Punching Shear with Moment 4

[dik]

In literature I was looking at, they use the formula:

and they use gamma v equal to 0.4. Can anyone explain the term? The article uses ACI, and I'm not familiar with the value. Thanks in advance.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[KootK]

I believe that gamma v represents the share of the unbalanced moment at your column going through the punching shear mechanism. The remainder is thought to be transferred by direct flexure (as you would in a moment frame).

 

[JoshPlumSE]

According to my college text. This equation is based on some research by Hanson and Hanson.

Gamma v represents the portion of unbalanced moment that is transferred by shear stresses. It sounds like setting gamma_v to 0.4 was done relatively arbitrarily, but that it helped to better match their test results.

 

[dik]

Thanks, gentlemen... if you are transferring the total moment, would the value be 1.0? I'm looking at a small jib crane mounted to a slab on grade...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[KootK]

indeed it would. Curious, why do you want to do that?

 

[dik]

the load is very small, and the client doensn't want to construct a real foundation... I've looked at the numbers and there is lots of capacity.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[JoshPlumSE]

The idea is that most of the moment will be transferred through regular flexure and won't affect the shear stresses in the punching shear perimeter. And, only a portion of the moment is manifested as shear stresses at the punching perimeter.

If you have an unusual situation where traditional flexure can't occur then sure it would be 100% transferred via these shear stresses. I cannot easily visualize a situation where this would occur.... Though I've never really taken a "deep dive" into this theory before.

 

[KootK]

So like column connection to an unreinforced footing or something?

 

[Agent666]

You don't just choose arbitrarily how much unbalanced moment is transferred by either the flexure or eccentricity of shear mechanisms. There are formulas and rules for determining gamma_v in most codes, usually being based on the geometry of the connection (column size) and whether you have internal, corner or edge conditions. I suggest you read your local code to see how this is treated in your area, simply arbitrarily assigning some factor as others have implied as okay isn't the intent.

https://engineervsheep.com

 

[JoshPlumSE]

Thank you KootK! I was racking my brain and couldn't come up with a valid case where someone might think the moment should be transferred entirely by shear stresses.

Maybe even column to footing connection where the footing has no top steel?

 

[KootK]

...simply arbitrarily assigning some factor as others have implied as okay isn't the intent.

Are these "others" that you speak of me and JP? Or the authors of whatever reference dik's looking at? I hope it's the latter as my role here is simply to help dik interpret what he's seeing, not pass judgment on whether or not it's appropriate for whatever his condition is. The 0.40 value may well be associated with a particular condition, or range of conditions. And I'm privy to none of that.

Thank you KootK! I was racking my brain and couldn't come up with a valid case where someone might think the moment should be transferred entirely by shear stresses.

Welcome. Even then some of the moment would likely be transferred by flexure as a result of the axial load in the column prestressing the joint and providing tension capacity. And, of course, if the moment would actually lift part of the column cross section from the footing, that's a whole different kettle of fish as well.

If somebody wants to assign 100% of the moment to the punching shear mechanism, I struggle to see how that's not a conservative choice in most cases.

Maybe even column to footing connection where the footing has no top steel?

Interesting, given that would represent most footings. Your starter dowels would point the undesirable way but, even at that, I'm sure that there's still some moment capacity in the joint.

 

[Agent666]

It was referring to you mostly in this thread suggesting in the factor arbitrarily by confirming you could use a gamma_v of 1.0 in KootK (Structural)9 Apr 21 19:28 post. That was my interpretation of the advice being offered, that you could arbitrarily assign all of the moment to shear or flexure mechanisms, but maybe I've misinterpreted what you meant. I was simply stating this advice is not actually correct in terms of how punching shear works or is designed for.

Some portion of the unbalanced moment still transfers via flexure even if the underlying structure is unreinforced. The reality is reliance on unreinforced structures for transferring punching shear leaves one on shaky ground, I'm not aware of any rules in codes or otherwise that allow for the assessment of this.

If somebody wants to assign 100% of the moment to the punching shear mechanism, I struggle to see how that's not a conservative choice.

I disagree, this simply isn't how it works in terms of modern design methods for punching shear.
For your classical punching shear formulations at no point would the gamma_v factor = 1 for example, nor gamma_f = 1.0 (the portion of unbalanced moment transferred (gamma_f = 1-gamma_v)).

https://engineervsheep.com

 

[dik]

I think that's how it will behave...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

Agree... but shear transfer is far more critical... if it works for that, then it easily works for flexure. I'm already conservative because the effective thickness of the SOG has been reduced by 2" for all calcs... I'm comfy... this one, the formula made sense, except for the factor, and I had no idea why it was introduced. I've used a factor of 1.0 and it's still safe.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[dik]

Thanks, gentlemen... design problem solved. Only difference is that I'm using Hilti epoxy on a bolt circle rather than LePages PL Premium...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[JoshPlumSE]

I should point out that the 0.4 may have been "somewhat arbitrary" in the Hanson and Hanson paper per the text book I was looking at. But, it sounds like it was done that way to better match their test data.

Now that the ACI code has that 0.4 is the only value given in the current version of ACI code (actually the commentary) it's pretty much a lock that you should use that here in the US. That being said, if someone is concerned that this isn't sufficient then he/she could arbitrarily use a higher amount to add conservatism to his/her punching shear check. That's conservative.

 

[dik]

Thanks Josh... that's what I did. I just didn't know why the value was there; I hadn't considered it as a 'fudge' factor.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Agent666]

Now that the ACI code has that 0.4 is the only value given in the current version of ACI code (actually the commentary) it's pretty much a lock that you should use that here in the US. That being said, if someone is concerned that this isn't sufficient then he/she could arbitrarily use a higher amount to add conservatism to his/her punching shear check. That's conservative.

Can you provide a reference for this being fixed at 0.4? I cannot seem to find this in ACI318-19.

My reading of clauses 8.4.2.2.4 & 8.4.2.2.6 is that it is still worked out indirectly form the gamma_f factor unless I'm missing something obvious. This is the way it has always been?

https://engineervsheep.com

 

[JoshPlumSE]

It's the commentary to 8.4.4.2:

Hanson and Hanson (1968) found that when moment is transferred between a column and a slab, 60 percent of the moment should be considered transferred by flexure across the perimeter of the critical section defined in 22.6.4.1, and 40 percnt by eccentricity of the shear about the centroid of the critical section.

 

[JoshPlumSE]

Actually, my quote is from the 2014 version of the code, not the 2019 version.