Civil Engineering/Cracked section stiffness modifiers
I read a lot (CSI website, ATKINS manual, Eng-Tips forums....) about stiffness modifiers for shear walls, but I have some doubts about what they report and what is required by ETABS.
In ETABS table is specified:
Membrane f11 modifier
Membrane f22 modifier
Membrane f12 modifier
Bending m11 modifier
Bending m22 modifier
Bending m12 modifier
Shear v13 modifier
Shear v23 modifier
and in CSI Analysis Reference manual at p.144 is specified that F11&F22 are axial forces along axis 1&2, F12 is shear within plane 1-2, whereas M11&M22 are bending moments out of plane, and M12 is torsional moment, and V13&V23 are transverse shear out of plane.
If F11&F22 are axial forces, why in CSI website
is stated that f11 controls EI, but EI is obviously related to bending (so to m11&m22), whereas f11=f22 control EA (axial behavior), and f12 controls GA (in plane shear behavior).
Is it a mistake or am I missing something?
According to ACI318 and ATKINS I should I use m11=m22=0.7. And what about m12=?
And what about f11=f22=? I believe I should use 1 to account elastic shortening correctly. So, to take into account cracking for shear should I modify f12? and what value? 0.7 uncracked and 0.35 cracked?
And for out of plane shear v13=v23 should I use 1 because the shear within the thickness is neglectible (thatís why ETABS doesnít allow me to change it)?
Thank you for your help
F11 and F22 are obviously related to axial stresses or stiffnesses. you need to use modifiers for moments M11 and M22. You will not need to use modifiers for other components (i.e. shear ) . For the case a shear wall being governed by moment rather than shear , which is easy to detect I suggest you to use 0.35 (cracked section) for M11 or even for M22. Obviously these factors are to tune the part of loading resisted by shear action which is the responsibility of engineer to decide about it according to amount of moment and shear resisted by the wall.
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