Rigid zone factor
If the Define Lengths check box is checked and you want to specify a rigid zone for the frame element, enter a value between 0 and 1 in the rigid zone factor edit box. The rigid-zone factor specifies the fraction of each end offset assumed to be rigid for bending and shear deformations. When a fraction of the end offset is specified rigid, the outside portion of the end offset is assumed rigid, that is, the portion at the end of the frame member. By default ETABS assumes the rigid end factor to be zero, that is, the end offsets are fully flexible and they have the same frame section properties as are assigned to the rest of the member.
The rigid zones of the end offsets never affect axial and torsional deformations. The full element length is always assumed to be flexible for those deformations.