Report - PEER - University of California, Berkeley

2.6 Preliminary DesignIn the preceding section, an estimate of the yield displacement was used to determinethe required base shear strength coefficient, C y , and corresponding period, T. Oncedetermined, it will often be convenient to use C y and T (rather than C y and ∆ y ) for thepreliminary design of the structure, thereby allowing conventional (elastic) structuralanalysis software packages to be used for design as is currently done in practice.Required member strengths can be determined by a simple rigid-plastic mechanismanalysis or using conventional design analyses in conjunction with an assumedoverstrength factor. As the mathematical model of the structure is developed, theactual period of vibration, modal mass coefficient, and modal participation factor canbe determined for the fundamental mode of response.A comparison of the fundamental period of the preliminary design and the periodof the ESDOF system provides a quick check on the validity of the assumptions usedin preliminary design. Differences in these periods indicate potentially incorrectassumptions regarding the estimated yield displacement and estimated modalparameters. If significant differences exist, the actual period, design strength, andmodal mass coefficient may be used to determine a new ESDOF yield point (usingEquations (2) and (3) to determine the parameters C y * and ∆ y * ). This yield point maybe found to have acceptable performance (i.e., if it is within the Admissible DesignRegion); if unacceptable performance is identified, a new ESDOF yield strength canbe identified for the revised yield displacement, and member strengths then may beincreased in proportion to the increase in base shear coefficient. The need for furtheriteration on the preliminary design would not be anticipated if member dimensions,span lengths, and material properties are kept approximately the same.Although the design approach is based on the use of ESDOF systems andnonlinear static (pushover) analysis, a nonlinear model of the structure is not requiredfor preliminary design. Strength may be assessed by means of a mechanism analysisor by using approximate overstrength factors. However, nonlinear static analyses canaid in understanding the behavior of the structure (e.g., Krawinkler and Seneviratna,1998), and once developed, the model used for the nonlinear static analysis may beuseful for dynamic analyses as described in the next section.3. DESIGN REFINEMENT AND PERFORMANCE ASSESSMENTThe preliminary design was determined to limit peak roof drifts and system ductilitiesfor a mechanism involving primarily first mode deformations. MDOF effects(associated with higher mode contributions) can significantly affect interstory drifts,story shears, and many other response quantities. For many structures, MDOF effectsshould be addressed to establish: (1) that the intended inelastic mechanism develops,(2) statistical distributions of performance parameters (e.g., interstory drifts anddeformation demands in inelastic components), and (3) the forces that must be489