A new spectral representation of seismic demand is described for use in the seismic design of new structures and in the evaluation and rehabilitation of existing structures. Yield Point Spectra (YPS) retain the intuitive appeal of the Capacity Spectrum Method (Freeman 1978) and join the Nonlinear Static Procedures of FEMA 273/274 (1997) and ATC 40 (1996) for use in estimating displacement demands. YPS also may be used to establish admissible combinations of strength and stiffness for the design of new structures to limit system ductility and drift to arbitrary values. Graphical procedures allow admissible design regions to be established to satisfy multiple performance objectives. YPS computed for 15 ground motions classified as Short Duration, Long Duration, or as containing near-fault Forward Directivity pulses are presented for bilinear and stiffness-degrading hysteretic models.

SUMMARYYield frequency spectra (YFS) are introduced to enable the direct design of a structure subject to a set of seismic performance objectives. YFS offer a unique view of the entire solution space for structural performance. This is portrayed in terms of the mean annual frequency (MAF) of exceeding arbitrary ductility (or displacement) thresholds, versus the base shear strength of a structural system having specified yield displacement and capacity curve shape. YFS can be computed nearly instantaneously using publicly available software or closed-form solutions, for any system whose response can be satisfactorily approximated by an equivalent nonlinear single-degree-of-freedom oscillator. Because the yield displacement typically is a more stable parameter for performance-based seismic design compared with the period, the YFS format is especially useful for design. Performance objectives stated in terms of the MAF of exceeding specified ductility (or displacement) thresholds are used to determine the lateral strength that governs the design of the structure. Both aleatory and epistemic uncertainties are considered, the latter at user-selected confidence levels that can inject the desired conservatism in protecting against different failure modes. Near-optimal values of design parameters can be determined in many cases in a single step.

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Although seismic design traditionally has focused on period as a primary design parameter, relatively simple arguments, examples, and observations discussed herein suggest that the yield displacement is a more stable and more useful parameter for seismic design. The stability of the yield displacement is illustrated with four detailed examples, consisting of moment-resistant frame buildings. Each frame is designed to limit roof drift for a specific ground motion using an “equivalent” SDOF model in conjunction with Yield Point Spectra. The effectiveness of the simple design method is established by nonlinear dynamic analysis. Yield displacements were stable and consistent while the fundamental periods of vibration (and lateral stiffness) required to meet the performance objective differed substantially.

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