It seems to be very common to drill small holes in any kind of structural concrete for mounting fixtures without worrying too much about any kind of reduction in strength of the structure, but I prefer to be safe than sorry…

We have a concrete beam (roughly 285mm wide x 540mm high including plaster coat) supporting a roof and part of a mezzanine floor.

I know nothing about the composition of it; it may encase a steel I-beam (the rest of the building structure is steel I-beams) or it may simply be reinforced concrete. The building is circa 1960s so it is not likely to be unreinforced concrete.

I wish to attach a 2 m horizontal length of steel pipe along the beam to attach stage lighting to. The pipe itself will weigh about 9 kg, and I will hang at most about 10 kg of lighting equipment from it.

In order to fix the pipe to the beam I would need to drill eight 6 mm diameter, 20 mm deep holes in the beam (four at either end of the pipe for the mountings). I would put M6 Dynabolt expansion anchors in these holes, which are rated for 3.4 kN (346.7 kgf) shear capacity each, giving a very large safety margin above the expected load.

Is it safe to drill small holes like this into structural concrete and use expansion anchors in them or should I consult an engineer before proceeding?

  • Alternatively, could you use something that clamps around the beam?
    – Steven
    Oct 11, 2012 at 22:33
  • I don't have access to all four faces of the beam, so I can't go around it.
    – jbg
    Oct 11, 2012 at 22:41
  • Are you going into the side? Or into the bottom? Oct 12, 2012 at 14:57
  • If the beam is metal, then you could try some large neodymium magnets (e.g. amazon.com/Applied-Magnets-Strong-Neodymium-Magnet/dp/…) Oct 12, 2012 at 17:41
  • @Chris Into the side.
    – jbg
    Oct 12, 2012 at 22:21

2 Answers 2


You can actually punch surprising large holes in the middle of a beam without diminishing its load-bearing capacity. This is because that capacity is largely a function of only the top and bottom portions of the beam and how far apart they are.

Engineered floor joists are a good example of this:
Engineerd Floor Joists

It would take a structural engineer to tell you exactly how much the load-bearing capacity will be reduced, but I can tell you that this amount will be at least an order of magnitude less than the margin for error in manufacturing the beam.

Don't worry about it. It's what these things are made for.

  • If the beam is pre-stressed, however, drilling in the center is the absolute worst place as that's where the cabling is.
    – DA01
    Oct 12, 2012 at 2:53
  • Correct me if I'm wrong, but the builders would have been somewhat ahead of their time to be using pre-stressed beams in the 1960s, wouldn't they?
    – jbg
    Oct 12, 2012 at 3:04
  • 1
    I think the cabling in a pre-stressed beam would be on the lower part of the beam, where they can take up tension. It wouldn't make sense to pre-stress the neutral axis. Oct 12, 2012 at 12:46
  • Wikipedia bears me out... en.wikipedia.org/wiki/Prestressed_concrete Oct 12, 2012 at 12:47
  • Unless you're talking about going up through the bottom. But I doubt a drill, with a masonry bit on hammer mode would get very far into steel. Oct 12, 2012 at 14:59

If it was certain that integrity of the beam was being compromised you could face the beam with a self supporting wooden structure that extended to the vertical supports on either end if the concrete beam. Then attach your lighting bar to the wooden structure.

If you do drill into the concrete beam the use of expansion type fasteners is best only for the portion of the beam that would be under a compressive load. In this case this would be in the upper portion of the beam.

It may be a better strategy to locate the part of the beam that is known to be in the neutral zone with respect to stress and then drill all the way through the beam and use long bolts that hold your brackets in place by applying compressive pressure across the beam.

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