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Etabs Shearwalls

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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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