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Individual directional results are automatically combined by CQC. And orthogonal loading effects are combined with SRSS. |
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90% of mass participation ratio is automatically controlled and reported by the program. |
12.9.1.1 Number of Modes
The natural modes of vibration for the structure is determined by structural analysis. A sufficient number of modes are obtained a combined modal mass participation of % 100 of the structure’s mass. The analysis is permitted to include a minimum number of modes to obtain a combined modal mass participation of at least 90% of the actual mass in each orthogonal horizontal direction of response considered in the model.
12.9.1.2 Modal Response Parameters
The value for each force-related design parameter of story drifts, support forces, and individual member forces for each mode of response, is computed using the properties of each mode and the response spectra defined in either Section 11.4.6 or 21.2 divided by the quantity R ∕ Ie. The value for displacement and drift quantities is multiplied by the quantity Cd ∕ Ie.
12.9.1.3 Combined Response Parameters
The value for each parameter calculated for the various modes is combined using the square root of the sum of the squares (SRSS) method, the complete quadratic combination (CQC) method.
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