Structures having significant physical discontinuities in vertical configuration or in their lateral force resisting systems are termed as vertically irregular structure. The vertical irregularities in structures are:
Stiffness irregularity (soft storey)
A soft storey is one in which the lateral stiffness is less than 70 % of that in the storey above or less than 80% of the average stiffness of the three storey above.
Mass irregularity shall be considered to exist where the effective mass of any storey is more than 150 % of the effective mass of an adjacent storey. A roof which is lighter than floor below need not be considered.
Vertical geometric irregularity
Vertical geometric irregularity shall be considered to exist where horizontal dimension of the lateral force-resisting system in any storey is more than 130 % of that in an adjacent storey, one storey penthouse need not to be considered.
In-plane discontinuity in vertical lateral force-resisting element
An in-plane offset of the lateral load-resisting elements greater than the length of those elements.
Discontinuity in capacity (Weak storey)
A weak storey is one in which the storey strength is less than 80 % of that in the storey above. The storey strength is the total strength of all seismic-resisting elements shearing the storey shear for the direction under consideration.