Struggling in understanding application of Stiffness modifiers for different building systems 2

[Usman3301]

Hi there, I am currently assigned to make the general layout of High rise building in ETABS, and once my senior reviews basic framing, he'll take over the model and carry on with the setting up Dynamic (Response Spectrum) parameters, etc and then on to design process. Thing is that I am very interested in this project and just don't want to constrain myself to model the structure and then hand it over. I am interested in understanding how seismic forces (Base shear) are distributed and attracted by shear walls based on their relevant stiffnesses. Moreover, I am struggling with the concept of using stiffness modifiers so as to model structure as close to reality as possible.

For this particular high-rise structure, my senior recommended using a Dual system with Intermediate Moment Resisting frames and Special Reinforced Concrete Shear Walls having a Response Modification Factor of 6.5. (ASCE 7-16, TABLE 12.2-1 (E2))
What I understand from this is that we have to model our structure in such a way that Shear walls resist lateral loads and the Moment resisting frame system carries the gravity load and as a backup, is capable of resisting at least 25% lateral forces.
Normally we use the following modifiers when designing shear walls for similar building systems.
f11=f22=0.7, f12=1, m11,m22,m33=0.01 (Which I assume suggests that we won't be designing our shear walls for out-of-plane bending moments and looks plausible.

Given these particular stiffness modifiers, I am doubtful about using f11 or f22 (whichever controls flexural behavior) equal to 0.7 for designing shear walls. In my opinion, by using these modifiers we are underestimating Seismic forces and it might lead to shear walls being under-design. I have been reading about Stiffness modifiers for the past few days and am a bit confused with my conclusion, therefore want someone to help me understand how to apply stiffness modifiers considering different structural systems.
I believe we should initially run f11 and f22=0.7 and see if our walls are cracking or not. If they are not cracking, then 0.7 would suffice. And if they are, we have to further reduce the stiffness of these particular shear walls to 0.35 as per ACI code requirements and design these walls for reduced stiffnesses. Moreover, I am struggling with understanding what role "f12" stiffness modifier plays in shear wall analysis/design.

And lastly, the reason for asking this here is that my senior isn't that helpful when it comes to sharing knowledge. Since I am a BSc graduate and we didn't study lateral systems during our studies, I struggle with these concepts but at the same time, this is exactly what interests me a lot. It would be great if someone can recommend some good books to develop a good understanding on this subject.
Thanks.

 

[OldDawgNewTricks]

Using lower stiffness modifiers is not conservative for determining seismic forces. For a high-rise building, your fundamental period is presumably longer than TS=SD1/SDS, so lower stiffness --> longer period --> lower seismic forces. However, lower stiffness would be conservative for determining story drifts.

The most accurate way to deal with this is nonlinear analysis. But for typical design, nonlinear analysis is rarely used. One simple approach to applying stiffness modifies would be to initially model all elements as uncracked. From the initial analysis, determine which elements are cracked or uncracked, and apply the 0.7 or 0.35 modifiers as appropriate to the subsequent analyses. An even simpler approach to determining story drifts would be to just use an average value of 0.5 for all walls as allowed by ACI 318 6.6.3.1.2.

A good reference for ETABS stiffness modifiers is the CSI Wiki. A good reference for dynamic analysis in general is Anil Chopra's book, Dynamics of Structures.

 

[HTURKAK]

The snip below ( sign convention for shell elements ) explain the membrane stresses and bending forces..

- f11, f22 , f12 modifiers are equivalent to modification factors on the thickness of the shell element.

- m11, m22 and m12 modifiers are essentially equivalent to modification factors on the t**3 of the shell element.

- ACI 318 suggests property modifiers for Walls Uncracked 0.70Ig , Cracked 0.35Ig but 1.0Ag for axial deformations..

- IMO, for uncracked 0.70Ig , and for Cracked 0.35Ig for f11, f22 , f12 modifiers literally means axial shortening and displacement incompatibility with adjacent frame columns..

- ACI commentary R6.6.3.1.1 (...... If the factored moments and shears from an analysis based on the moment of inertia of a wall, taken equal to 0.70Ig,indicate that the wall will crack in flexure, based on the modulus of rupture, the analysis should be repeated with I = 0.35Ig in those stories where cracking is predicted using factored loads )..IMO, implicitly restricts the use of proposed modifiers 0.70Ig,0.35Ig for the analysis based on the moment of inertia

- I will suggest you to look also to the tutorials ..

 

[centondollar]

You write "f11=f22=0.7, f12=1, m11,m22,m33=0.01 (Which I assume suggests that we won't be designing our shear walls for out-of-plane bending moments and looks plausible. ".

Please note that stiffness in a structural model should reflect the stiffness of the real structure when it is in use. Arbitrarily lowering bending and torsional stiffnesses to zero does not mean that the wall actually behaves that way; presumably, the wall is tied to the floor or beams, which means that it will receive some moment, and consequently that it will experience less in-plane forces than a "zero bending stiffness model" predicts. Structural response is estimated by physico-mathematical models, and the quality of the estimate depends on how well the engineer can predict the constitutive equations, kinematics, loading and boundary conditions of the structure.

If you want to account for cracking, the technically correct way is to perform nonlinear analysis: inserting the nonlinear strain-stress curve and performing a stepwise calculation, where at each load step (say, "TotalLoad/100" in each step) the stiffness is updated after convergence (almost zero residual) of the internal force vector (tangent stiffness multiplied by displacement) and load vector. Stiffness modifiers provide only crude approximations and do not necessarily reflect actual behavior of the structure very well, particularly for reinforced concrete. In a high-rise building, the effects of crude modelling may be very significant.