Flexural and axial behavior for wall elements modeled using shell properties can be modified in ETABS by using either f11 or f22 property modifiers, depending on the orientation of your local axes. The shear behavior is controlled by f12 property modifier. In column and code terms f11 or f22 would correspond to modifications of EI or EA and f12 would correspond to modifications to GA shear. The code recommendations in ACI318-05, Section 10.11 are related to slenderness effects where flexural deformations govern so the code recommends modifying EI (corresponding to f11 or f22 for shear walls). Furthermore, ACI318-08 Section 8.8 includes recommendations for member’s properties modification factors to be used for lateral loads analysis. There is no recommendation for reducing the GA shear. Modifiers for f12 can be used where deterioration of shear stiffness is expected.
The above discussion applies when the local axes 1 and 2 of the shear wall area object are either vertical or horizontal. This is under user control. When drawing walls in ETABS, the default is to have the 1 axis horizontal and the 2 axis vertical. This means that the flexural modifier for EI should be applied to f22 for wall piers and to f11 for spandrels. If you apply the modifier to both f11 and f22, it hardly affects the results.
