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Re: Etabs Shearwalls

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Jake

All I meant that with the wall elements being modelled as membrane elements if you mesh the wall membrane say 1/2 way between story levels which you can do (you said you were manually meshing walls - without seeing the model I was only guessing you may have done this) you lose out of plane stability of the membrane element though this may not be  a problem if you have zero mass -  ie a similar effect would occur if you used membrane elements for floors without null beams to mesh with to pick up the out of plane stiffness. 

However, I think the issue is still with your pier assignments and how you are reading the output - it could be any number of effects that is occurring especially if your flange/web element thicknesses are changing between levels.  That said, try labelling the piers as a whole and see if the total moments in each pier sum up properly and make sense - you can actually still look at individual wall "element" forces even if you group the shell elements together into piers. A hand calc should be sufficient to calculate the shear flow between flanges and web anyway?  

Tim.

>>> jwatson(--nospam--at)utahisp.com 21/04/2006 12:43:26 am >>>
I deliberately assigned different panel numbers to the flanges so I 
could more easily determine internal forces in each piece.  We are 
concerned about vertical stress between flanges and webs along with 
several other issues.  If I assign the flange the same number, I will 
lose the internal detail.  That said, I will go back and renumber the 
group I am most concerned about and see what that tells me.  I strongly 
suspect the flanges and how I understand the out-put are the problem.

Your comment about using shell instead of membrane elements is 
interesting.  It seems to imply that there may be out-of-plane buckling 
issues.  Is that possible?  The way I understand the Etabs 
documentation, the membrane elements don't contain an out-of-plane 
stiffness (or degree of freedom) which is considered in the stiffness 
matrix.

I can't find an explicit definition of what the pier moment, shear, and 
axial out-put considers.  Does it take the corner forces of each plate 
and sum them only at the top/bottom of the pier?  Or does it somehow 
consider forces on the edge of the pier?  My primary concern is the 
discontinuity in the pier moment diagram.  If it sums membrane corner 
forces, then summing moments at the bottom of the elements above a floor 
should give exactly the opposite of summing forces at the top of the 
elements below.  Remember there are no vertical loads in my model which 
could affect the sums.

Thanks for your insight,

Jake Watson, P.E.
Salt Lake Ci

Tim Shannon wrote:
> Jake
>
> It could be your definition of the pier assignments for the output.  How have you assigned pier numbers to the flanged walls?
>
> Also, have you tried modelling the walls as shell elements - especially if you are meshing the walls they will likely require out of plane stiffness unless all of your nodes meet perfectly with the floor levels (ie no holes in walls or coupling beams etc) - I usually find it beneficial to use shell elements for walls for this reason. 
>
> I could take a look at the model if you want.
>
> Tim Shannon (MIEAust)

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