SYMBOLS
Ki = Stiffness defined at the i'th storey
1a. Stiffness–Soft Story Irregularity
For any of two earthquake directions perpendicular to each other, stiffness–soft story irregularity should be checked. It is defined as there is a story in which the lateral stiffness is less than 70% of that in the story above or less than 80% of the average stiffness of the three stories above.
1b. Stiffness–Extreme Soft Story Irregularity
For any of two earthquake directions perpendicular to each other, stiffness–soft story irregularity should be checked. Stiffness–extreme soft story irregularity is defined as there is a story in which the lateral stiffness is less than 60% of that in the story above or less than 70% of the average stiffness of the three stories above.
According to Table 12.6-1 - 1a and 1b type irregularities are the irregularities that are effective in the selection of the analysis procedure selection.