Frequently Asked Questions (FAQs)
Masonry Home > Frequently
Asked Questions > Ties and joint reinforcement for concrete
masonry walls
Q:
What kind of ties should be used with concrete masonry walls?
A: There are many ties available in
the marketplace. Not all ties are appropriate for all masonry uses;
they should be selected based on the application. Corrugated ties
are supposed to be used only with veneer over wooden studs; unit
metal ties including regular ties and adjustable ties are used with
two-wythe construction for cavity walls; and metal ladder-type ties
are to be used for multiwythe composite action walls. Masonry headers
are no longer recommended for connecting wythes (MSJC 2008, PCA
2008).
A recent advisory from the Masonry Advisory Council cautioned those
involved in designing, building, specifying, or inspecting masonry
for residential or commercial properties about the inappropriate
use of corrugated ties (MAC 2007). These light gauge metal ties
are simply not robust enough to be used with brick veneer over concrete
or concrete block. If the wall has been laid, it is possible to
install retrofit anchors either before or after brick veneer has
been installed. These tend to be costly repairs and should be prevented
by the proper specification and use of ties at the outset.
Unit metal ties are available in one-piece or two-piece (eye and
pintle sections) varieties, with the two-piece ties accommodating
some difference in alignment between wythes.
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| Adjustable ties connect two wythes of masonry
in this cavity wall detail. (PCA EB008 2008) |
Additional information about wall ties can be found in:
Q: What is the purpose of joint reinforcement
in masonry walls?
A: In a previous issue of the Masonry
enewsletter, we showed the above drawing with eye-and-pintle ties
on a two-wythe wall. These two-piece tie assemblies are designed
to connect one wythe to the other while allowing for some adjustment
for joints that do not align at the exact same height. A brick expert
who read the item, John Bufford of Acme Brick, sent a comment about
the ties suggesting that the use of continuous wire reinforcement
and anchors was a far more common tie detail as it serves the dual
function of connecting the two wythes together while simultaneously
providing longitudinal reinforcement in the concrete masonry backup
system for crack control purposes. On further investigation with
Jason Thompson of the National Concrete Masonry Association, we
agreed that the unit ties originally shown, which do not have longitudinal
reinforcement as part of the tie assembly, are not an effective
means of providing crack control measures for the concrete masonry
wall backup.
To accommodate shrinkage due to temperature or moisture fluctuations,
or to account for differential movement between different portions
of a wall assembly, concrete masonry walls need longitudinal reinforcement,
either in the form of joint reinforcement or bond beams, to minimize
the potential for cracking in the wall assembly. When bed joint
reinforcement is used, it is frequently placed in every other course.
In the case of the original detail, assuming bond beam reinforcement
was not used, it would have been appropriate to illustrate the use
of a tie assembly that included bed joint reinforcement along with
the double pintle and hook system to attach brick veneer.
Continuous metal ties are also called prefabricated joint reinforcement,
mesh, or more commonly, joint reinforcement. Joint reinforcement
normally consists of two or more parallel longitudinal wires to
which cross wires are welded. Joint reinforcement may be used for
the following reasons:
- To act as structural steel reinforcement to increase masonry’s
resistance to flexural, shear, and tensile stresses
- To act as longitudinal reinforcement for the control of cracking
due to drying shrinkage and temperature changes
- To bond adjacent masonry wythes together in composite, faced,
cavity, and veneer wall constructions.
Referring to the drawing above, the reason the question was raised
is that these distinct accessories do not provide longitudinal reinforcement
along the wall’s length (reason #2 above).
 A
variation on this type of reinforcement is shown at right, which
combines the need for crack control of concrete masonry along the
length of the wall with the ability to tie two wythes together.
The adjustable ladder wire (with double hooks) is laid in the CMU
as the wall is built, and the pintle section is used to attach the
brick veneer, which is constructed later. The hooks (and pintles)
are typically spaced 16 in. (406 mm) on center horizontally. Because
the reinforcement is placed in every other course, that makes for
a 16-in. (406 mm) vertical spacing, too. This results in one tie
for every 1.77 sq ft (0.16 m2) of wall area (16 in. x
16 in.) The Building
Code Requirements and Specification for Masonry Structures
(LT313) requires at least one tie for every 2.67 sq ft (0.25
m2), so this requirement is satisfied by the 16-in. spacing
horizontally and vertically. The adjustable ladder type has built-in
eyes to hold the pintles to accommodate some vertical difference
between wythes (like the unit ties shown above).
Other related types of reinforcement are show in Figures 3 and 4.
While these do serve to connect two wythes of masonry, neither of
these styles allow for vertical adjustment between wythes. In addition,
both wythes would need to be laid at the same time, but that is
no longer very common.
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| Figures 3 and 4. Prefabricated
ladder T’s (left) and tab ties (right) connect two wythes
of masonry but are not adjustable. (PCA EB008 2008) |
Additional information about wall ties can be found in PCA’s
Concrete Masonry
Handbook for Architects, Engineers, Builders,
2008 edition (EB008) and in several TEK Sheets from NCMA.
Return to Masonry FAQs.
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