additional punching shear due to unbalanced moment 2

[h-badawy]

dear experts

regarding to the additional punching shear due to unbalanced moment,
are the finite elements programs(sap,etab or safe) taken into considerations the effect of the unbalanced moment during analysis or i have to use the ACI methods (EFM or DDM) to get this moment, because when i am checking the punching i get the normal force and moment from the columns and use any spread sheet to check. but if the normal force from the finite elements programs included the additional shear due to unbalanced moment the loads will be duplicated .


Thank you

 

[JoshPlumSE]

There is no set answer to this question. You have to really read the documentation of the software you're using.

My impression is that most of them will account for this in some way. But, that even when they do you will want to investigate the results to make sure you agree with them. These calculations aren't as straight forward as you think and you should make sure you agree with the method used in your software.

 

[structural87]

It will depend on your model and assumptions. If u dont release the moments and have unequal soans then the software will give u the requested unbalanced moment.
If u release ur columms then u will get no moment transfered to the columms

 

[h-badawy]

thnx structural87
the problem is not how to get unbalanced moment , my quetion is the FE program taken into consideration the unbalanced moment by increasing the shear force and in thiss case i will check punching on the shear force only or i have to get the moment in two direction and check the punching on shear force + moments


Thank you

 

[hokie66]

I would want to query the program results myself. Use the reaction and the moment to develop the shear profile, then compare with what the program output says. Trying to determine exactly how a FE program is working can be very frustrating.

 

[PSEPK]

or i have to get the moment in two direction and check the punching on shear force + moments

IMO, yes.

It would be better to check it yourself manually, to make sure you are accounting all the force effects properly. This check would also clarify your basic question i.e., whether the software is considering the effect of unbalanced moments or not? and, to which extent?

A good structural engineer is often a blessing for others.

 

[KootK]

but if the normal force from the finite elements programs included the additional shear due to unbalanced moment the loads will be duplicated .

None of the commonly used FEM software packages do this in their reporting. There's no need to be concerned about doubling up demand.

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.

 

[h-badawy]

thnx KootK
you want to say that i can take only normal force on the columns and check the punching without the moments ?
i agree with you but what is making me confused when (safe) check punching u will find the moments in two directions Mu2 and Mu3 have been taken into consideration with the shear force so i think that the shear force haven't been increased due to unbalanced moment.
check the attachment



Thank you

 

[KootK]

you want to say that i can take only normal force on the columns and check the punching without the moments ?

I am saying the opposite of this. Reported axial loads transferred from the slab to the columns are not adjusted to include unbalanced moments.

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.

 

[rapt]

I agree with Kootk on this. Analysis software will normally report only the moment attracted to the column by stiffness. It will not allow for Code Design Rules that require the designer to allow for further Moment Transfer.

Some FEM programs that do the design as well may or may not allow for the moment transfer depending on how good the programmers are! But the analysis results will be for the stiffness analysis results only.

 

[h-badawy]

after study the CSi program (safe) Manuel , check of punching have been done by basic code method by take the full moment transfer and multiplied by (gamma V) and get the stress from (Q and M) but i found anther method i think it may be better and accurate by the finite element , if we get the stresses (S13 & S23 ) on d/2 from the face of the columns and these values already included the the effect of (Q and M) and it should be less than Vc .

note :
S13: Out-of-plane shearing stress (force per unit area) acting on the positive and negative 1 faces in the 3-axis direction.
S23: Out-of-plane shearing stress (force per unit area) acting on the positive and negative 2 faces in the 3-axis direction.

check attachment


Thank you

 

[JoshPlumSE]

Haithambadawy -

First, let me clarify that I'm not all that familiar with what CSI / SAFE is doing for their punching shear calcs. My thoughts here are more generalized based on what I have seen done in RISAFoundation and RISAFloor ES. My educated guess is that CSI is doing something similar, but I can't claim special knowledge on this.

In concept the idea of looking at shear stresses from an FEM analysis instead of directly looking at the forces and moments is valid and rational. However, I don't think it is easy. To me it's the type of thing to do for a PhD project where you want to propose a change to the ACI formulas on how these stresses are calculated. You build an FEM analsyis and see what stresses are produced in your simulation of reality. Then eventually you test it out in a lab to see if the reality matches up with the FEM results. But, this is not something that you would generally do as a practicing engineer.

Some reasons to stick with the force and moments and then calculate the punching shear stresses from those:
1) This is what the code tells us to do. You can argue that the direct stresses might be more accurate. However, you're going out on a bit of a limb. If you're coming up with more conservative results using the direct stress method, fine. But, if you come up with less conservative results then it would be very easy for another engineer (or plan check engineer) to state that your design did not meet the code requirements.

2) The code calculations are based on an assumed punching shear perimeter. Actually, multiple punching shear perimeters (edge, corner, or such). The reality is probably different than what is assumed in the code. It's just been demonstrated that these perimeters give reasonable results when matching up with test data. But, what sort of perimeters are you going to assume when you look at the direct FEM stress results? Using the code perimeters may not always be the right way to do it.

3) What direction are your local stress results reported? Do these line up with the assumed local axes of the punching perimeters? You can probably make it work if they don't, but it will be more work.

4) Is your mesh fine enough to capture the variation of stresses along these elements? Because you're not concerned as much with the average stresses as you are with peak stresses. Right? So, you have to be careful with your model to make sure you can adequately capture the variation of the stress and arrive at a reasonable peak stress at one corner or edge.

 

[h-badawy]

Thank u JoshPlum for your precious advise
But when we design the flat slab (slab without internal beams) we used the FEM methods(moment capacity from csi ) and we didn't use the traditional code method (column strip and field strip) even the layout is uniform with equal spans ,so i think we can use also the stress of the shear acoording to code and your recommendations above.

Thank you

 

[rapt]

Haithambadawy,

What do you mean by "even the layout is uniform with equal spans" when you design flat slabs by FEM methods?

I agree with what Josh is saying. To extend it, how are you going to allow for the effect of flexural cracking around the shear head on the shear stresses in your FEM analysis?

 

[h-badawy]

i meant that when i design the flat slab in case of regular distribution of the column or not i use finite element methods (safe or sap) by get the maximum moment at the face of the columns from the contour and calculate the reinforcement so if we can use this way to get the reinforcement why not we can use also the remaining stresses result to check on the shear (Q and T).
and if you talking about crack section for nonlinear analysis these options can be done by both methods on CSI program 1- by put modifiers sections as per ACI codes (0.35 beams ,0.7 columns , 0.25 slab ,......... ) 2- the new ver. of safe can make nonlinear analysis without modifiers section.

check compressed attachment file

Thank you

 

[rapt]

From your Sage analysis, how do you distribute your reinforcement in the slab? Do you still divide it into "strips" with more concentrated areas?


To base shear design on FEM I would have thought you would have to do a Non Linear Solid Model to determine the shear distribution throughout the depth as well as different zones around the column.

I would have thought that Safe is simply using plate elements which cannot do this for you.

Then you would have to determine a way of relating those solid model results to real world design code strengths and determine suitable factors of safety etc and probably do some comparisons with real world tests to confirm the results.

It is a lot more complicated than just getting some numbers from an FEM plate analysis.

 

[h-badawy]

no i didn't divide it into strips i put only one top and bottom mesh (starting with min. reinforcement ) and then put additional above the columns and at mid span if i need

Thank you

 

[rapt]

So your flexural reinforcement distribution does not match the distribution of stresses and moments within the slab!

If Safe is doing a non-linear analysis, on what reinforcement distribution is it basing the non-linear analysis? Is it the same as your reinforcement distribution?

If you do not place at least 60+% of the top reinforcement within the column zone, you cannot even justify the slab flexurally using yield line solutions. Forgetting about shear, deflections and crack control.

 

[JoshPlumSE]

I could also have stated the following:
5) The real behavior is a 3D issue not a 2D issue. It's reasonable to assume 2D plate elements capture the overall behavior of the relatively thin slab system. But, the actual punching shear failure will occur on an inclined surface. So, it's possible that the peak stresses shown in your analysis using plate elements might not capture it.

Again, I'm not saying that you can't do this. Just that it will take a good deal of engineering judgment. And, that it would be wise to also do it the traditional method as a "reality check" against the numbers that you're getting doing it the other way.

Personally, I don't have enough confidence in my engineering judgment to think that I could currently do it correctly. After a few months of digging into the issue, running test models and reading research papers I think I could probably get there.

 

[KamalSafa]

I arrive a bit late to this thread but I would like to share my opinion about punching shear and shear.

1- Punching shear is a local process, with failure mode different than that of shear. Punching shear failure develops a strut-and-tie scheme at the vicinity of the column, independent of the surrounding spans. It is not wise to read shear forces at d/2 from the support, and to deduce punching shear forces. You have to consider the ultimate force at the column AND bending moments My, Mz at the top of the column.Moments at the top of the column are referred in the Code with the name "unbalanced". These moments provide additional shear stresses to be superposed to those resulting from the axial force.

2- Shear is read at distance "d" from the face of the column. But you have to read principal shears V1 and V2, and not Vx and Vy, because Vx and Vy are arbitrary, they are not necessarily the biggest shears! The notion of "X" and "Y" is wrong in flat slabs where you have infinity of directions to read moments and shears. And the shear reinforcement should be placed in the direction of the principal shears. Otherwise you may fall in a situation where the maximal shear forces may fall in the part of the slab located at the corners of the shear links cross disposition, as per ACI. Last word: If punching shear is verified, that does not mean that shear is verified as well. You have to check both.

Kamal Safa, PhD
Ecole Polytechnique