211 Some Issues in Current Seismic Design

The first question that comes to mind when introducing a new methodology is the obvious one: why do we need a new procedure? What is inherently wrong or inadequate in the existing provisions for design that warrants a new look at the entire process? Hence, the task of introducing PBSE is more easily accomplished by highlighting the limitations and drawbacks of existing seismic design procedures.

Since the early development of seismic design codes, global response modification factors (or R-factors) have remained at the core of seismic force formulas. The main purpose of the force-reduction factors used in seismic design is to simplify the analysis process so that elastic methods can be used to approximately predict the expected inelastic demands in a structure subjected to the design loads. They account for reductions in seismic force values due to a variety of factors including system inherent ductility, overstrength, and redundancy. Of these, only the ductility component of the R-factor is generally implied in the design provisions because systems with larger expected ductility have the lowest reduction factors. Current codes also specify a displacement amplification factor Cd that quantifies the expected inelastic displacement of the system. Both R and Cd factors are global response measures that do not provide an assessment of structural performance at the component level. There is growing awareness that force-based design using R and Cd factors has serious shortcomings. For instance, these factors are independent of the building period and ground motion characteristics. Additionally, the same R-factor is used for moment-resisting reinforced concrete (RC), steel, and braced frames. It is clear that a single global response modifier cannot capture the progressive distribution of nonlinearities between various structural elements, the resulting redistribution of seismic demands inside the structure, and the changes that occur during the course of the seismic motion. In addition to differences in seismic demands and failure mechanisms, the damage distribution is also likely to vary from one structure to the other even though they have all been designed for the same R-value. A coherent description of the meaning and basis of establishing force-reduction factors is outlined in two papers by Uang (1991) and Uang and Bertero (1991). Figure 21.1 shows the base-shear versus roof displacement response of a typical building structure. The vertical axis in Figure 21.1 shows the base shear coefficient, which is the total shear normalized by the seismic weight of the building (V/W). The design base shear coefficient of the building is Cs, while the corresponding elastic strength is Ce. As is evident from the response of the structure, the yield strength of the building is Cy (assuming a bilinear idealization as shown). First yielding in a member in the system should typically commence at Cs, though material overstrength and member sizing

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