Vertical Irregularities in Structures
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:
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Type
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Name
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Description
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I
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Stiffness irregularity (soft storey)
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 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.
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II
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Mass irregular
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 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. |
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III
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Vertical geometric irregularity
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 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. |
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IV
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In-plane discontinuity in vertical lateral force-resisting element
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 An in-plane offset of the lateral load-resisting elements greater than the length of those elements. |
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V
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Discontinuity in capacity (Weak storey)
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 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. |