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Re: Etabs Shearwalls

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Jake,
 
I have had a similar problem with shear. However, it was caused due to a "propped cantilever" effect wherein I had a wall that was going up 14 stories while the shorter wall was going up only 6 floors. There was a increase in shear at the 6th floor since the taller shear wall was a "propped cantilever". 
 
I do not know the configuration of your walls, but this was something I did come across during an analysis. I did not have membrane elements. My shear walls were shell elements that included both in-plane and out-of-plane stiffnesses.
 
~Anantha
 
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ANANTHA NARAYAN, E.I.
Structural Engineer
Bliss and Nyitray Inc.
Miami, FL - 33134


----- Original Message ----
From: Jake Watson <jwatson(--nospam--at)utahisp.com>
To: seaint(--nospam--at)seaint.org
Sent: Tuesday, 18 April, 2006 11:43:06 PM
Subject: Etabs Shearwalls

We are modeling some very convoluted shear walls in Etabs and trying to
check the model by hand.  Walls in the model without any flanges behave
as anticipated: continuous moment diagrams, forces above and below floor
lines match, shear diagrams increasing / decreasing as appropriate.  Our
walls with flanges have "jumps" in the wall moment diagrams.  The moment
above a floor will be one number (say 1000 ft-k) and below the floor it
might be be 500 ft-k.  These points are the bottom of wall panel and the
top of the panel below.  My hand calc of P/A +/- Mc/I to check wall
stress in an individual panel gives substantially different results.  
Meanwhile the Etabs panel plate corner stress plots are realistic and
continuous.  Can anyone explain what Etabs is telling me?

A few model notes:
We didn't model any gravity or "self-weight".  I like to check this
stuff by hand.
All walls are "membrane" elements, no out-of-plane stiffness
The diaphragm is modeled as infinitely rigid (we didn't mesh it and have
turned off that "feature").
I have hand meshed the walls to get appropriate element size and shape
(this is not my problem, I have a fair amount of experience with FEA meshes)

Jake Watson, P.E.
Salt Lake City, UT

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