ETABS - layered wall with modal analysis

[jorWT]

Hi,

I am working on a high-rise building model that I would like to do some nonlinear time-history analysis with. I am currently working on getting the modal analysis right before I move to this stage, but I am having issues using the layered wall type.

The building is a core wall system, so I am trying to use a layered wall section to define my concrete and rebar, which I am assigning nonlinear material properties to. However, when I use this type of section, the natural period of my building increases to almost double compared to using a shell-thin element for the walls. Has anyone else had this issue or have any suggestions as to why this might be happening?

Thanks!