Large circular opening in large circular shaft

[MPETER]

I'm building a 30 foot tall 10 foot diameter reinforced concrete shaft structure. How do I allow for a 4 foot diameter penetration for a sewer pipe into the structure. What sort of reinforcing approaches can I use. The concern is primarily the hoop stress not being transmitted through the opening. I was hoping to Tie the hoop rebar to added reinforcing around the structure as one might do in a wall panel, however the project engineer wants to use a stainless steel sleeve at the opening and positively connect this sleeve with the hoop steel. Since the sleeve must be stainless steel due to the wastewater combined with the circular geometry of the shaft itself this is an expensive and problematic fix. We also looked at placing an embedded channel frame in the form work around the circular PIPE opening and tying rebar to this. The simplest and easiest solution is just adding Reinforcing around the circular opening but we seem to not be able to find any examples on how to do this.

 

[BAretired]

If the stresses are not critical in the remaining 3' sections each side of the opening, you don't need any additional reinforcement. If they are critical, you need to either add wall thickness or additional vertical bars each side of the opening to compensate for the hole. Additional vertical bars must be fully developed in bond above and below the opening.

In addition to that, it would be a good idea to place diagonal bars at 45 degrees above and below the opening on each side.

BA

 

[JStephen]

A suggestion on the reinforcing approach- see attached file- add bars above and below the opening to account for the bars that are cut, add vertical bars as shown, then check that rectangular area as a beam for shear and bending.

In steel tanks and vessels, the analysis essentially consists of adding additional steel area around the opening to replace the material removed. The idea is simple, sometimes the details get involved. That always assumes that the assembly will behave in a ductile manner.

They do make large concrete pipe and fittings, they do put manways into concrete tanks, so it might be informative to do more research on how that's handled.

If the hole is too large relative to the cylinder, you start getting some additional out-of-plane effects going on.

 

[MPETER]

Good input.

I reread my question, failed to mention the 4 foot diameter penetration is for a pipe that runs through the base of the 10 diameter foot shaft , in other words there are 2 ea 4 foot penetrations ( one at 12 o'clock and one at 6 o'clock) all down at 30' deep

To add to the issue there are also 24 inch and 30 inch diameter penetrations at 3 o'clock and 9 o'clock respectively. With the base having a significant amount of holes (4 total) , concerns about the hoop stress is warranted, we just feel it can be addressed with reinforcing steel. Note; The shaft is completely buried. The shafts concrete wall is being proposed at 2' feet thick. The rebar is a double wall of #6 BW@ 12" oc . We may just abandon the round shape and go to a rectangular shaft to eliminate the hoop stress concern and deal solely with lintel/beam forces

 

[KootK]

Your "hoop stress" is really compression in the shaft walls, right? Normally hoop stress is envisioned in the tensile sense. While a rectangular shaft might be appealing because it seems simpler to analyze, I think that it will produce a weaker, less efficient structure. Here's another alternative:

1) keep the circular shaft.
2) imagine a strong ring of concrete placed just above your highest opening. Design it for the forces that I mention next.
3) treat the panels between openings as vertical elements spanning between the base and the strong ring.

That's about it. With 2' walls, this should be completely manageable. JStephen is right, you're not blazing new trail here. You just gotta figure out how the original wheel was invented.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[MPETER]

yes I agree on the hoop stress definition- tension not compression. Issue is the engineer of records fixation on hoop tension developed by fluid surcharge from the interior of the structure (and for a buried structure??)

Good input on treating the remaining shaft walls of the swiss-cheesed lower as vertical panels- may get me past this hoop stress argument.

 

[BAretired]

The fact the your holes are placed symmetrically is an advantage because it means there is no eccentricity on the shaft as a whole.

What is causing the hoop stress? Do you have internal pressure or are you talking about soil pressure? Why is the wall two feet thick? What kind of forces do you have acting on this shaft?

BA