Modify Section 220.127.116.11.2.1 by adding Exception 3 as follows:
- 3. The following two-stage static analysis procedure is permitted to be used for structures having a flexible upper portion supported on a rigid lower portion where both portions of the structure considered separately can be classified as being regular, the average story stiffness of the lower portion is at least 10 times the average story stiffness of the upper portion and the period of the entire structure is not greater than 1.1 times the period of the upper portion considered as a separate structure fixed at the base:
- 3.1 The flexible upper portion shall be designed as a separate structure using the appropriate values of R and r.
- 3.2 The rigid lower portion shall be designed as a separate structure using the appropriate values ofR and r. The reactions from the upper portion shall be those determined from the analysis of the upper portion amplified by the ratio of the R/r of the upper portion over R/r of the lower portion. This ratio shall no be less than 1.0.