Dual-level seismic design: a reliability-based methodology

The seismic design provisions of most building codes in the United States specify ground motion parameters for various regions of the country and provide simple formulae to determine a distribution of lateral forces for which the structure should be designed. Although the code provisions are very simple to use, they oversimplify a complex problem and are based on many implicit assumptions which many designers may not appreciate. Furthermore, the reliability of the final design is not easily determined. This paper describes a reliability-based seismic design procedure for building structures. It is a performance-based design procedure which requires the designer to verify that a particular structural design satisfies displacement-based performance criteria. An equivalent system methodology and uniform hazard spectra are used to evaluate structural performance. The performance criteria are expressed in probabilistic terms, and deterministic design-checking equations are derived from these criteria. The design-checking equations incorporate design factors (analogous to lead and resistance factors) which account for the uncertainty in the seismic hazard, the uncertainty in predicting site soil effects, and the approximate nature of the simplified models of the structure. The alternative procedure should enable designers to achieve code-specified target performance objectives for moderate and severe levels of earthquake excitation.