Reconciling Eurocode 8 Part 1 and Part 2 Two-component Record Selection

ABSTRACT

Eurocode 8 (EC8) allows the use of dynamic analysis for the design and assessment of structures and provides some constraints for the selection of acceleration time-histories as seismic input. However, when it comes to the selection of two-horizontal-component natural ground motion records, parts 1 and 2 of EC8, stipulate apparently different criteria. The first aim of this paper is to build up on previous studies and investigate whether this difference in provisions translates into the selection of systematically different sets of records. A series of record selection case-studies presented in this study, corroborate the preliminary findings of previous work, and show that this is not the case, that is, record sets chosen according to one group of criteria tend to satisfy the other by default. A second aim is to investigate the different options for selecting multi-component ground motion records in part 1, which turn out to be equivalent. Finally, a third issue tackled is the effect that different definitions of spectral acceleration, that the design spectrum refers to, can have on spectrum-compatible record selection, when two horizontal components are involved. The results indicate that, for some of these alternative definitions, such as maximum or random component spectral acceleration, sets obtained via direct spectrum compatibility may not always agree with a simple application of Eurocode 8 provisions. On the other hand, when spectral acceleration is defined as the geometric mean of the two components, consolidated record selection algorithms appear to guarantee spectrum compatibility.

KEYWORDS:

• Seismic input
• bi-directional dynamic analysis
• spectrum
• design
• assessment

Acknowledgments

The work presented in this article was developed within the activities of the DPC-ReLUIS (Rete di Laboratori Universitari di Ingegneria Sismica) 2019-2021 research project, funded by the Department for Civil Protection (DPC) of the Italian Presidency of the Ministers’ Council.

Notes

1. Although seismic ground motion recorded at a single station exhibits six components, three translational and three rotational, the effect of rotational components is usually considered negligible (Kubo and Penzien 1979). The influence of the vertical component on structural response is a research topic in its own right and has been extensively discussed elsewhere (e.g., Chopra 1966; Munshi and Ghosh 1997; Papazoglou and Elnashai 1996; Vamvatsikos and Zeris 2010). However, considerations on the vertical component of ground acceleration are beyond the scope of this paper, which focuses on the two horizontal components.

2. While Part 1 of EC8 is mainly focused on buildings, it includes a series of more general provisions that also pertain to the other parts of EC8 (e.g., seismic action, performance objectives, analysis procedures, general rules). At the time of writing of the present article, a new version of EC8 is at draft stage; since a finalized version of the new EC8 is not yet available and the current EC8 is not expected to be formally superseded by the new version for a few more years, the authors chose to refrain from making explicit references to the Eurocode revision underway, apart from this note.

3. The limit states CLS, ULS, DLS considered correspond to the ones that are defined in the Italian Code (CS.LL.PP 2018) and are labelled SLC, SLV and SLD, respectively. At ULS the structure has to retain its structural integrity and a residual load bearing capacity while at DLS the performance objective is that of limiting the occurrence of damage that could provoke high repair costs and/or interruption of use. In this sense, if one were to follow the nomenclature used by the American standard for assessment of existing buildings (ASCE 2017), these would correspond to collapse prevention, life safety and damage control limit states, respectively.

4. Using the EC8 Type 1 spectrum for all cases is an intentional simplification, in the sense that disaggregation of seismic hazard for DLS at Milan hints at lower magnitudes, for which the Type 2 spectral shape could have been considered.

5. Moment magnitude.

6. In this case, this search is limited to permutations of the two orthogonal components, as they would ostensibly be found in a database. Other researchers have taken this a step further, by also rotating the components until a match could be found (Grant et al. 2008).

Additional information

Funding

This work was supported by ReLUIS [ReLUIS-DPC 2019-2021 research program].