Q: I’m reviewing some anchor tests submitted on a project of ours. There are aluminum anchors that carry both dead load and wind load, but I only see testing performed in one direction. Why would they not be tested in both directions of load?
A: Many of the anchors that we use for stone cladding are bidirectional anchors. In the case of the aluminum J-Bar anchor, its ability to carry the gravity load of the stone panel is quite easily calculated, since it’s merely a function of the bending resistance of the aluminum, which is quite predictable. It would be rather rare to see this actually tested. The anchor’s ability to carry lateral loads is not easily calculated, since the failure will likely be in the stone, so we typically mock this up and test it per ASTM C1354, Standard Test Method for Strength of Individual Stone Anchorages in Dimension Stone. Other anchors, such as Type 31s or precast hairpin anchors are usually tested in two directions, although not simultaneously. Simultaneous bidirectional loading during the test would seem like the most prudent method, but we usually avoid it for two reasons. The first reason is that it’s really difficult to fixture in the laboratory. When necessary, we usually establish the gravity load as a constant, because that’s the easier load to predict. But fixturing the test specimen with a simulated, constant gravity load while applying a steadily increasing lateral load is difficult, because the simulated gravity load tends to vary during the test. The second reason for doing bidirectional testing infrequently is because it’s rarely necessary. We use very high factors of safety, anywhere from 4 to 12, for stone anchorage design. So if the gravity load is loading the anchor in shear, it is loading it to 25% of its shear capacity (or possibly much less), and this isn’t likely to have much influence on the tensile capacity of the anchor that is resisting the lateral load. There are a few exceptions, notably liner blocks, in which I’ve seen bidirectional loading result in significantly reduced capacities due to the prying action on the liner block. If bidirectional loading is needed, it might be worthwhile considering doing a C1201 test procedure, in which case the dead load of the stone is already represented.