PROPERTY MODIFIER IN ETABS 2

[Mkasem]

HI
Modeling cracked behavior of shear walls in ETABS
1. For shell elements pier-shear walls with default orientation of local axes, the main modifier affects directly on flexural stiffness "EI" is "f22".

2. For shell elements spandrel- beam with default orientation of local axes, the main modifier affects directly on flexural stiffness "EI" is "f11".

ACI318-08 code declared in its commentary “R.8.8.2“ that the modulus of shear modulus may be taken as 0.4Ec, so the shear stiffness modifiers "f12" could be reduced as well.

In general, we can use the following stiffness modifiers for pier-shear walls:
f11=1 , and f22=f12=m11=m22=m12=0.7 for un-cracked walls.
f11=1, and f22=f12=m11=m22=m12=0.35 for cracked walls.
For spandrel shell-modeled beams:
f22=1, and f11=f12=m11=m22=m12=0.35

For shell-modeled deep wall spandrel-outriggers under high level of horizontal and vertical stresses:
f11=f22=f12=m11=m22=m12=0.35

Sometimes, the designer may go lower than those values of stiffness modifiers mentioned in code. This decision depends on designer's judgment on the degree of cracking and the expected degradation in element's stiffness under the cyclic loading and level of developed stresses.
It is good to highlight the followings:
1. Against the expected, ACI318-08 code doesn't discuss the issue of reducing the flexural stiffness modifier under chapter "21" adopted for Earthquake Resistance Structures, even though this issue is quite related to the ductility and design of structures under the attack of earthquake waves.
However ACI code discuss this issue under the clause of slenderness effect in compression members, and to be more specific, when it talks about the design of long/slender columns which are extremely affected by the second order displacement/moment result from lateral load such wind & earthquake load. In this regard: it is so clear that reducing the flexural stiffness will lead to increase the lateral displacement caused by lateral load and then increasing the second order moment effect "P-Delta" called-phenomena.
2. Reducing the flexural stiffness affects directly on structure stability index (equation 10-10 in ACI318-08).
3. Ductility of structure may measure by the degree of flexural cracking takes place under the reversal/cyclic seismic load.
These cracks grow up from cycle to the other result in degradation in element’s stiffness. And for high-ductile special structures the degree of degradation quite differs from this observed for low-ductile structures. However ACI code releases up to 2005 edition have no such distinction in the value of stiffness modifiers between special, intermediate and ordinary structures, whereas the latest edition ACI318-08 start show such difference as shown on equations “10-8” & “10-9”.

 

[KootK]

Here are a few documents that discuss the issue a bit. Shear stiffness in particular gets punished in regions of high plasticity. Stiffness modifier recommendations come in as low as 1/20 x uncracked.

Link

Link

Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Mkasem]

Thanks Kootk

 

[slickdeals]

Important thing to note is that you cannot decouple axial and bending behavior in shell elements. If you apply a f22 or f11 factor (depending on your local axis), the axial and bending stiffnesses are affected simultaneously.

 

[KootK]

That's interesting Slick. Can you elaborate at all? If one modifies both multipliers, do they compound?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[slickdeals]

In ETABS, for example, bending and axial stiffness is controlled through F22. You can have any value in the F11 parameter and it won't make any appreciable difference.

If you applied a F11 factor of 0.01, you will find that the lateral and gravity deflections of that wall are the same as an uncracked wall (i.e. F11 = F22 = F12 = 1.0)
If you only applied a F22 factor of 0.1, you will find that you reduced the lateral stiffness (and axial stiffness) by 90%.
If you applied a F12 factor of 0.1 in conjunction with F22, you will find that lateral deformations may increase a little more or a lot more (depending on shear deformations). F12 factor will have a larger effect on a short squat wall compared to a long, slender wall.

 

[slickdeals]

Bending and axial stiffness for WALLS is controlled through F22. For spandrels (horizontal) it is controlled through F11.

 

[bookowski]

This isn't unique to Etabs. Bending stiffness is the same as axial (tension/compression) stiffness and can't be decoupled in a shell.

MKasem - When modeling spandrels using shells in Etabs beware of the affect of meshing and the internal functions of Etabs (internal meshing and auto line constraint). If you play around with line elements vs shell elements and varying meshes for both you will see results all over the place. Shells as spandrels usually require intermediate meshing to get a reasonable behavior, while line elements connected to shell walls actually require that you not mesh too fine or the fixity is thrown off (counter-intuitive).