ETABS 9.7.4 - Wall stiffness modifiers 1

[dadomago75]

Hi everybody, please help me cause I'm getting frustrated with all these different explanations!

I read a lot about stiffness modifiers for shear walls (CSI website, ATKINS manual, Eng-Tips forum threads....), but I have some doubts about what they report because they say different things, and they look different to what is required by ETABS.
In wall's ETABS table is required to specify:

Membrane f11 modifier (supposed to be axial force along axis 1)
Membrane f22 modifier (supposed to be axial force along axis 2)
Membrane f12 modifier (supposed to be in-plane shear)
Bending m11 modifier (supposed to be bending moment about axis 1)
Bending m22 modifier (supposed to be bending moment about axis 2)
Bending m12 modifier (supposed to be torsional moment about axis 3)
Shear v13 modifier (supposed to be out-of plane shear)
Shear v23 modifier (supposed to be out-of plane shear)

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, M11&M22 are bending moments out of plane, and M12 is torsional moment, and V13&V23 are transverse shear out of plane, according with what I have written above.

If F11&F22 are axial forces, why in CSI website Link is stated that f11 controls EI? 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 is the 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 (that’s why ETABS doesn’t allow me to change it)?

Furthermore, just thing if I decide to use a membrane element, which means m11=m22=0, what should be the values for the other parameters? what do you usually use, and where/how did you get them?

Thanks in advance for your help
PS sorry if I am honest .....but I don't like ETABS at all!

 

[chekre]

dont use membrane for walls, it will create problems if u are performing a P-delta analysis !

what is recommended is to assign 0.7 for f22 (keep 1.0 for other) and where there is a shear deterioration, u can modify f12 !

 

[PicoStruc]

F11 & F22 control membrane EA Stiffness, but I = sum (A*d^2) for inplane bending ! So modifying F11 is the right thing to do.

M11 & M22 are for out of plane bending

 

[chekre]

u assign modifiers for f11 if u are dealing with spandrel. Per default and when u draw a wall, the local axis 2 will be parallel to the Z direction. Since the wall will experience the biggest moment according to its strong inertia, f22 is the one to the be modified.

 

[dadomago75]

@PicoStruc
Yes, I know that modifying f11 and f22 and (if you want to be extremely precise you could modify f12 as well) is the right thing to do because the shear wall has in-plane actions, but my question is "why in the CSI link I posted it is stated that f11 is related to EI and controls bending? f11 to me is related to axial behavior along axis 1, exactly like it is stated that f22 controls axial behaviour along axis 2!
If you look at page 144 of the CSI Reference manual, it says that F11 and F22 are membrane forces, M11 and M22 are bending moments, V12 is in-plane shear, V13 is out-plane shear, and M12 is the torsional moment which means in-plane bending.

How do you explain the discrepancy between the two manuals?
Thanks

 

[PicoStruc]

As i said, the In-plane bending generate in-plane axial deformations.

So the INPLANE bending stiffness of the wall (EI) equal (EI where I = Sum (A*d^2)

 

[MrHershey]

M11 and M22 are out of plane bending moments. I will often apply them anyways because if your wall is cracked in plane, it'll be cracked for out of plane as well and I don't want to be attracting more moment than I should (even though it's not much).

Sketch it out:
If your shell element is in the YZ plane with X pointing out from the plane of the shell, M11 and M22 (bending modifiers for the shell element) will affect bending about Y and Z axes. Which isn't really what you're interested in because your shear wall isn't typically used to resist forces out-of-plane. You'd be more interested in making sure crack factors are applied to rotation about the X axis, because those are the forces the shear wall will primarily be resisting. To accomplish this, you'll need to modify the axial stiffness factors of the shell element. You would modify horizontal stiffness (F22) for piers in shear walls because the horizontal section is what is resisting the bending moment. You would modify vertical stiffness (F11) for spandrels/beams in shear walls because the vertical section is what is resisting the bending moment.

 

[dadomago75]

I think I found how it really is
According to ETABS manual and ATKINS manual, " The f11, f22 and f12 modifiers are essentially equivalent to modification factors on the thickness of the shell element. The m11, m22 and m12 modifiers are essentially equivalent to modification factors on the thickness3 of the shell element."

For what I understood, if you modify f11 you are modifying the thickness (only for stiffness calculation) of the wall for the face perpendicular to axis 1, whilst when modify f22 you are modifying the thickness of the wall for the face perpendicular to axis 2.
This means that for a wall WxDxt (length x depth x thickness), for f11=0.7 you will have an inertia of 0.7*t*D^3/12, and for f22=0.7 an inertia of 0.7*txW^3/12.
Whereas if you modify m11 or m22, you will have D*(0.7*t)^3/12 or W*(0.7*t)^3/12 as stated by ETABS manual.

When you have a pier, this is behaving like a vertical beam, so you will have to modify f22 because the design moment is acting on the face perpendicular to the local axis 2 (axis 2 vertical, face horizontal), whereas when you have a sprandel, this is subject to bending moment on the face perpendicular to axis 1 (axis 1 horizontal, face vertical), and you have to modify f11. This is in accord with ETABS online manual (the link I posted).
But becasue axial and bending behaviour cannot be de-coupled, you have to modify both factors.

Thanks guys for your considerations.

 

[elance300]

But becasue axial and bending behaviour cannot be de-coupled, you have to modify both factors.

When you say both factors, do you mean both factors F22 and M33 for piers? And likewise F11 and M33 for spandrels?

 

[dadomago75]

Correct, both f22+m33 for piers and f11+m33 for sprandels.

 

[msegerman]

Dadomago75,

when you say M33 for piers, I don't see M33, did you mean a different parameter?

 

[dadomago75]

Hi msegerman,
you are right, I just replied without realizing that there was a mistake.
you have to modify the factors f & m relative to the type of element you are considering, so f11 m11 and f22 m22 I guess.
Sorry but it is long time I don't use etabs, so I cannot recall straight away, but if you do a quick model to se the difference in changing those parameters, you will find out.
Sorry I cannot be too helpful.