How to model cracked section in Etabs 2

[rejoicer]

According to 10.11.1 of ACI318-05, concrete shear wall should use 0.70Ig in the model. How to consider 70% reduction in Etabs? Reduce modulus of elasticity to 70% or set stiffness modifiers to 0.7.

Thanks.

 

[structuresguy]

Set stiffness modifier to the appropriate amount, 0.7 in your case for walls, unless they are cracked, then it would be 0.35 for walls.

 

[slickdeals]

The best way is to use stiffness modifiers

Use f11,f22 and f12 on walls
use m11,m22 and m12 on slabs

 

[sreichwein]

I set the modifier for f22 equal to 0.70 when modeling the cracks of a shear wall. For framing elements (like coupling beams and moment frames), I set I33 equal to 0.35 to model the cracking.

 

[saburo]

For beam/line elements, it is quite simple, I33 or whatever can be set to be 0.35. Fine.

But for walls that modeled with shell elements, the problem is more complex. If the wall is one straight wall, tweaking f11, f12,f22 would make sense in a way.(I would not touch m11, m22 modifier because this usually cause local vibration when doing RSA) I still do not believe the validity of setting these with the same value tho.

Anyway, my point is, if you have c-shaped wall or box-shape core wall, what would you do? there is flange effect etc and the whole section will be working as one section.For the flange, really, the axial stiffness should be modified.

The other issue for wall is, most of the case, only the bottom couple of storeys are cracking, rest might stay elastic. So blindly setting stiff modifier is troublesome.

CODE typically states, 0.7Ig? for wall in ACI, right?. this means that the flexual deformation at the top(consider a cantilevered wall for argue sake) will be 1/0.7 times larger than totally elastic when taking account for cracking. So after modifying these f11,f12 modifiers, we should get similar deformation. but if we blindly input 0.7 for all f11,f12 and f22. I do not think the deflection will be 1/0.7. this is all to do with wall configuration, external force distribution along height hence ratio of inplane shear/flexural deformations.the CODE seems well behind than what we can do now with all sophisticated programme like ETABS. It seems to me that they are talking when we modeled walls with line elements with rigid arm or brace replacements!!!.

Appreciate your thoughts on wall stiffness modifiers. To me it's a million dollar question. Nothing is relevant IMO LOL.

Sorry my English