Are accidental-eccentricity load cases not required when using semi-rigid diaphragms?
Extended Question: IBC code requires consideration of additional seismic load cases with 5% eccentricity when designing rigid diaphragms. Can this design check be avoided by using semi-rigid diaphragms?
Answer: Accidental eccentricity must be considered because reinforced-concrete slabs which are designed without the ETABS rigid-diaphragm feature are not necessarily flexible diaphragms, as explicitly defined by IBC 2006 and ASCE 7-05:
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- 12.3.1.3 Calculated Flexible Diaphragm Condition. Diaphragms not satisfying the conditions of Sections 12.3.1.1 or 12.3.1.2 are permitted to be idealized as flexible where the computed maximum in plane deflection of the diaphragm under lateral load is more than two times the average story drift of adjoining vertical elements of the seismic force-resisting system of the associated story under equivalent tributary lateral load as shown in Fig. 12.3-1. The loadings used for this calculation shall be those prescribed by Section 12.8.
See Also
- Rigid vs. Semi-rigid diaphragm article