huangyhg

holes in concrete walls or tanks
i am designing several concrete tanks and there will be penetrations through the tanks to accomodate large pipes (say 36" in diameter). does anyone have any design examples or design methods for determining reinforcing around penetrations through concrete tanks or concrete walls?

all the ones i've seen have rebar all around the perimeter. also, rebar at corners at 45 degrees.
see
this thread,
there's a lot of subjective judgement involved in analyzing an opening in a wall. what i like to do is use some rational method, like using the thickness of the wall on each side of the opening and checking the forces from the opening plus the forces from that section. it usually, but not always, is enough to add an amount of reinforcing equal to the interrupted reinforcing.
as a rough rule of thumb i would aim to "move" the rebar which is in the hole to either side, locally reinforcing it, and use diagonal framing bars at the corners as bagman suggested. i think the size of the hole and its effect on the integrity of the structure would determine how much effort you need to invest in designing and detailing the rebar.
also, depending on how you are placing your inserts, box outs as opposed to cast in, i have often seen an internal rebate within the box out which serves to act as a waterstop and a mechanical key for groutings afterwards. a hydro expansive waterstop is also a good idea around the cast in item.
i think the method of design for the wall depends on the size of the opening with respect to the wall. if it is large you would need to consider the wall in terms of strips and panels supported on two sides. if small you can consider the wall, as a wall and just make local modifications to suit the opening. it all comes down to a judgement call from the engineer as to whether the wall acts like a wall or not.
we have also used the same methods used on steel tanks in that we make a beam and column frame-work around the opening. this is more intuitive and is probably conservative but it works. we do this for "large" openings. we would just provide steel in the wall itself to make up the framework around the opening and size it for the wall loads accordingly that it will be seeing, spanning over the opening.
article 13.1.1 of aci 350 reads: "provisions of chapter 13 shall apply for design of slab systems and straight tank walls reinforced for flexure in more than one direction, with or without beams between supports."
the commentary to 13.1 reads: "the fundamental design pronciples contained in chapter 13 are applicable to all planar structural systems subjected to transverse loads." excluded are one-way slabs and slabs-on-grade.
moving the reinforcing to the sides and top and bottom of an opening (together with diagonals) essentially creates a set of beams within the slab to carry the forces around the opening.

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