Punching Shear ACI | Calculation Method

[bookowski]

Question(s) regarding calculating punching shear capacity per ACI, primarily this is in regards to corner columns:

Is there a clear direction/requirement on how to calculate max stress for corner columns per ACI? I have seen this approached many different ways and have never found a satisfying answer. Looking at 318-11 here but no difference that I know of from previous editions:

- 11.11.7.2 lays out a V/A + M/J procedure for calc'ing max stress. They provide this for an edge condition and state that "similar equations may be developed... for columns located at the corner of a slab". This leaves the accepted procedure at a corner open.

- I have seen many texts and software (SAFE and RAM) which treat corner columns by finding the principal axis and transforming moments to find the M/Jz stress where z is the principal axis.

- I have seen hand calcs that do M/Jx + M/Jy and superimpose the results.

- PCA Notes on 318-11, page 16-15 "where biaxial moment transfer occurs, research has shown that the method for evaluating shear stresses due to moment transfer between slabs and column in R11.11.7.2 is still applicable. There is need to superimpose the shear stresses due to moment transfer in two directions". This is allowing us to check one direction at a time, x and then y.... doesn't seem to be much logic in this other than it possibly has shown to be ok relative to testing.

- From the CSI wiki page for SAFE on "why do results differ from hand calcuations" - "For corner columns, results should differ from ACI and PCA formulation because SAFE incorporates the I23 moment-of-inertia term, leading to more conservative results. We believe this approach to be more theoretically sound." This I23 is the principal axis component mentioned above. SAFE has a white paper where they have verification calcs and show the PCA method vs SAFE.

- Decon, a manufacturer of studrails, has free software - this follows the SAFE approach of finding the principal axis

So I'm seeing 3 general approaches: Check X and Y independently (PCA and maybe ACI?), Superimpose X + Y, Find the principal axis and find M/Iz.

Beyond just being curious this has a very large impact on the results. A corner column in SAFE that has a demand/capacity ratio of 2.0 which would put it beyond even shear reinforcing per ACI. The same condition (same moments, shear, geometry etc) done by X and then Y independently may be well below D/C 1.0 meaning it doesn't need anything. The X + Y approach falls in the middle.

This is a tough sell if I have corner columns that need to be 30x30 for punching shear and the guy down the hall can pull off 16x16.

Am I missing something where this is clarified?

 

[bookowski]

KootK - Thanks for those references, very interesting. The 0.5 phiVc is especially interesting since this would be much easier to satisfy.

This problem really sticks in my craw, hard to let it go. Usually when there is such a discrepancy it's from modeling assumptions, i.e. I fixed the base and you assumed it was pinned. It's not often that you can start with a set of forces, go to a particular section of code, and end up with different answers by a factor of 3. I could do a job in Risa and get peer reviewed by someone using safe and they'd sound the alarm that I am failing by a huge factor. Strange.

 

[KootK]

@bookowski: if you're still feeling ambitious, I would love to see how the 0.5 x phi x Vc stacks up against the other data that you've tabulated.

@Josh: I've always felt that there was a real opportunity in the software world for more engineer friendly punching shear reporting. Pretty much every firm that I've worked with has had an internally developed, excel based post processor for punching shear. I'd love to see FEM software that would do the following:

1) Output an excel data file indicating LOCATION, LOAD CASE, CORNER/EDGE/INTERIOR Mux, Muy, Pu etc.
2) Manually or automatically read back in a data file indicating LOCATION, LOAD CASE, UTILIZATION RATIO.

This would allow designers to program their own punching shear algorithms as they see fit but still handle it efficiently and be able to view the results graphically.

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.

 

[JoshPlumSE]

Thanks KootK, I'll write that up as a request.

 

[bookowski]

I am and I will.

I am also going to play with modeling assumptions for col height and far end fixity, col cracking, and using linear uncracked slab moments verus fem determined cracked slab moments. I am thinking that the far extremes of the ps method combined with modeling assumptions will probably take it over a factor of 5. At which point I will never have to check punching again because I'll be satisfied that no one knows how to deal with it.

 

[KootK]

At which point I will never have to check punching again because I'll be satisfied that no one knows how to deal with it.

Right?? Long ago, I was taught that the preliminary rule of thumb for corner columns was Pu <= 0.5 phic Vc. Different offices seem to use slightly different versions. Turns out thats all that needs to be done! Another nice feature of that method is that it's almost completely independent of modelling assumptions.

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.