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Factors
to Consider in Selecting and Detailing Stone
By Mike Picco
Being involved with
stone detailing and design for several years, we have come across
several projects requiring complicated and costly engineering
solutions because not enough planning and research was done at the
initial stages of the stone design.
It is essential for the design professional to consult with
experts and when using stone
THE STONE
In choosing a stone, several factors must be considered.
Although colour, texture, veining and/or grain size are always
critical aspects to architects and designers, there are other aspects
that must override these, when selecting a suitable
stone. Some of the more
obvious aspects are:
- Strength is the stone
strong enough to withstand all of the design loads at the
thickness and size specified?
- Block size is the
stone specified available at the size detailed on the drawings?
- Finish is the finish
suitable for the proposed application?
- Exposure conditions Is
the stone exposed to high levels of pollution?
Will the stone undergo several freeze thaw cycles?
Is the stone exposed to high levels of sodium (coastal
exposures)
TESTING
Most stones quarried and sold today, have readily available
ASTM test data including density, compressive strength, flexural
strength, absorption, and abrasive resistance.
This test data is valuable for providing information for the
initial evaluation and selection of a stone for a particular
application in a given location.
Current test data must be performed in order to
determine the actual strength of the stone being quarried and used for
the project. In most
cladding applications, the most common test giving the most useful
information is the ASTM C880 Flexural Strength Test for Dimension
Stone. Individual anchor
tests should be performed with the proposed anchors being suggested
for the project. These
test are also outlined in the ASTM standards.
BACKUP
The types of back up used by the building designer can, in
many cases, determine the success or failure of a cladding project.
Commonly used back ups are:
- Concrete
the most preferred back up for exterior and interior cladding.
Anchors can be placed were required.
Anchor capacities are highest when installed into concrete.
- Masonry
(CMU) When used for interior cladding where panels are
stacked, it is a very suitable back up.
If used for exterior cladding, the CMU must be solid
filled with masonry grout to ensure anchors can achieve the
required capacity. If
masonry is not properly filled, positive anchorage is very
difficult to achieve and may require the use of adhesive or epoxy
anchors. These anchors
are more costly and require time to set up, particularly when
ambient temperatures are low.
- Steel
sub frames When designed by the stone installers engineer,
can be a very effective and economical back up.
However, if the frames are not designed to accommodate the
stone designers anchors, they can be ineffective and very often
require either additional steel, or the stone designer to engineer
an expensive anchoring system to suit the steel provided.
- Metal
Stud - When used for interior cladding where panels are
stacked, is an acceptable back up.
When used for exterior cladding, it is by far the least
preferred and not recommended.
The capacity of anchors into metal studs is limited.
The placement of the studs is critical; the service life of
the anchorage into metal studs is generally much lower than the
stone and can be dramatically reduced by corrosion at the anchors.
ANCHORS
In several cases, anchors are misinterpreted by architects and
designers as masonry ties. In
the transition to thinner veneers, the traditional masonry tie and
stacked stone has continued to be incorrectly used in many thin
cladding projects. Anchors
must be capable of independently supporting all the loads imposed by
each stone. Stones should
not be stacked. The
failure or breakage of a stone should not affect any of the adjacent
stones.
It
is highly recommended that all anchoring components be manufactured
from grade 304 stainless steel. Hot
dipped galvanizing and the use of dissimilar metals can result in
corrosions problems. The
cost to upgrade to an all-stainless anchoring system is usually
negligible when compared to the total cost
of the cladding project. All
stainless offers peace of mind that the anchors will perform as
detailed for life of the building.
Stone can be an
everlasting maintenance free material when designed properly or a
disaster if used and detailed incorrectly, as evidenced by many
re-cladding and re-installations required because of either poor
design and installation or wrong stone selection. ’
Michael
Picco, President of Picco Engineering is a professional engineer with
over 15 years experience in the stone industry.
Michaels experience includes stone selection, testing,
manufacturing and installation, as well as design and detailing of
some anchors and stone shop drawings.
Michael can be reached at (905) 760-9688 or by email at mpicco@picco-engineering.com.
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