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I had an argument with a friend about the safe location for openings in reinforced concrete beams. I've chosen position 1, a little above reinforcement steel, as at this location, the beam is subjected to tension in the lower fibers which is resisted by reinforcement steel and, assuming failure case, the concrete at lower fibers is cracked.

Any other opinions?

beam

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    $\begingroup$ I like (1) and (4) personally, since they'd be in the tension portion of the beam. I'm not so hot on (3) since that might be in an area where you'd expect diagonal cracks from shear, and (2) and (5) might intrude on the compression block (but not likely since the compression block is usually within 2" of the surface anyway). $\endgroup$
    – grfrazee
    Mar 18, 2016 at 13:40

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This is the sort of thing that is best defined by each country's structural codes. I'm from Brazil, so I'm going to use the NBR 6118, our concrete code. That being said, the general gist behind all of the codes tends to be pretty similar.

Doing a free translation, this is what is described in the Brazilian code:

13.2.5.1 Openings that cross a beam along its width

In any case, the minimal distance between an opening and the nearest beam-face must be greater than 5 cm (~2") and twice the concrete cover adopted for the beam. The remaining section in this region, having discounted the area occupied by the opening, must be able to resist the design loads, as well as allow for a good concrete pour.

So that no special check is required, the following conditions must all be satisfied:

  1. openings must be in the area under tension and at a distance of at least $2h$ from the support, where $h$ is the height of the beam;
  2. the opening must be lesser than or equal to 12 cm and $\frac{h}{3}$;
  3. the minimal distance between aligned opening faces is $2h$;
  4. concrete covers must be satisfied and steel reinforcement is not disrupted.

So, of the positions you present, only position 1 is automatically valid (and maybe 3, depending on the beam's cross-section and where exactly it is, in relation to the support). Position 2 is in the compression area, and 4 and 5 are too close to the support.

That being said, there's nothing to stop you from making openings at these other positions, so long as you certify that the structural integrity of the beam will not be affected.

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Looking in ACI 318-14, Building Code Requirements for Structural Concrete, I find the following:

22.5.1.7

R22.5.1.7

Doing a word search for "opening" only shows specific results for openings in one-way and two-way slabs and walls. However, reading the above Commentary section R22.5.1.7, you can use the provisions of Chapter 23 to design using the strut-and-tie model.

strut and tie

Otherwise, you can use any rational analysis to determine how the hole in the concrete web affects the strength of the member. For your cases (1) and (4), these appear to be in tensile regions of the beam, though (1) would be more ideal since there is less shear stress.

Update: concerning the strut-and-tie discussion in the comments and applicability to slender beams, the Code has this to say:

23.1

Thus, it appears that the strut-and-tie model can be used for slender beams so long as the member or local region are checked as an idealized truss following the provisions of that Chapter.

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    $\begingroup$ I'd not have considered this beam to fit into a strut and tie model, especially considering OP's implied aspect ratio. This made me look for a aspect ratio limit in ACI and I couldn't find one. Also, OP didn't specify the application of loading. Is Strut and Tie valid for distributed loading? $\endgroup$
    – William S. Godfrey- S.E.
    Mar 18, 2016 at 15:39
  • $\begingroup$ @WilliamS.Godfrey, most likely not. Thanks for the clarification. I'll add a caveat to the answer. $\endgroup$
    – grfrazee
    Mar 18, 2016 at 15:45
  • $\begingroup$ @WilliamS.Godfrey (I'm Brazilian, so I apologize if I'm talking nonsense, since I'm not used to US codes). I understand the reference to strut-and-tie models for openings as applying as a local analysis of a beam. I'd use the Mörsch truss analogy (used in reinforced concrete beams) to create a local strut-and-tie model for a long beam around the opening. I just find it odd that the code would point to strut-and-tie for openings if they're only valid for short elements. $\endgroup$
    – Wasabi ♦
    Mar 18, 2016 at 18:07
  • $\begingroup$ Thanks for the further info. I only own up to 318-11 and the strut and tie chapter was still Appendix A at that time. Doesn't Section 23.1.1 preclude the application of the chapter to slender beams though since slender beams (almost always) have a liner strain distribution (regions around openings withstanding)? Either way, this is good info. $\endgroup$
    – William S. Godfrey- S.E.
    Mar 18, 2016 at 23:21
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    $\begingroup$ @Wasabi, I agree it's odd that this chapter is the only place that openings are mentioned. I'd have never thought to use a truss analogy in practice, I would have just reasoned it out just like grfrazee did in his original comment on OP's question. Good stuff. $\endgroup$
    – William S. Godfrey- S.E.
    Mar 18, 2016 at 23:24
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First preference is #1 , as long as not too near to the edge (affecting rebar). No problem eliminate concrete at this location since the load is on rebar.

Second preference (if #1 means close to the edge / affecting rebar) is #3. #3 is less preferable since half of the hole is eliminating compressive concrete (upper half beam).

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