CONCRETO & Construções | 141
They could be constructed either with
a composite concrete topping or not.
Constructed flat, the mild reinforce-
ment was set into channels on the
lower face of special blocks or in the
head joints of standard units. Ge-
nerically, prefabricated panels were
known as Dox planks taken from the
name of the most popular system.
Dox planks were discontinued in the
1970s with the introduction of prefab-
ricated, prestressed concrete planks.
Figure 8 shows a cross-section of a
Dox plank.
On occasions, there is still a need
to recreate masonry Dox planks. Fig-
ure 9 shows a prefabricated roof panel
being lifted. One photograph is inside
the fabrication shop and the other is
at the site. The panel is a replacement
for a roof section of Dox planks from a
school that was originally constructed
in the 1960s. The overhang of the
building is constructed with AAC.
The structural portion of the plank is
CMU. The replacement is to repair
a roof section damaged by a falling
tree during a wind storm. Since the
Dox planks in the building are archi-
tecturally exposed showing the CMU
jointing on the underside, the decision
was made to replicate the appear-
ance of the original Dox planks for the
replacement.
During the damage assessment,
it became clear that the original Dox
planks sagged due to creep deflec-
tion. The planks were 20cm thick,
had no topping, and spanned 7.6m.
So, the challenge was to provide the
appearance of the existing planks, but
with a stronger, stiffer section. The
choice was made to use a prefabri-
cated panel reinforced with post-ten-
sioning. The masonry units selected
were 25cm thick with a 5cm topping
slab. See the recess in Figure 9 for
the topping. In addition, the panel
was cambered for dead load. The ten-
dons were placed near the bottom of
the CMU cells and laterally-restrained
with grout plugs. Since there are no
standards in the United States for hor-
izontally post-tensioned elements, the
design used aspects of the current
masonry standard wall provisions,
precast concrete slab provisions, and
first principles.
Currently in the United States, the
Prestressed Masonry Subcommittee
of the Masonry Standards Committee
(TMS 402) is developing code provi-
sions for post-tensioning horizontal el-
ements. New research from Medellín,
Columbia is providing some useful data
helpful in the development of design
criteria for post-tensioning masonry ele-
ments including beams and panels.
LESSONS LEARNED FROM
DEVELOPING A MASONRY
STANDARD FOR
POST-TENSIONING
1 – Producing any new masonry stan-
dard by committee is difficult. An
excellent series of state of-the art
papers on prestressed masonry
was prepared by Schultz and
Scolforo
2, 3, 4
. These papers gave
the committee the resources to
begin its work in the 1990s.
2 – The code development process
in the United States (American
National Standards Institute pro-
cedures) requires consensus and
adds a lot of time compared to
some countries where standards
are developed by a committee of
scholars and then adopted direct-
ly by agencies.
3 – Research on the topic of post-
tensioned masonry has occurred
in several countries by only a few
published researchers. Because
those international researchers
were willing and able to share
their work, the US masonry com-
mittee was able to obtain enough
data to develop a standard.
That professional cooperation is
continuing.
4 – Acceptance by American engi-
neers to post-tensioned masonry
has been slow since few univer-
sities teach masonry, in general,
and almost none teach masonry
post-tensioning. Therefore, few
projects are ever designed as
post-tensioned masonry for pub-
lic bidding.
5 – The limited amount of post-ten-
sioned masonry designs by engi-
neers results in limited opportuni-
ties for contractors as well. A few
industrious masonry contractors
u
Figura 9
Roof plank with horizontal post-tensioning (Photograph: Dave Muirhead)
