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EXCEPTION: This requirement need not be applied to a story where the dead <br /> weight above the story is less than 10 percent of the total dead weight of the <br /> structure. <br /> <br /> Structures may be designed using the procedures of this section under the <br /> following conditions: <br /> <br /> 1. The entire structure is designed using the lowest R of the lateral-force-resisting <br /> systems used, or <br /> <br /> 2. The following two-stage static analysis procedures may be used for structures <br /> conforming to Section 1629.8.3, Item 4. <br /> <br /> 2.1 The flexible upper portion shall be designed as a separate structure, <br /> supported laterally by the rigid lower portion, using the appropriate values <br /> of R and p. <br /> <br /> 2.2 The rigid lower portion shall be designed as a separate structure using <br /> the appropriate values of R and p. The reactions from the upper portion <br /> shall be those determined from the analysis of the upper portion multiplied <br /> by the ratio of the (R/p) of the upper portion over (R/p) of the lower portion. <br /> This ratio shall not be taken less than 1.0. <br /> <br /> Sec. 9.67 SECTION 1630.7 OF THE BUILDING CODE AMENDED: Section <br />1630.7 of the Building Code is hereby amended to read as follows: <br /> <br /> Section 1630.7 Horizontal Torsional Moments. Provisions shall be made for the <br /> increased shears resulting from horizontal torsion where diaphragms are not <br /> flexible. The most severe load combinations from each element shall be <br /> considered for design. <br /> <br /> The torsional design moment at a given story shall be the moment resulting from <br /> eccentricities between applied design lateral forces at levels above that story and <br /> the vertical-resisting elements in that story plus an accidental torsion. <br /> <br /> The accidental torsional moment shall be determined by assuming the mass is <br /> displaced as required in Section 1630.6. <br /> <br /> Where torsional irregularity exists, as defined in Table 16-M, the effects shall be <br /> accounted for by increasing the accidental torsion at each level by an <br /> amplification factor, Ax, as determined by the following formula: <br /> <br /> ~max n 2 <br /> Ax = ~- 1.2 ~avg J (30-16) <br /> <br />Atty/Ord/Ord. 167 <br />102402 8 <br /> <br /> 2241 <br /> <br /> <br />