can long load bearing concrete shear wall be designed not to take lateral forces?

[4dmodeller]

Dear all,
I was wondering if you can help shed some light in my dilemma about Etabs as a lateral stability analysis tool.
Previously under my first company, I used to do a full etabs model and extract individual wall pier forces for design in a manual spreadsheet. This was all clear and is similar to how Etabs online tutorial have suggested.

At my new company, They have a design methodology that is radically different. They asked me to only draw lateral resisting shear wall (neglecting columns and other walls that may also long but are transferring and does not reach ground/ or precast wall that has minimal horizontal joint dowel connections - and thus assume not to be rigid enough to take lateral load in ultimate condition)+assign slab as membrane element with mesh option of rigid diaphragm so that it does not transfer any vertical load down the wall.
In effect, they want to extract lateral forces out of Etabs only and combine it with manual hand calc load rundown for design. Group of wall are grouped as single pier and designed as a box etc.

I was wondering if there's anything wrong in the second method?
Things that concern me about this approach is that
1. I found that despite no load transfer from slab (due to mesh option described earlier) I was still getting significant bending moment in the pier from self weight which doesn't seem right. I feel that the geometry of the slab that's connected to the wall are what causes the bending moment. But why would there be much if there's no load transfer between them?
2. can long load bearing concrete shear wall be designed not to take lateral forces? I feel that minimum connection to slab alone would cause the wall to take lateral load. perhaps it will fail and the load redistribute to other walls that are designed to take load? is that an acceptable way of design?

Sorry for the long message. Thanks all for your opinions

 

[KootK]

If you draw a free body diagram of the bent plate stitch plate assuming that no moment is transferred around the corner as torsion, you will find that the moment and shear are identical to the non-corner case. Effectively there is still double curvature and the effective eccentricity is the same.

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.

 

[4dmodeller]

KootK,

If the moment cant be transferred around the corner as torsion, then moment and shear would not be double curvature correct? as it can accept any moment..then it is a single curvature and effectively a cantilever stitch plate.

Please clarify.

 

[KootK]

If the moment cant be transferred around the corner as torsion, then moment and shear would not be double curvature correct?

If you draw a free body diagram of the bent plate stitch plate assuming that no moment is transferred around the corner as torsion, you will find that the moment and shear are identical to the non-corner case.

Post the free body diagrams for the corner and non-corner cases, then we'll talk.

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.

 

[4dmodeller]

 

[KootK]

Perfect. And you've noticed that:

1) In both cases, shear = V?
2) In both cases, moment = V x ed?
3) The right hand FBD is identical to the right half of the left hand FBD?
4) The shear and moment diagrams for the right hand FBD will be identical to the right half of the left FBD?

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.

 

[4dmodeller]

KootK,

Sorry i think i misunderstood you. Double curvature basically gives 2 BMD like 2 cantilevers...which is what i meant. thanks for clarifying.

 

[KootK]

Right, but the important takeaway from that is that, if the moment and shear are the same for both cases, then the connection eccentricity is the same for both cases (e=m/v. That's the bit that was giving you grief above, right?

But if the connection is in the corner with the return wall, eccentricity can increase because you dont get double curvature in the stitch plate anymore (it will be a cantilever from one wall receiving v* from the return panel)

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.

 

[Enrique2015]

This process is not uncommon, multiple consulting companies that I am familiar with in the NYC area use this approach, they prefer to do load take down for gravity loads and use a "lateral only" model for lateral loads. Also, please notice that the fact that you are using membrane elements doesn't mean you don't have any transfer of gravity loads, it only means that you do not have any out of plane bending stiffness on your floors. The confusion arises for engineers when using shell type area elements to model floors and when they try to match hand calculations due to load take down with results of the program, they usually won't math specially for tall buildings. This is because shell elements have continuity within vertical elements, hence reactions are not the same as oversimplifications assumed for load take downs assuming simply tributary areas. The main problem I find with EATBS, is that it has so many parameters and variables for preferences and overwrites , and not all intermediate calculations results are displayed, hence it makes difficult for the engineer to check design. I have worked for companies that used both approaches, the one you mentioned to use ETABS only for lateral loads and then use a spreadsheet to combine loads and design walls using full iteration diagrams, and others that design everything in ETABS. The issue I find with the second approach is that errors during modeling are overseen, since the program will not check your structure for you to make sure things make sense, for sure I wouldn't trust anyone without a great experience in ETABS design to do this, whereas the first approach lets you have a feeling of the structure and realize if your results make sense. Lately I been using a program from

http://www.structuralaps.com

caled EZ-Shear wall, which lest you check results easily, it also allows for individual pier results to be imported from ETABS and checked.