huangyhg

holes in concrete beams
i need to place an 18"wx10"d hole in a 12"wx24"d concrete beam. of course, i am placing it where i have the least shear (about midspan of beam) and based on the tension reinforcing in the beam (3#7 bottom) i calculated "a" from
a = asfy/0.85(f'c)(b)
my question is: as long as my calculated "a" is smaller than the actual "a" in the field, i should be okay. is this correct? in this case, my calculated "a" was 3.53 inches. i placed the hole in the beam such that i have 8-inches of concrete on the top, 10-inches for the hole depth and 6" of concrete below the hole providing encasement to the beam's bottom reinforcing. it would seem to me that since the compressive stress is satisfied it should be okay to place the proposed hole as described. am i missing anything?
thanks for any assistance.
8" above and 6" below the opening does not seem like enough. you will need top & bottom steel plus stirrups in each piece. i don't don't how you can squeeze all that in.
it is not just the principal moment that you are designing for. it is possible that the beam could be half loaded. this shear developed by the half loading can also add a secondary moment with the shear lag caused by the opening that you have not taken into account.
additionally, you may need additional stirrups in the upper and lower segments. the shear thru each segment needs to be designed for.
it is probably a good idea to also check deflections.
i would ask your supervisor for some help on this one!
6" (150mm) isn't a lot of space to accomodate the longitudinal bars, stirrups and cover to each 'face'. you may need to sketch the geoemetry of the beam.
a couple of things to consider:
- if the hole is large wrt to the beam depth, then you may get a vierendeel-like moments at the corners of the opening.
- suggest you have filleted, rather than sharp, corners.
jike: how would half loading develop? theoretically i'm at a point of zero shear. my ultimate moment is small (mu= 60 ft-kips. i understand that if the hole was wider perhaps i'd developed vierendeel-like behavior as dbuzz points out. but as is, it seems to me that the only forces would be the resulting couple from the moment. in fact i left 8-inches on the top to add a couple of small bars below the top reinforcing and tie them with perhaps no. 2 ties simulating a short column. but i had not plan on doing anything to the bottom since it would be in tension...
any other thoughts? anyone has literature on this subject?
thanks for the info.
here's a book on the subject:
i don't know what the use of your particular building is, but just because you design a beam for uniform load does not mean that in its lifetime, it will not see partial loading. uniform loading is a theoretical concept, but reality may be completely different. you should design the opening for the controling condition, whether it is uniform loading or partial loading.

in an office building, the owner could elect to locate his furniture on one side of the beam and put a hallway on the other side. in a storage building, half could be loaded and the other half unloaded.
you might also consider stress concentration around the hole which will increase your normal stresses. roark chapter 17 deals with this.
i've found a very good article in the aci structural journal, volumn 93 no. 4 july-august 1996 titled "design procedure for reinforced concrete beams with large web openings authored by k.h. tan and m.a. mansur. from the same authors there is another article on aci journal vol 96, no. 6 november-december 1999 titled "effects of creating an opening in existing beams". with these 2 articles i should be able to resolve the issue. thank you all for your interest and assistance.
dario gonzalez, p.e.

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