Factors to Consider in Selecting and Detailing Stone By Mike Picco

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. Michael’s 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.