August 27, 2018
by Charles Muehlbauer
Natural stone products have perhaps the longest history of use of any construction material. Their usage was often governed by empirical design methods because everyone was comfortable with “the way things were being done.”
Materials introduced in modern eras typically underwent more vigorous testing and engineering to prove their worthiness as a construction component. For example, the number of standards and test methods for the use of steel in construction far outweighs the ones for stone. One of the reasons for this is steel, having been used in construction for only a couple of centuries, needed those quality control (QC) assurances to gain acceptance in the industry. Stone, with several millennia of historic use, did not require a similar validation of its usefulness in construction.
However, stone construction methods have changed; the days of loadbearing masonry walls built with massive, cubic sections of stone are no longer. Driven by cost-reduction desires and energy-efficiency requirements, today’s stone cladding is usually quite thin, frequently in the 30-mm (1 3⁄16-in.) range. This thin shell of stone then needs to be attached to the structure of the building by one of two methods: adhesion or mechanical anchorage.
Both adhesive and mechanical anchorage are used and accepted. However, there are limitations on what can be adhesively attached. Adhesive attachment of any masonry unit is limited to products not exceeding 73 kg/m2 (15 lb/sf) unit weight, 0.5 m2 (5 sf) in unit area, and 0.9 m (3 ft) in either the vertical or horizontal dimension. Stone products heavier or larger than these parameters must be mechanically anchored to the building.
Surprisingly, on smaller projects of less than perhaps several hundred square meters, the capacity of these anchors may not actually get tested. This is particularly true when an engineer’s stamp is not required on the stone anchorage design. Similar to the spacing of brick ties in a façade, there are industry-published “rules of thumb” about how many anchors are to be used for a given area of stone panel.
For instance, Chapter 15 of the Natural Stone Institute’s (NSI’s) Dimension Stone Design Manual recommends using four anchors for panels up to 1.1 m2 (12 sf), and two extra anchors for every additional 0.75 m2 (8 sf). The recommendation is accompanied by a host of cautions, as it should be, since multiple anchorage locations on a stone panel do not always equate to uniform loading of those anchors, and there needs to be some discretion used in the selection and performance verification of the anchor in the stone. However, for someone applying a stone base course to a retail store front, following this rule of thumb is likely adequate. If the stone is being installed at higher levels or in regions of greater loads, building professionals need to get serious about verifying loads, anchor capacities, and safety factors.
Testing stone anchorage
ASTM C1242, Standard Guide for Selection, Design, and Installation of Dimension Stone Attachment Systems, governs natural dimension stone attachments. This guide is written from the perspective of “best practices” as opposed to a “minimum practices” guide. It is also written with frequent usage of permissive language, as opposed to mandatory language. Therefore, much of the guide’s content is to be interpreted as suggestion rather than rule.
It is also unlikely it will ever be referenced in building codes, since they are always written in mandatory language and focus on minimum standards required for public safety, rather than best practices. Being a guide makes the enforcement of ASTM C1242’s content much more difficult, yet the document contains valuable information. It is one of the few documents with recommended safety factors for stone attachments.
Predictably, the recommendation is accompanied by a host of cautions and warnings saying the factors of safety may not be adequate in all cases, but they at least provide a starting point from which one can make modifications. The recommended factors of safety in ASTM C1242 vary greatly by the geological type of stone, as seen in Figure 1. It may seem alarming to some people to know travertine, for example, is recommended to have almost three times the factor of safety as granite. However, the recommendations are influenced by the variability of the stone and the concern of strength degradation over time. Sedimentary stones, such as limestone, travertine, and sandstone, are typically the most variable types.
Physical and mechanical test results in these stones will frequently have noisy data sets with large standard deviations. Additionally, there is no published test method within ASTM to assess degradation of strength due to weathering exposure. Both the variability of the material and the concern of possible degradation of strength over time are unknowns which the authors of ASTM C1242 have guarded against by recommending a high factor of safety.
There are two methods within ASTM to test stone anchorage. One is ASTM C1354, Standard Test Method for Strength of Individual Stone Anchorages in Dimension Stone. The other is a far more involved procedure titled ASTM C1201, Standard Test Method for Structural Performance of Exterior Dimension Stone Cladding Systems by Uniform Static Air Pressure Difference. ASTM C1201 was originally published in 1991, and the ASTM C1354 procedure did not arrive until five years later. This does not mean anchors were not tested prior to 1996, but that there was no standardized, published protocol for the laboratory to follow.
The simpler and more economical ASTM C1354 procedure will suffice for a vast majority of projects. As far as test procedures are concerned, this is a very loosely written, generic procedure, as opposed to the typically rigid test protocols. ASTM C1354 is a simple, five-page document on the basic setup, cautions, and reporting requirements of testing a stone anchor. It is intended to be applicable to all anchors used with dimension stone, so it necessarily allows some flexibility in the building of fixtures and the application of load to accommodate a myriad of anchor types. It also establishes significant recommendations.
For instance, it recommends all stone specimens are tested in a saturated condition because if a stone exhibits different strength based on its saturation level it will nearly always test lower when wet. It also mandates a minimum of five specimens be tested for each condition, and this should be adhered to, as the level of statistical confidence is related to the number of specimens tested.
One often-questioned recommendation in the procedure is eliminating fillers, or at least, eliminating the capability of the filler material to bond to either the stone or the anchor. Anchor preps in stone are always oversized to some extent, and some type of filler, commonly silicon, polyurethane, or epoxy, is used to displace the resultant void. The rationale for intentionally divorcing the bond between this filler and the stone/anchor was the knowledge most stone façades are designed for relatively long service lives. Therefore, testing the anchor without the benefit of the bond would ensure the anchor would still perform if the adhesive bond of the filler material failed during the building’s lifetime. Considering most ASTM documents take about five years from conceptual development to publication, one can assume ASTM C1354 was drafted in the early 1990s, which would mean the consensus of the committee was based on experience from projects in the 1980s and before. The recommendation to prevent bond has never been modified since the original publication, so it is really based on the performance of adhesives of roughly 40 years ago. Obviously, there has been great advancement in adhesive technology over this period of time, so today building professionals tend to allow the bond to exist for the test more frequently than in years past. This is especially true if adhesive manufacturers can provide some assurances on the useful life of the adhesive.
ASTM C1354 does not address simultaneous bidirectional loading even though many of the anchors are subjected to it in the building façade. For a majority of them, bidirectional loading is unnecessary, as designers have found their performance in the two directions is very similar if loaded simultaneously or loaded in each direction separately. Since smooth, uniform application of simultaneous bidirectional load is very difficult to achieve in the laboratory, testing is typically avoided. In the author’s experience, the stone liner support detail is an exception as it will yield falsely optimistic values if tested with load applied in only one direction (Figure 2).
The ASTM C1201 procedure should be specified sparingly, as the cost per specimen can be 20 times more than the ASTM C1354 procedure. ASTM C1201 is similar to ASTM E330, Standard Test Method for Structural Performance of Exterior Windows, Doors, Skylights and Curtain Walls by Uniform Static Air Pressure Difference, in setup and scope. Since ASTM C1201 only addresses one material—stone cladding—it is a little simpler, and also provides some notes and cautions.
The ASTM C1201 procedure calls for a sealed mockup chamber into which pressure and vacuum are alternately applied. It can provide valuable information by replicating the behavior of various components not included in the setup for ASTM C1354. This basically justifies the specification of the more expensive ASTM C1201 procedure. If there are other components involved that may deform under load and adversely influence the performance of the stone anchor, then running the ASTM C1201 procedure is the best way to replicate those influences. Consider the two anchors shown in Figure 3.
The strap anchor on the left is not going to perform any differently in ASTM C1201 test vs. ASTM C1354. It is affixed to an unyielding concrete support, which will have negligible deformation. The back anchor on the right, however, may perform quite differently from one test method to the other. To test the back anchor in tension, as per ASTM C1354, a straight tensile pull will be applied directly on the anchor, using a cylindrical restraint to avoid introducing any flexural stress into the stone specimen. But the in-situ arrangement is not likely to result in a straight tensile pull. The clip angles and miscellaneous metal frame will deform to some degree under load, and this will result in a prying action on the anchor.
The ASTM C1201 test method includes Procedure A and B. In Procedure A, deflections are recorded only at maximum load, while in Procedure B deflections are recorded incrementally to establish a load/deflection curve. For the example mentioned above, one should use Procedure B to better understand the influence of the deformation of the metal components. Let us assume the system failed prior to reaching the required proof load (anticipated load times the factor of safety). If substantial deflection was recorded prior to reaching the design load (without factor of safety), then there is perhaps a design issue here. However, if substantial deflection occurred only at higher loads, then perhaps the system is adequately designed because the steel components are not designed to the same factors of safety as the stone components, so running them up to those loads is somewhat unfair. This is a case where professional judgment is required.
ASTM C1201 can be run until the proof load has been met, or at the option of the specifier, loading can continue after the proof load has been met until failure of the specimen. Some engineers prefer to discontinue testing upon reaching the proof load. There is sometimes a concern if the system performs well past the required load they will be accused of grossly overdesigning the system, and subsequently asked to redesign it to reduce costs. Conversely, there may be a concern if the system fails soon after meeting the proof load. The clients may be uncomfortable with the results because the system barely passed the requirement. The author personally prefers to take the system to ultimate failure. A lot can be learned from seeing the mode of failure, witnessing if mode of failure is consistent across all specimens, and how consistent the load values are at failure. In some cases, however, the laboratory equipment is not capable of producing enough pressure in the chamber to produce failure of the mockup.
When specifying for an anchored stone cladding project, think about how the anchor capacities will be documented. One, or possibly both, of the test methods discussed here should be in the specification.
Charles J. “Chuck” Muehlbauer is the technical director of the Natural Stone Institute (NSI). Muehlbauer is now in the fourth decade of his career in the stone industry, with experience in quarrying, fabrication, installation, anchorage design, stone selection, industrial stone applications, laboratory testing, stone forensics, estimating, and both domestic and international sales. He is a voting member of seven national technical committees, including ASTM Committee C-18 on Dimension Stone, on which he recently completed his term as committee chair. Muehlbauer can be reached at firstname.lastname@example.org.
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