Q: We are installing a small exterior façade using stainless steel strap anchors. The specs require us to test the anchors in the stone. We argued against that requirement, since strap anchors have been used in stone for generations and surely they’ve been tested before, but we were unsuccessful in that argument. We’re now getting ready to test the anchors. The single tab anchors aren’t a problem, but we’re not sure how to test the split-tail anchors. We think the anchor should be tested while engaged in two pieces of stone, just as it is in the job, but the guys at the lab are saying that that’s too difficult to set up in the lab, so they want to test the anchor with one stone only. Wouldn’t this give us low results, since the load is not balanced?
A: I’m glad to hear that the specification was enforced and that you will be testing the anchor in the actual stone species. Yes, it has been tested before, but perhaps not in that stone, that thickness, with that anchor slot size, and that joint width. The cost of the testing is well justified.
Assuming that the lab is looking at ASTM C1354, Standard Test Method for Strength of Individual Stone Anchorages in Dimension Stone, they are probably getting confused by the graphics contained in that standard. C1354 is a very generic procedure by necessity since it needs to address testing of a myriad of different anchor types and conditions, and there is no way that a graphic can be provided for every possible configuration. Unfortunately, all of the diagrams in this standard for testing edge anchors show the anchor being rigidly affixed while the load is applied to the stone. In many cases, it is simpler to fixture the test in the opposite way, so that the stone is restrained while the load is applied to the anchor. In doing it this way, it is not any more difficult to test it while engaged in two stones.
Having said that, however, I still wouldn’t recommend testing with the anchor engaged in two stones. The loads on the two tabs of the split-tail are rarely balanced. If the split tail occurs between a course that is 3 ft (~1 m) high and one that is 1 ft (~300 mm), obviously the load is far greater on the tab that is engaged in the larger stone. Even if the stones are of the same size, windload, while we think of it as a uniform load, is usually not uniform. There are always “hot spots” on a façade which may have significantly greater windload than regions which might be right next to them. There are also field installation tolerances to consider. Most draftsmen and/or engineers will draw a beautiful detail of that anchor being perfectly centered in the joint, with exactly the same depth of penetration into each stone. Yet having been on countless jobsites during my 36 years in this industry, I have yet to see a split-tail anchor actually installed like that – they are nearly always tight to one side of the joint, providing maximum penetration into that stone while providing minimal penetration into the opposite stone. My recommendation is then to test the anchor engaged in one stone only, representing the most eccentric load condition possible.
While some in the industry avoid light gauge strap anchors altogether, I’ve used strap anchors and split-tail strap anchors my entire career and consider them to be a practical and effective means of anchoring stone. However, we need to be mindful that they are of a very limited capacity, and that capacity can be further reduced by some commonly encountered conditions in the field. So to protect ourselves, we need to mockup the anchor including some of those conditions that will compromise its capacity, so that we are getting an honest value for the anchor which is representative of how it will perform in situ.
Below is my standard protocol for fixturing this anchor type in the lab (Ref: Included figure):
- Test the anchor while engaged in one stone only.
- Provide blocking to replicate the maximum joint width specified on the project, and you may or may not have to cut the 2nd tab from the anchor to avoid interference with the blocking.
- Restrain the stone specimen and apply the load to the anchor. Ensure that the restraint on the stone is positioned far enough from the anchor slot to prevent interference with the failure spall.
- Position the anchor to represent the MINIMUM engagement into the stone’s kerf or slot.
- If using a filler in the anchor prep, intentionally divorce the bond between the filler and the stone. I usually do the test without the filler, as I’ve run the test both ways and rarely have I found significant difference. This recommendation is also in the C1354 procedure, with the assumption being that since most stone-clad buildings are designed for extended service lives, the bond will likely degrade over the useful life of the building.
- Always test a minimum of 5 specimens per anchor type and load direction.
