Is it safe to drill and tap an i-beam in order to mount wood to it, to then mount a TV to the wood? TV and mount will weigh about 100lbs combined. I plan on using 1/4 bolts, and the two pieces of would will be glued together. The picture below roughly represents what it is I want to do.

The i-beam is vertical. This is a warehouse, the i-beam is about 22' by 3", it's holding up the roof of said warehouse.



I see an S3X5.7 (the smallest S3 section) that has a weak-axis section modulus of 0.383 in^3. Assuming 36 ksi steel and that it only supports your 100# at 12" eccentricity and at mid-height (mid-height nullifies any P-delta effect), I get a demand-capacity ratio of 100#/[(0.383 in^3)(36000 #/in^2)/(12 in)] = 0.087. I can't imagine those 100 pounds could fail your column assuming that it is indeed an S3X5.7.

As for your connection detail, don't tap the holes, just bolt through to a nut on the opposite side of the web. That web is probably paper-thin, so tapped threads will limit the bolt to far below its rupture capacity, and drilling 3 holes at 5/16" through the web shouldn't hurt the column any more than 3 holes at 1/4" (negligible either way).

Also on the connection, I would strive for sufficiently thick blocking between the flanges so that the tightened bolts leave an unfilled gap between the flange tips and your outer board. Otherwise you could end up transferring the load by friction between that outer board and the flange tips, where any slippage would activate the bolts in bending (bad) instead of shear (good). And the strength of the connection depends on the distance between the top bolt and the bottom bolt, so try to maximize that distance (8" between 1/4" bolts sounds more than adequate to my intuition). Oh, and skip the glue.

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  • I don't have access to the back side of the beam, would it be prudent to use toggle bolts here? – DipreSantana Nov 6 '18 at 17:37
  • I'm no machinist, but my intuition says that the thickness of a nut is chosen to develop the rupture strength of the bolt in pure tension (with a little safety margin). Assuming that the bolt and web are made from similar materials, a tapped web at 25% the thickness of a 1/4" nut would develop 25% of the bolt's strength. I would drill the hole, observe the material's thickness, and make a judgement from there. You could also through-bolt to the exposed flanges to work out an alternative connection detail (I suspect that they're tapered, however, and I like my hardware to lay nice and flat). – popham Nov 6 '18 at 17:40
  • I don't trust toggle bolts, but they would work theoretically. – popham Nov 6 '18 at 17:48
  • I just looked up the S3X5.7 web thickness. It's 0.17". A 1/4" nut is 3/16" thick, which is 0.19". While that seems reasonable, A307 bolts use 60 ksi steel whereas your column is probably 36 ksi steel, so I would want bolts whose nuts have a thickness of (0.17 in)(36 ksi)/(60 ksi) = 0.10". I expect that these tiny bolts would be unreliable, getting damaged during installation. Instead I would opt for 6 #12 self tapping screws, 3 clustered at the top and 3 clustered at the bottom to attach a 1.5" block to the web, and then wood screws to attach your cap plate to that block. – popham Nov 6 '18 at 19:22
  • Thank you, you have been a lot of help. Does it matter that the beam is standing vertically, and I will be drilling holes at one of the ends, and not near the middle? – DipreSantana Nov 6 '18 at 20:18

It shouldn't be an issue, two 1/4" holes in the web of the beam are not significant. (I'd only bother with two holes - more than enough strength.)

I would just use four beam clamps so it's readily adjustable, and because it's much easier, and secure enough.

Either way, I'd just bolt the mount straight to the beam clamps, or straight to the beam, no need for the wood.

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  • At my plant we are not allowed to touch the flanges except with beam clamps (very handy) device. We can drill and tap the web 1/4-20 bolts are quite common. I have quite large 3 phase panels and motor starters nena class 5 (large) hanging from 3 or 4each 1/4-20 bolts and conduit going up 30' the conduit is strapped but the load is supported by the enclosure attached to the beam+ – Ed Beal Nov 6 '18 at 18:50

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