A Reliability-Based Dual Level Seismic Design Procedure for Building Structures

structural design, Limit design, Spectra

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 formulas to determine a distribution of lateral forces for which the structure should be designed. The simple formulas typically involve the use of one or more “factors” to account for anticipated inelastic behavior of the structure, relative importance of the structure, and site soil effects. Although these 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 known. This paper describes the key features of an alternative seismic design procedure in which the underlying assumptions are more clearly defined and which provides a more uniform level of reliability in the final design. The procedure requires the designer to consider two levels of earthquake excitation. An “equivalent” single-degree-of-freedom model and uniform hazard response spectra are used to predict structural performance. The alternative procedure should enable designers to achieve code-specified target performance objectives for moderate and severe levels of earthquake excitation.