Understanding Cracked Section Modifiers in Modeling

[Respawn]

I am having a hard time understanding how these work in modeling. I know for standard concrete modeling for beams it is 0.35Ig. What if this was 0.9Ig? Does it resemble what member yields under EQ?

 

[chris3eb]

Concrete members have a nonlinear stress-strain (or force-deformation) profile. To simplify analysis, the "elastic" portion of their stress-strain curve is idealized as being a single linear curve. The analysis program will be able to calculate the uncracked stiffness of the concrete section based on the dimensions of the section and the modulus of elasticity. Concrete sections are designed to go beyond their cracking capacity, and the stiffness is reduced after cracking, so a stiffness modifier needs to be given to the program so that it will consider a more realistic stiffness.

The stiffness modifier doesn't impact member capacities, but it does affect the overall building stiffness which will increase the building period and may reduce the seismic loads. It can also change the load distribution through the building if some elements have different stiffness modifiers than others.