In ACI318-19 section R6.3.1 and its commentary, "member stiffness GJ should be reflected the degree of cracking and inelastic action that has occurred along each member before yielding".
On table 6.6.3.1.1(a) the value for I-cracked for each member is recommended. However, I cannot find any recommendation for cracked torsional stiffness.
I am analyzing the structure subjected to gravity, wind, and earthquake load according to ACI using ETABS, and I am struggling about this issue
https://wiki.csiamerica.com/display/etabs/Modeling+concrete+cracked+section+properties+for+building+analysis
Please explain what is the best practice for applying cracking stiffness modifiers for torsion in
beams (Torsional constant, J),
columns (Torsional constant, J),
walls (in-plane f12), and
slab sections (m12).
It would be really appreciated if supportive reasons are provided.
Thank you.
Correction factor of torsional stiffness of concrete beams
A very common case in reinforced concrete buildings is the placement of side beams on the main beams. The problem that designers face in this situation is that the beams turn red due to excessive twisting. In these conditions, the solution is not to increase the cross-sectional dimensions, but to consider the torsional stiffness correction coefficients in them.
According to the philosophy of applying stiffness correction coefficients, which we examined in the previous article of stiffness coefficients of beams and columns, in the current situation, due to torsional cracks, the main beam rotates and its torsional stiffness decreases. Therefore, a coefficient should be considered to correct the torsional stiffness of these beams.
Unlike the flexural stiffness correction coefficients of beams and columns, the ninth section does not specify a specific number for the torsional stiffness correction factor; instead, it clearly states that the torsional stiffness of cracked members must be properly calculated and considered in structural analysis. Therefore, the process of determining the torsional stiffness correction factor is opposite to the computational flexural stiffness correction factor.
In cases where it is possible to reduce the torsional anchor by replaying the internal anchors to a member of an indeterminate structure, the Tu value can be reduced to 0.67Tcr.
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