Slab Penetration in Center of Column Strip

[KBVT]

I have a slab penetration that the architect wants to slap directly between two columns. Bay spacing is approximately 20'-0" x 29'-0", penetration would be located in a column strip-middle strip intersection. I took a look at the ACI slab opening provisions, 13.4, and I cannot use the prescriptive methodology here because I interrupt more than 1/4 of the reinforcement in the middle strip.

How do you usually analyze the design strips for this opening? Since the calculated moments are usually averaged over the width of the strip, is it accurate to calculate the required reinforcement assuming a decreased strip width due to the opening. Add 50% of the interrupted reinforcement each side of the opening and add any additional bars required for strength?

 

[kslee1000]

Yes, & yes.

 

[BAretired]

Well, I think you have to be a little bit careful about doing that. It could be all right provided the opening is not too big.


BA

 

[KBVT]

Well, I would think that if the slab opening becomes too large than you would have to look at detailing the slab as two cantilevers, but at point do you make that transition from spanning around the opening to slab cantilevers? 50% strip section removal? Anyone have a rule of thumb they use?

 

[lutein]

what we typically do is analyze it using FEA program, such as adapt. it would allow you to correctly monitor the stress around the opening and provide sufficient reinforcing.

 

[msquared48]

Just tell the Architect that the addition of the opening will require the addition of two new columns , one to either side of the opening in the column strip area.

It's Friday - play with him and have some fun with this one.

Mike McCann
MMC Engineering

 

[KBVT]

Mike - maybe 4 coluns will do the trick, and I'll frame the opening with perimeter beams. I have a feeling they would stick with the middle stip - middle strip locations in that case.

I have used RAM Concept to analyze the slab, but although it is a FEA program, it averages the stresses within the strip, so essentially I receive an extra #6 bar. Not too much to sneeze at. Another engineer in the office has major concerns concerning the required continuity bars extending through the column zones. But in my opinion, this interruption occurs all the time at elevator, stair openings, etc.

 

[kslee1000]

This openning will be truely critical if it is located in the area bounded by the column strip in each direction. Your case is not desirable (from structural eng's view), however, as you have admitted, it happens quite often in reality. Anyway, other than strength, check the deflection diagram, with an eye on stress diagrams, closely, you will detect areas that require add'l strengthening to smmoth out the stress flows.

 

[Tomfh]

Use RAM concept to find areas of high stress and deflection and then reinforce appropriately from first principles.

RAM concept's reinforcing mindset is strip based, despite it being a 3D package, hence you shouldn't expect it to choose particularly good reo arrangement for you.

 

[kslee1000]

Note, in general, high stresses are likely to occur at the corners (you now that well). Also the columns surrounding the openning will see higher stresses as well.

 

[NS4U]

I think FEA is the way to go. the strip method is meant as an approximation, but I think a good FE model will show you what is really going on. But be aware that the strip method allows you to ignore twisting in the slab. You will need account for twisting in the FE model.

 

[csd72]

Be careful in assuming that FEA is much more accurate. FEA is an elastic analysis while concrete is not an elastic material. It has its uses though and I expect it will be more accurate than any other method for this case.

 

[asixth]

Design the slab to span around the penetration. This would include framing the penetration with beams. The Effective depth parameter of the column strip will lower a little but typically, you will need to displace the reinforcement that would pass through the penetration elsewhere within the column strip so it may become quite heavily reinforced. The penetration is going reduce the stiffness of the column strip so deflections may increase, another good reason for framing out the void with beams.

How does Adapt go as a FEA concrete package, I have never used it.