Determination of Acceptable Structural Irregularity Limits for the Use of Simplified Seismic Design Methods
Approximations to the exact response of a structure under design level excitation may be obtained by conducting a number of 3-D inelastic dynamic time history analyses considering all relevant effects and using the best information available. Factors that should be considered include foundation effects, floor diaphragm effects, and the likely variation of earthquake demand and structural capacity.
In general, this type of approximate analysis is too complex for design engineers, so simpler, and hence more approximate analysis methods are commonly used. These have been calibrated based on the response of regular structures. When it is felt that a structure is too irregular, some approximate analysis methods are not permitted to be used. The amount of irregularity permitted has generally been selected based on engineering judgment.
This paper describes a quantitative method to determine limits for structural irregularity for structures designed using different analysis procedures. The method considers both the design approach estimations and the response from inelastic dynamic time history analysis. Irregularity limits based on a specified level of confidence are selected and these are then proposed for the use in codes. The method is illustrated by an example considering mass irregularity.