Clause 7.1 classifies buildings as regular or irregular based on their geometric configuration and stiffness/mass distribution. Irregular buildings require dynamic analysis and may need more stringent design checks. Plan irregularities (Table 5) include torsional irregularity, re-entrant corners, diaphragm discontinuity, and out-of-plane offsets. Vertical irregularities (Table 6) include soft storey, mass irregularity, vertical geometric irregularity, and in-plane discontinuity.
Key Requirements
•Buildings shall be classified as regular or irregular before selecting the analysis method
•Plan irregularity types are defined in Table 5
•Vertical irregularity types are defined in Table 6
•Irregular buildings in Zone III, IV, or V with specific irregularities shall be analyzed by dynamic analysis (Cl. 7.7)
•Soft storey buildings require special design provisions as per Cl. 7.10
•Buildings with vertical irregularity shall not have response reduction factor R greater than 4 in Zone V
Reference Tables
Table 5 — Plan Irregularities (Clause 7.1)
Type
Irregularity
Defining Criterion
i
Torsion Irregularity
Maximum storey drift at one end > 1.5 × average of storey drifts at two ends of the structure
ii
Re-entrant Corners
Both projections beyond re-entrant corner > 15% of plan dimension in that direction
iii
Diaphragm Discontinuity
Abrupt discontinuity or opening area > 50% of gross enclosed floor area
iv
Out-of-Plane Offsets
Discontinuity in lateral force path due to out-of-plane offset of vertical elements
v
Non-Parallel Systems
Vertical lateral force resisting elements are not parallel to or symmetric about major orthogonal axes
Table 6 — Vertical Irregularities (Clause 7.1)
Type
Irregularity
Defining Criterion
i
Soft Storey (Stiffness Irregularity)
Lateral stiffness of a storey < 70% of that of storey above, or < 80% of average of three storeys above
ii
Mass Irregularity
Seismic weight of any storey > 200% of that of adjacent storeys (excluding roof)
iii
Vertical Geometric Irregularity
Horizontal dimension of lateral force resisting system in any storey > 150% of that of adjacent storey (excluding penthouses)
iv
In-Plane Discontinuity in Vertical Elements
In-plane offset of lateral force resisting elements > than plan length of those elements
v
Weak Storey (Strength Irregularity)
Lateral strength of any storey < 80% of that of storey above
A 'soft storey' is a common issue in Indian buildings with open ground floors (stilt parking). Such buildings require special treatment per Cl. 7.10.
Practical Notes
✓Open-ground-storey buildings (stilt parking) are the most common type of soft storey irregularity in India
✓Re-entrant corners are common in L-shaped, T-shaped, and U-shaped building plans
✓Torsional irregularity often arises when the centre of rigidity is far from the centre of mass (e.g., shear wall on one side only)
✓Even if not strictly irregular, symmetric plan layouts perform significantly better in earthquakes
Common Mistakes
⚠Not checking for irregularity before choosing the analysis method — irregular buildings need dynamic analysis
⚠Treating stilt-parking buildings as regular when they have a soft storey
⚠Not verifying torsional irregularity numerically — it requires computing storey drifts at both ends
⚠Ignoring mass irregularity when adding a heavy floor (e.g., swimming pool, mechanical floor) at an intermediate level