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Modal Response Spectrum Analysis per ASCE 7-16 §12.9.1

Number of Modes

The natural modes of vibration for the structure are 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.

Modal Response Parameters

Force-related design parameters, such as story drifts, support forces, and individual member forces for each mode of response, are computed using the properties of each mode, and the response spectra are divided by the quantity R ∕ Ie. Displacement values and drift quantities are multiplied by the quantity Cd ∕ Ie.

Combined Response Parameters

The value for each parameter calculated for the various modes is combined using the complete quadratic combination (CQC) method.

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