Modeling building with elevator and stair shaft

[Adrdm]

I'am trying to figure out a way to model an elevator shaft that does not contribute very much to the rigidity of the hole structure as to avoid causing too much eccentricity. Since it's my first time I wondered if anyone had any tips. My structure consists of rigid concrete frames with concrete slabs supported on beams, the model is 7 storeys high. Slabs are modeled as shell-thin elements.