonry) are
used synonymous since the masonry
is always built first and then post-ten-
sioned afterward. There are no known
cases of prestressing tendons and
building masonry around them. Until
then, we use the terms prestressed
masonry and post-tensioned masonry
interchangeably.
POST-TENSIONING CONCEPT
Simply put, post-tensioned ma-
sonry uses induced precompression
to compensate for the possible ten-
sile stresses that may develop from
structural loads. In conventionally re-
inforced masonry, engineers account
for the mass of structure to provide
David T. Biggs
B
iggs
C
onsulting
E
ngineering
