The Spatial Variability of Ground Motion and Its Effects on Multi-Supported Structures

Abstract eng:
Observations have shown significant incoherence in earthquake ground motions measured at different locations within the spatial dimensions of large horizontally expanded structures. In Eurocode 8, the effects of incoherent ground motions are addressed, but without the detailing needed for practical applications in engineering design. The objective of this paper is to present a simplified model for design purposes to account for the spatial variability of ground motions. Strong-motion effects measured at different locations within the dimensions of an engineered structure are typically different, even for structures of moderate size. However, the current engineering practice assumes routinely that the excitations at all support points are the same or that they differ only by a wave propagation time delay, i.e., excitations at all locations are assumed to be fully coherent. These assumptions ignore the natural incoherence in the ground motion, which may lead to incorrect or inaccurate results. An improved model should include the main effects governing the spatial structure of strong ground motion i.e. wave passage effects, incoherence effects and local site effects. This study emphasises the horizontal incoherence of ground motion. Selected records from shallow strike-slip earthquakes obtained at rock sites in events with magnitude 6.5 are used to facilitate the study. The horizontal components of the records are transformed into principal coordinates and coherence estimates computed from the strong motion phase of acceleration containing 90% of the wave energy. It is seen that the loss in coherence increases on the average with increasing frequency and increasing separation distance, which is in accordance with results reported in the literature. The presented model is found to be applicable in response calculations of horizontal structures.

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