I have a project that involves mounting a ceiling fan to an exposed masonry ceiling. The ceiling is sections of precast concrete (I think the right term is “flexicore”) 24” wide and about 8” think. The plan calls to mount a standard 30-50lb ceiling fan fixture, under an appropriately-rated electrical junction box, directly to this masonry. My initial though was to simply use a couple of Tapcon masonry anchors to secure the box, as that seems like standard industry practice for securing small-to-medium fixtures to masonry. It’s also what appears to be used for other fixture and conduit runs in the same building. However, after thinking a bit more I’m worried about the fact that a ceiling fan creates an small dynamic load through minor vibrations that are not inherit in other static mounting applications. In theory this dynamic load could weaken the integrity of the anchor’s threaded contacts over many many years, even for an application that is otherwise well within load limits. So I’m trying to figure out if this effect is so small as to be totally negligible, or if it’s something that really necessitates a different type of anchor.
A pair of 1/4” Tapcons at 1-3/4” depth seems to be sufficient to hold 20+ ceiling fans (load chart here) assuming factor of safety of 4 and lowest PSI of 2000 (I don’t actually know the PSI rating of this concrete, so I’m just going with the lowest value). This seems like a bulletproof approach, but again, I have no idea to what extent the dynamic load factor comes into play. I was hoping someone with industry knowledge of masonry anchor failure would be able to shed some light on that.
I’ve done some light exploratory drilling and probing in a hidden section of the ceiling to get a sense of where the hollows are, and where I would therefore expect the pretension cables to be. This of course means that I also have the ability to drill directly into a hollow (they are about 6” in diameter and thus come within about 1” of the surface). This would allow me to use some form of toggle anchor or SnapToggle instead of a Tapcon. This of course requires pretty big holes, and I suppose may come with other risks, but it's a very inciting option.