Stone Veneer Anchorage
Most stone exterior facades of the last several decades are no longer heavy load bearing stone construction, but rather thin veneers of stone applied in a method similar to a curtain wall. This stone is often cut to thicknesses from ¾” to 2”. The long term performance of these systems rests both on the method of attachment and the stone itself.
Thin-stone cladding must resist wind, gravity, dynamic loading, seismic loading, climatic forces, the chemical attack of acid rain, the forces imposed during installation and the movement of the building itself.
In the early 1990’s, stone anchor systems attracted renewed attention as an increasing number of problems occurred on thin-stone clad buildings. One widely discussed problematic façade was Chicago’s Amoco building, where 43,000 slabs of Italian Carrara Marble had to be replaced after individual pieces bowed by as much as 1 ½ inches.
To design a successful thin-stone cladding system, a thorough structural analysis of the stone itself and its proposed anchoring system should be prepared early in the design process. Since stone is a natural material with inconsistencies, its structural behavior cannot be predicted by standardized data. Even stones of the same type, from the same quarry can vary. Stone is brittle and unforgiving; a few issues that can cause stone veneers to spall or shatter are:
- Insufficient or over restrictive anchors
- Excessive water infiltration
- Anchor corrosion
- Selection of an incompatible anchoring system
While there are numerous types of anchors available, they can be divided into 2 categories:
- Anchors inserted into a kerf or slot cut into the edges of stone panels
- Anchors inserted into a drilled hole in the sides or rear of panels
In general, anchors should be made of stainless steel or other non-ferrous metal to avoid corrosion.
Both the selected anchor type and the specific stone to be used should undergo full scale testing before installation begins.
Kerf Anchors
These anchors include straps, disks and rods, and spilt ear anchors. One benefit of this type is the kerf can be milled into stone economically in the shop. Kerf anchors allow for maximum erection tolerance in the field. One end of the anchor is inserted in a kerf or slot and the other is directly attached to the structural substrate. It is important that the kerf is deep enough to ensure that the anchor will not disengage if distorted, but shallow enough to minimize weakening of the stone. The kerf must be sealed with nonstraining sealant to prevent the stone from shifting. An expanding mortar or sealant should not be used.
Rod and Dowel Anchors
These anchors use rods inserted into drilled holes on the sides or rear of stone panels. Rod and dowel anchors are often preferred over slot type anchors as the mechanical stresses in the stone are better distributed around a hole than a slot. It’s recommended that anchor diameter not exceed ¼ of the stone’s thickness and that it be located in the center of the stone’s thickness. The depth of the hole drilled is typically 2/3 the thickness of the stone. 2 or more of the rods are set at opposing angles to reduce the chances of pull out. In rod and dowel systems, the bottom anchor of a piece of stone is typically combined with a steel angle to provide gravity support.
Wire Ties
Wire ties are used to connect stone to poured in place concrete. These anchors are not appropriate for high rise exterior use; they are best suited for low rise exterior or interior applications free from outdoor elements.
Allowing for Movement
One of the most critical criteria in designing stone anchorage systems is accommodating movement without stressing the stone. One cause of movement is varying thermal reactions of the stone, its anchor and the structural backup. Seismic and wind loads can also cause movement. Another stress to to be provided for are the forces imposes during installation. After the stone is installed, it should then be carefully inspected as cracks will expand over time. An additional movement that must be provided for is construction tolerance to ensure joint can align as intended.
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Reader Comments (3)
hi
Stone anchor testing is used as a method of confirmation of an engineering design or a regulatory system, without engineering is essential in the construction market increasingly complex where many subcontractors are responsible for different parts of a given wall. Specific project stone anchor testing not only provides a documented measurement of results, but may also give some peace of mind to the facility and equipment design and development.
Thanks..
Great post. The option not mentioned is the use of natural stone on an aluminum honeycomb panel such as StonePly: www.StonePly.com . The advantage of the StonePly system is that the stone is bonded to the honeycomb panel continuously over every square inch with aviation grade epoxy. The StonePly panels are about 60 times stronger than slab stone and about 1/5th the weight.
Nice post. I read your post. It’s very simple and informatics. Thank you for sharing..............
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