The following limitations shall be imposed on the use of some structural systems and configurations:
1) Vertical irregularities of structures
Type I : 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.
Type II : Mass irregularity :
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.
Type Ш : 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.
Type IV : 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.
Type V : 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.
2) Plan irregularities of structures
Type I : Torsional irregularity (to be considered when diaphragms are not flexible):
Torsional irregularity shall be considered to exit when the maximum storey drift, computed including accidental torsion, at one end of the structure transverse to an axis is more than 1.2 times the average of the storey drifts of the two ends of the structure.
Type II : Reentrant corners:
Plan configurations of a structure and its lateral force-resisting system contain reentrant corners, where both projections of the structures of the structure beyond reentrant corners are greater than 1.5 % of the plan dimension of the structure in the given direction.
Type Ш : Diaphragm discontinuity:
Diaphragm with a abrupt discontinuities or variations in stiffness, including those having cutout or open areas greater than 50 % of the gross enclosed area of the diaphragm, or changes in effective diaphragm stiffness of more than 50 % from one storey to next.
Type IV : Out-of-plane offsets:
Discontinuities in a lateral force path, such as out-of-plane offsets of the vertical elements.
Type V : Nonparallel system:
The vertical lateral load-resisting elements are not parallel to or symmetric about the major orthogonal axes of the lateral force-resisting system.