A Note on Selection of Time-Histories for Seismic Analysis of Bridges in Eurocode 8

Abstract

Eurocode 8 (EC8) allows the use of real earthquake records as an input for time-history analysis of structures. In its Part 2, the code discusses the preparation of seismic input for bridges; although referring to the same target spectral shapes of Part 1, which applies to buildings. The prescriptions are somewhat differently specified in the two drafts. However, the main requirement the chosen record set should satisfy in both cases, is the compatibility of the horizontal average spectrum with the target in a broad range of periods. The set has to be made of at least three recordings, but seven is the minimum set size to consider the mean structural response as the design value. The code, at least for bridges, seems to indicate real records as the principal source of ground-motions the practitioners should rely on; however the selection of real records is not straightforward. In another study, the authors discussed the record selection prescriptions of EC8 Part 1 with respect to the current best practice, and the actual possibility of finding real record sets compliant with EC8 spectra was investigated. This paper represents an extension of the same study to EC8 Part 2 and bridges. To this aim the European Strong-Motion Database is searched to identify real record sets matching the design spectral shapes for several hazard levels and site conditions in a broad range of periods up to 4s. It resulted that combinations well approximating the target may be found for some soil classes, at least for low-to-moderate seismicity sites and if the condition of matching specific source parameters is released and large record-to-record variability is accepted. Finally the record sets presented have been used to compare spectral compatibility prescriptions of EC8 Part 1 and Part 2, which have been found to be equivalent to some extent.

Keywords:

• Real Records
• Bridges
• Seismic Isolation
• Dynamic Analysis

Acknowledgments

The study presented in this article was developed within the activities of Rete dei Laboratori Universitari di Ingegneria Sismica – ReLUIS for the research program founded by Dipartimento di Protezione Civile (2005–2008).

Notes

¹In January 2008, a new seismic code was released in Italy [CS.LL.PP., 2008], which supersedes the seismic classification of the territory in zones considering the actual seismic hazard at the site to determine seismic actions on structures. Nevertheless, in other seismically prone European countries zone-classification is still enforced with similar anchoring values of the deisgn spectra [Garcia-Mayordomo et al., 2003].

This paper is a revisited and extended version of that originally presented by Iervolino et al. [2007] at the First US-Italy Bridge Workshop held at EUCENTRE (Pavia, Italy) in 2007.

²In the rest of the paper all calls and verbatim citations of Eurocode 8 will be simply indicated in italic.

³After this point the code also specifies that: The scaling factor derived from the previous step shall be applied to all individual seismic motion components.

⁴For example, Part 2, for the vertical component, has the same prescriptions of Part 1 for horizontal ground motion.

⁵This may change in the future, and probabilistic hazard analysis may be easily available in an increasing number of countries. For example, the recent Italian hazard mapping of the Istituto Nazionale di Geofisica e Vulcanologia provides hazard curves and disaggregation of seismic hazard in terms of magnitude, distance and epsilon; although disaggregation is only available for the PGA (see http://esse1.mi.ingv.it/).

⁶For example: 30% means that the average spectrum of a combination has to be above 1.3 times the code spectrum in the considered range of periods to be acceptable.

⁷For example: 100% means that the average spectrum of a combination has to be below 2 times the code spectrum in the considered range of periods to be acceptable.

⁸For example: 10% means that the average spectrum of a combination has to be above 0.9 times the code spectrum in the considered range of periods to be acceptable.

CS.LL.PP, DM 14 gennaio, [2008] “Norme Tecniche per le Costruzioni,” Gazzetta Ufficiale della Repubblica Italiana 29. (In Italian).