Stiffness Reduction for Walls 1

[Roy88]

Hi Guys,

I am sure most of us here are familiar with the stiffness modification factor stipulated in ACI 318-14.

For beams and columns, the application of stiffness modifiers can be easily incorporated into ETABS analysis by modifying the respective moment of inertia.

My question here is how can we correctly apply stiffness modification for walls. As per CSI Wiki Page, it basically says that we modify f11=f22=0.70 for uncracked wall.

I am a bit skeptical with CSI recommendations. The reduction of f11 and f22 also indirectly corresponds to a reduction in axial stiffness of the wall. This contradicts ACI 318 recommendations that there should be no modifications to gross cross-sectional area.

I am wondering how do you guys approach this. Do you just use CSI recommendations of reducing f11 and f22?

 

[wannabeSE]

I don't know what wiki you are referencing. But this WIKI says

The above discussion applies assuming the local axes 1 and 2 of the shear wall area object are either vertical or horizontal. This is under user control. When drawing 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.

I typically start by modifying the stiffness in f11 and f22. If there is significant axial loads on the walls, I will consider going back and double checking the local axis and changing the modifier in the axial direction back to 1.

 

[slickdeals]

One of the ways to counteract this is by introducing a weightless frame member at the centroid of the wall that "replaces" the area of wall that's taken out by application of crack factor. For a planar wall, this would be at the middle of the wall. For a 3D wall, it would be at the centroid of the wall or one each in each leg of the wall.