Take the 2-minute tour ×
Home Improvement Stack Exchange is a question and answer site for contractors and serious DIYers. It's 100% free, no registration required.

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?

share|improve this question
    
Alternatively, could you use something that clamps around the beam? –  Steven Oct 11 '12 at 22:33
    
I don't have access to all four faces of the beam, so I can't go around it. –  Jasper Bryant-Greene Oct 11 '12 at 22:41
    
Are you going into the side? Or into the bottom? –  Chris Cudmore Oct 12 '12 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/…) –  James Van Huis Oct 12 '12 at 17:41
    
@Chris Into the side. –  Jasper Bryant-Greene Oct 12 '12 at 22:21
add comment

2 Answers

up vote 3 down vote accepted

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.

share|improve this answer
    
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 '12 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? –  Jasper Bryant-Greene Oct 12 '12 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. –  Chris Cudmore Oct 12 '12 at 12:46
    
Wikipedia bears me out... en.wikipedia.org/wiki/Prestressed_concrete –  Chris Cudmore Oct 12 '12 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. –  Chris Cudmore Oct 12 '12 at 14:59
show 2 more comments

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.

share|improve this answer
add comment

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.