This report investigates the issue of selecting and scaling ground motions as input excitations for response history analyses of buildings in performance-based earthquake engineering. Many ground motion selection and modification (GMSM) procedures have been developed to select ground motions for a wide variety of objectives. This report focuses on the selection and scaling of single, horizontal components of ground motion for estimating seismic demand hazard curves (SDHCs) of multistory frames at a given site.
Chapter 2 develops a framework for evaluating GMSM procedures in their ability to provide accurate estimates of SDHCs. The notion of a benchmark SDHC is introduced, enabling biases caused by GMSM procedures to be isolated from other sources of bias. More importantly, the ability to quantify bias facilitates the identification of intensity measures (IMs) that are sufficient; an IM, which may be scalar or vector-valued, is defined to be sufficient for a response quantity, or engineering demand parameter (EDP), when the EDP is sensitive to only this IM and no other features of the ground motion 1. However, the application of the framework in this chapter is limited by the availability of recorded ground motions and prediction models for EDP of structures.
The framework developed in Chapter 2 is applied to synthetic ground motions in Chapter 3, where biases in estimates of SDHCs caused by GMSM procedures can be estimated for any structural system and any EDP. However, the use of synthetic ground motions gives rise to the issue of developing benchmark-consistent ground-motion prediction models. Based on the results from Chapters 2-3, it is hypothesised that the potential bias in any SDHC estimate is caused directly by two important properties of the particular selection of ground motions: (i) hazard consistency and (ii) IM sufficiency.
A novel ground-motion selection procedure, rooted in the theory of Importance Sampling, is developed in Chapter 4 that allows: (i) hazard consistency of the selected motions to be directly enforced for a user-specified collection of IMs, and (ii) SDHCs of a structure to be estimated from a single ensemble of ground motions, with the option of avoiding record scaling altogether. This procedure, together with two other contemporary GMSM procedures – (i) “exact” Conditional Spectrum and (ii) Generalized Conditional Intensity Measure – are evaluated in Chapters 5-6 for several structural systems and EDPs at a specified site. In these chapters, the amount of effort involved in implementing these procedures for estimating SDHCs is summarized in a step-by-step form, and the magnitude of biases caused by these procedures are documented.
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