Report - PEER - University of California, Berkeley

2. SYSTEM SELECTION AND PRELIMINARY DESIGNA design must be developed in order for its performance to be assessed analytically.This section describes relatively simple techniques that can be used in systemselection, schematic design, and for the design of the members in the preliminarydesign of the selected system. The design relies on the use of an “equivalent” SDOFsystem to establish the base shear strength required to limit roof drift and systemductility demands to desired values. Roof drift is related to nonstructural damage andsystem ductility is related to structural damage.2.1 Modal Property EstimatesModal properties such as the first mode participation factor (Γ 1 ) and the modal masscoefficient (α 1 ) depend on the relative distribution of stiffness and mass rather thantheir absolute values, and therefore can be estimated prior to the detailed design of thestructure. Estimates of adequate precision may be made on the basis of assumed modeshapes and mass distributions, or using simplified models as described by Miranda(1997). The values of Table 1 are based on the deflected shapes of Figure 1 andassume the floor masses to be uniform and the roof mass to be 80% of the floor mass.Somewhat similar values (or their inverses) are reported in Appendix I of the 1999Blue Book (SEAOC, 1999).Numberof StoriesMoment-ResistantFramesTable 1. Modal property estimatesDual Shear Wall-Moment FrameSystemsSlender CantileveredShear Walls andBraced FramesΓ 1 α 1 Γ 1 α 1 Γ 1 α 11 1.00 1.00 1.00 1.00 1.00 1.002 1.21 0.94 1.24 0.89 1.24 0.763 1.27 0.90 1.33 0.85 1.35 0.704 1.30 0.87 1.37 0.83 1.42 0.685 1.32 0.86 1.40 0.82 1.46 0.6610 1.35 0.82 1.45 0.79 1.54 0.6315 1.37 0.81 1.47 0.77 1.57 0.6220 1.37 0.80 1.48 0.77 1.59 0.622.2 Yield Displacement EstimatesThe yield displacements of structures in first-mode pushover analyses often can beestimated as a fixed percentage of the height of the structure, for a given structuralsystem and material (e.g., Aschheim, 2002). Furthermore, the yield displacement isstable even as the lateral strength of the system is changed, in many design contexts.This is illustrated in Figure 2 for two four-story moment resistant frames. In contrast,482