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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.

Alternative Approaches

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.

  • I should also note that building management has already approved the installation and the use of "appropriate", "industry standard" masonry anchors, but they will not impose exact requirements regarding what anchors are used. – rjacobs Sep 14 '16 at 16:06
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It reads like you are at 'industry standard' as most installs wouldn't go further than a concrete anchor. I understand your concern, and appreciate the level of detail you're delving in to figure out alternatives. In terms of industry knowledge, most of the folks here have experience with residential, single family home classification, not industrial, or commercial application, it's all wood.

If you're looking for other options, find the exact weight of the fan, and find a hard rubber (or soft plastic) washer that only partially compresses from the weight(very partially, gross up your concrete anchor by a 3/16th (or whatever the net distance increase is from the washers, and add first a metal re-enforcing washer, followed by the soft washer. this will provide a break between the fan and the anchor.

One important point to cover everything is that you'll need about a week of curing before you'll hit the 2000psi (assuming 20mPa). A day or two less if you're using 25mPa.

Disclaimer of course is that I'm not a fastener guy, and do not have extensive knowledge of the failure rate of in tension concrete anchors with vibrations applied.

  • Yeah, my own experience is residential as well and I've done installs like this before, just never in concrete. I just had a look at the mounting bracket of the actual fan (what will be attached to the junction box) and see that it already has a layer of rubber lining it. This should help a bit to dampen some of the movement transferred up to the box and anchors, though I'll also see if its practical to add flexible washers to the fan mounting bolt connection itself. That's a good point -- it's not just about the concrete anchors. – rjacobs Sep 15 '16 at 21:16

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