ΣA e = Effective shear area in the direction of the earthquake considered in any floor
ΣA g = Total of cross-sectional areas of the
structural system elements working as a curtain parallel to the earthquake direction considered at any floor ΣA k = masonry filling parallel to the earthquake direction considered in any floor total of wall areas (excluding door and window openings)
ΣA w = Total effective body areas of column cross-section at any floor
η ci = Strength Irregularity Coefficient defined at i'th floor

B1 - Strength Irregularity Between Neighboring Stories (Weak Stories): In reinforced concrete buildings, in any of two earthquake directions perpendicular to each other, the Strength Irregularity Coefficient defined as the ratio of the total effective shear area on any floor to the total effective shear area on the next floor η ci is less than 0.80.
η ci = (ΣA e ) i / (ΣA e ) i + 1 <0.80]

Definition of effective cutting area in any floor:

(ΣA e ) i = (ΣA w ) i + (ΣA g ) i + (0.15 ΣA k ) i

Note: A k = 0 for walls defined in (b) . - In buildings with B1 type irregularities, if the sum of the infill wall areas on the i'th floor is higher than that of the upper floor, the filling walls will not be taken into account in the calculation of η ci . Considering the floor where η ci is the smallest, the load-bearing system behavior coefficient given in Table 4.1 in the range of 0.60 ≤ (η ci ) min <0.80 shall be multiplied by 1.25 (η ci ) min value and applied to the whole building in both earthquake directions. However, it will never be η ci <0.60. Otherwise, the earthquake calculation will be repeated by increasing the strength and stiffness of the weak floor.