What is structural irregularity:
During an earthquake, failure of structure starts at points of weakness. This weakness arises due to discontinuity in mass, stiffness and geometry of structure. The structures having the discontinuity are termed as irregular structures. Weight (Mass) Irregularity: It is defined to exist where the effective mass of any story is more than 150% of the effective mass of an adjacent story. A roof that is lighter than the floor below need not be considered.
As per ASCE 7 -16 Code (Chapter 12)
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