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Interstory Strength Irregularity (Weak Story)


ΣAe = Effective shear area on any floor in the direction of the earthquake considered
ΣAg = Total of cross-sectional areas of the structural system elements working as a curtain ina direction parallel to the earthquake direction considered at any floor

ΣAk = masonry filling parallel to the earthquake direction considered in any floor total of wall areas (excluding door and window openings)
ΣAw = Total of column cross-section effective body areas at any floor
ηci = Strength Irregularity Coefficient defined at i'th floor

  • Interstory Strength irregularity, B1, defined as Strength Irregularity Coefficient ηci should be less than 0.80, which is defined as the ratio of the total effective shear area on any floor to the total effective shear area on the next floor in either of two perpendicular earthquake directions in reinforced concrete buildings. The Interstory Strength Irregularity Coefficient ηci is defined as follows.

ηci = ( ΣAe )i / ( ΣAe )i+1 < 0.80]

  • Definition of effective shear area in any floor:

(ΣAe)i = (ΣAw)i + (ΣAg)i + (0.15 ΣAk)i

  • If this calculation is to be controlled manually, firstly, the areas of columns, and shearwalls must be determined on each floor. To be detailed with an example for the X direction;

  • For the column, there are a total of 14 *60 * 60 and 2 *80 * 80 columns on the 11th floor.

Aw = 14 * (0.6 * 0.6) + 2 * (0.8 * 0.8) = 6.32 m2

  • For the shearwall, there are 4 shearwalls with 8 m openings and core shearwall arms on all upper level floors.

Ag = 4 * 8 * 0.4 + 4 * 0.4 * 3.4 = 18.24 m2

  • In the earthquake regulation report, B1 type irregularity is checked and necessary conditions are applied.

  • There is no B1 type irregularity in the building. Details of the calculation are given in a table below.

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