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2. The second approach uses the "Dynamic Procedure". This procedure modifies the dynamic characteristics of the structure so that the dynamic amplification of the accidental torsion is directly considered. This modification can be done by shifting the mass on each floor 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. This procedure is automated for ETABS (2015 and later versions) only. Where this "Dynamic Procedure" is used, amplification of the accidental torsion in accordance with the ASCE 7-10 section 12.8.4.3 is not required. See ASCE 7-10 Commentary C12.9 and C12.9.5.

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Static Procedure — Implementation as part of a Response Spectrum load case

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2- Define nonlinear static load case Lcase2, Lcase3 etc. with loads applied that correspond to same used as in the mass source. Make sure to select the correct mass source while in nonlinear static case i,e, MsSrc2, MsSrc3...for nonlinear static load cases Lcase2, Lcase 3 etc. While in the nonlinear static cases, under Other Parameters select Pdelta from the Geometric Nonlinearity Option to account for updated Pdetla stiffness in the subsequent modal cases as defined in step 3 below.

3- Define > modal cases and add new case, say ModalCase2, ModalCase3 etc. and make sure to choose radio button "use nonlinear case..." and select Lcase2, Lcase3... respectively.

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• Variation between computed and actual values of structural properties.

References

• Fahjan, Y., Tuzun, C., Kubin, J. (2006). An Alternative Procedure for Accidental Eccentricity in Dynamic Modal Analyses of Buildings. First European Conference on Earthquake Engineering and Seismology, 1166.
  

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