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?
Best Answer
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:
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.