Punching Shear Of Shear Walls In Case Of Eq "important Issue"

[mhdhamood]

Dears
I argued with a senior Engineer about the punching shear check for shear wall; As far as I know :

1. The shear wall will attract high moments.
2. Also in the case of design itself, the controlling case from a combination that contains Earthquake will be the tension axial force not the compression (Please Verify or disagree!!!)
3. If I want to check punching for this shear wall I will take the moment at the Top of the shear wall (pier)
which will be very high and I will take the axial force as the difference between the top of that wall and the bottom of the wall above so to achieve the axial force from that story only.
4. I found that I need to put shear reinforcement and some places it fails.
5. Now, the senior engineer told me no need for shear reinforcement in case of shear walls because of length of shear wall.!!!!!!!
I said: Sir your case is right in Ultimate dead+Live combination not in case of earthquake because in
this case there is high moments attracted by shear wall.
He said : No tht moment in case of earthquake will not affect the slab , it will be taken by thear wall
only !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

6. Please help me in this subject. I am sure the shear wall in earthquake will suffer from high moments which in case cause punching need to be counted for .

Regards

[Badar (BAZ)]

I am not sure what your query is. I infer that you are talking about dealing with shear  strength of floor slab around the shear wall. My question is: are you using flat plate as floor, or beam-slab system? If you are using flat plate, then you need to be worried about the shear failure of slab.

[mhdhamood]

Dear ;
I am speaking about flate plate.... if there is a shear wall .... I will check it against punching from the combination that includes dead and live and earthquake. Then because it will attract high moment from earthquake then the punching will be critical???

[WR1]

Well, I am not sure what you are asking, but punching is when the load acts lateral (perpendicular) to the surface...Like gravity load on slab and then below is the column that can punch through.

 

But EQ is lateral loading which comes on vertical elements let's say a shear wall which is supported by top and bottom slab.

 

So you mean to say, you wanna check punching of shear wall through slab horizontally?

[Muneeb Badar]

Dear mhdhamood,

 

1- In case of seismic loading, the shearwall will take high moment and high shear.

In case of seismic, it is not necessary that the design combination will be tension. If you talking about alone seismic load case, it can be tension. but when you design for load combs, it can be tension if seismic increases from DL and compression if not.

3- First of all you need to check the punching of slab for gravity loading. The thing which you are saying is 100% correct. But what your senior saying is also correct. I ll explain.

 

What your senior are saying: This is right but not for code based design. You do not need to design the thickness of slab for seismic as in ACI code, you can design the members for gravity loading for the structures which are in high seismic areas ACI 21.13. You are designing the slab for out of plane forces. AND these out of plane forces as per ACI 21.13 we can design for gravity loading including shear and flexure.

 

What you are saying: BUT you need to design the slab  (diaphragm design) for inplane forces which are based on seismic demand. So yiu are not required to provide the thickness of slab for seismic loading as it will be out of plane forces. But you need to design this slab near shearwall are for inplane forces

 

Now what you are saying is the next step. After providing the thickness of slab on gravity loading, we need to check the thickness of slab by checking it  on slab beam behaviour and checking its rotation which is the next step and its details are not given in any code. There are guidelines which can be used to check these details like ATC 72.

 

Thanks

 

Muneeb

[mhdhamood]

Thank you all ...
Mr Rana Waseem ..
I am spkng about that I want to check the punching of shear wall through a flat slab, Do I have to take the loads form the combination
(1.2Dead+1Live+1EQ) or (1.2 Dead+1.6 Live) .
The normal steps to check punching :

1. We read the moments from ETABS at the top of column or shear wall and the axial from the difference of the top of column or shear wall and the bottom of the above column or shear wall .

2.. If we took the loads from the 1st combination then the moments is high for example 1600 kn.m but from 2nd comb. 260 Kn.m (a 
real numbers) so from where we shall compute the punching shear????

 

[mhdhamood]

Dear Mr Muneeb;
You are saying that
1. I only design for punching from garvity loads only ??.
2. If that is only then when the EQ comes it will cause the shear wall or column to rotate at its top causing the connected slab also to rotate then makes punching, So why not to design for punching from EQ loads also ..??
 

[WR1]

You must check punching for all load cases.

 

If (1.2Dead+1Live+1EQ) giving you more forces you should check punching on that.

 

But also remember that, you should check both axial plus moment. So may be you are getting more M in in this combination but axial force is less. So you should check which load combination is giving you more shear stress.

 

You should all load combinations whether it is EQ or gravity or W anything.

[mhdhamood]

THANK YOU Mr Rana Waseem ;

But please clarify my doubts about the following:
If I want to check the punching by reading the forces and moments from Etabs then how I will get the moments that it is affecting:
1. At the Top of the shear wall or column. 
2. Or At the bottom of the above shear wall or column because it also make rotation for the slab connected below .
3. Now in SAFE we take the Moments readings from the floor itself neglecting the columns or walls above ...is that safe?

Regards

[Muneeb Badar]

Dear Rana,

 

I can not understand your first comment clearly. Actually the lateral forces transferred to vertical elements through beams and slabs as axial forces and shear and moment. We are only required to design the slabs for gravity not for these lateral loadings but we need to check the punching by some other way which is not code based. So what you are saying to check horizontally, then I think he is not asking for this. He is asking to check the slab shear for lateral loading which code does not require. Sorry if I can not clarify 100%

[Muneeb Badar]

Dear mhdhamood,

Yes I am saying this to check the shear for gravity. Actually, what you are saying is 100% correct but this design should be inplane. I ll explain again.

 

When earthquake comes and there will be rotations then these rotations will not be out of plane. These will b e inplane. So thats why you need to design the slab for inplane forces for lateral loading and thickness of slab for out of plane forces. Thats the concept of code. This is the reason the ACI chapter 21 allows us to design the slab member on gravity for out of plane forces and then design the slab for lateral forces for inplane forces.

 

So there are two types of design for Slab in case of earthquake if building is in high seismic zone

 

1- Out of plane design which is based on gravity analysis which includes the ordinary flexural reinforcement and shear reinforcement if required

2- Inplane design which includes the Tension reinforcement, shear reinforcement and shear fricktion reinforcement.

 

 

Seconldy if your building is in high seismic areas you can not use these same load combinations which you wrote above. If as per ASCE if your building is in seismic deisign category D, E and F then you need to modify these load combinations.

[Muneeb Badar]

Dear mhdhamood, in your next comment you are asking which axial force to use, top shear wall or bottom shear wall whcih will cause rotation. Dear these shear forces can not be used to check the slab thickness as these are inplane shear forces when transferred from shaer wall to slab or from slab to shear wall. Even IF YOU PROVIDE THE THICKNESS FOR THIS SHEAR, IT WILL NOT BE EFFECTIVE IN CASE OF EARTHQUAKE AS YOU PROVIDED THE THICKNESS WHERE IT IS NOT REQUIRED. You have to increase the thickness in the basis of inplane bending not on the basis of out out of plane bending.

 

Thanks

 

Muneeb

[mhdhamood]

Muneeb Badar

  Rana Waseem

Thank you so much dears for this amazing discussion.

because previously I was checking the slab in shear wall punching form the earthquake load combinations (it was wrong and over design) and because of high moment's values due to earthquake I was encountering punching shear problems .....

So from now I will just check the punching from the gravity loads 

 

[Muneeb Badar]

Dear mhdhamood,

 

Sorry I forgot to add one thing.

 

You can design the Flat Slab based on the gravity loading and then design the diaphragm for lateral forces. BUT there is one condition you need to check. In case of Flat Slab, if you are designing it on gravity loading then you need to check the story drift ratios. Please check ACT 21.13.6 (b ). We need to satisfy this condition if we want Flat Slab to be designed as gravity member.

 

Thanks

 

Muneeb

[mhdhamood]

Dears ;
But I still not satisfied and please look at the attached figure. that one is clearing my opinion.
See the statement that says : seismic forces tend to push the shear wall over causing an overturning moment.....that means the connection area of the slab connected to shear wall will also rotate due to that moment so because of that it behaves the same as if the moment coming form gravity loads...causing punching so we shall check punching due to seismic loads !!!!! Am I right??

[Muneeb Badar]

Dear mhdhamood,

 

My dear as I told you before that you are 100% right about this.... .....But these are inplane forces not out of plane forces. The picture you showed me is for inplane forces.

 

As I told you before that you need to design for gravity for out of plane by satisfying 21.13.6 and then inplane forces (which you showed me) you nedd to design as well, which itself is a very complex kind of design.

 

You not only need to design for collector forces as you showed but also for Shear Friction, Shear Reinforcement and Tension Reinforcement.......BUT ALL OF THIS DESIGN IS FOR INPLANE FORCES NOT FOR OUT OF PLANE FORCES

 

 

You are confusing inplane and out of plane design.

 

I ll summarize again:

1- Out of Plane: You need to check the thickness of slab (for flexure and shear )by satisfying 21.13.6 using gravity loads

2- In Plane       : You need to check the thickness and provide shear reinforcement and flexural reinforcement for inplane design uding Lateral Loads. These reinforcement includes Tension (Chord) Reinforcement, Shear Friction Reinforcement, Shear Reinforcement, Retaining Wall Distribution Reinforcement: Retaining Wall Vertical Reinforcement

 

The second type of design (for inplane forces) is not easy to handle. It involves a lot of knowledge and engineering judgement and is not common at all.

 

If you want to design for this I can provide you guidelines.

 

Hope it will be more clear now

 

Thanks

 

Muneeb

[mhdhamood]

Thank you so much Mr Muneeb , I get your point. Thank you for your patience.
 

[WR1]

Very confusing discussion to me!

 

Some notes: 

mhdhamood; please be clear and easy in your questions! 

 

Let me say what I think about slab system for this issue;

 

 

3 different cases

 

1. Only gravity load is present

Slab is checked against out-of-plane bending

Punching is checked against P and M arising from these loading

 

 

2. Only E loading (actually assume E as horizontal load because E and gravity loads are always accompanies together).

Slab will resist this by axial stiffness (in-plane or diaphragm design). And columns will resist this load by bending stiffness.

 

So in slabs, no out-of-plane bending..right? No slab will also have bending moment out-of-plane. Model a one-bay frame and apply a horizontal load of 100KN at LEFT-TOP joint and see the moments in beam.

 

In this case, you will check slab design against axial load and this out-of-plane bending.

For punching in this case, you will use this same M. And for axial force (P)

P = 0 for one story frame (refer to frame model)

P = P from column above for multi-story

 

3. When E and gravity both acts. For example in combinations including D+L+E

There will be additional component of moment here called, P-Delta

 

So you will use M = moment from p-delta + moment from step 1 + moment from step 2

and use P = P from step 1 + P from step 2 of the column above the slab

 

you will use this P and M for calculating punching anywhere in slab location. If you are calculating punching for shear wall, it will follow the same procedure.

 

Having said all this, please correct me if my understanding of this is not upto the highest standard. Thanks.

 

Simplicity is sexy

[Muneeb Badar]

Dear mhdhamood,

 

I am happy to help............If you still not clear just let me know.....I ll send you some literature regarding this.

 

 

What Rana Waseem summarized is absolutely perfect.........I just wanted to add in his Point # 1 that during this out of plane design we need to satisfy ACI 21.13.6 for Flat Slabs. In case of Beam supported slab no need to consider ACI 21.13.6.

 

Thanks

 

Muneeb

[UmarMakhzumi]

This is a good discussion because of the fact that it reminds us how the use terminology/ terms can lead to difference of approach.

 

For example, when we talk about column punching shear, actually the limit state in that case is that the slab is punching through the column. Its not the column that requires checking but the slab which we always do.

 

Similarly, the slab would be punching through the shear wall too, not the other way around as summarized above. 

 

Thanks.

[mhdhamood]

Thanks all   
Dear Mr Muneeb I will not mind if you send me 

[Muneeb Badar]

Dear mhdhamood,
 
I ll send you. Please tell me how can I send as I do not know I can attach here........and my name is Muneeb not Mr. Muneeb   and I am not too old to be called Mr. I think I should upload my picture as well..........
 
I think it is better if I create another topic/discussion and summarize all what we have discussed here under the title Diaphragm design and the post. It will be a complete concept regarding diaphragm design. But I do not know how to attach the documents and to how much capacity. What do you say........

 

 

Rana Waseem what do u say
 

 
Thanks
 
Muneeb

[WR1]

Muneeb thanks for your efforts. You can attach by going to "More Reply Options"