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- Shift the center of mass for each rigid floor diaphragm by the distance required by code or specification. Building code often takes this distance to be a fraction of maximum story dimension, typically 5% or 10%, depending on regional standards. Center of mass may shift along either direction and along either lateral axis. Each of these changes in structural configuration also changes the global stiffness matrix, modal parameters, and dynamic properties of the structure. Therefore, separate static analyses must be conducted for each eccentricity considered.
For each direction of eccentricity, the lateral static load pattern is then applied to the shifted center of mass to generate accidental torsional loading. These results may then be enveloped to obtain a maximum condition. Static torsional response is then combined with the results of dynamic analysis results analyses to produce design forces which account for accidental eccentricity. - The second approach is to apply, at each story level, torsional load which approximates the effect of accidental eccentricity. Static response is then combined with dynamic analysis results.
Fahjan et al. proposes an alternative procedure which utilizes modal superposition by modifying global force vectors to include the effect of accidental torsion within each separate mode shape.
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