Exemption from the application of Eurocode

Clauses 2.2.1 (4), Eurocode 8 itself states that its provisions need not be applied in cases of very low seismicity.

3.2.1 (5) As for cases of low seismicity, which combination of categories of structures, ground types and seismic zones in a country will qualify as cases of very low seismicity is left to the National Annex. However, it recommends (in a note) the same criterion as for the cases of low seismicity: either the value of the design ground acceleration on type A ground (i.e. on rock), aa, or the corresponding value, agS, over the ground type of the site. It goes on to recommend a value of 0.04g for ag, or of 0.05g for agS, as the threshold for the very low-seismicity cases. As the value of ag includes the importance factor 7,, certain structures in a region may be exempted from the application of Eurocode 8, while others (those housing essential or high-occupancy facilities) may not be. This is consistent with the notion that exemption from the application of Eurocode 8 is due to the inherent lateral force resistance of any structure designed for non-seismic loadings, neglecting any contribution from ductility and energy dissipation capacity. Given that Eurocode 8 considers that, due to overstrength, any structure is entitled to a behaviour factor, q, at least equal to 1.5, implicit in the value of 0.05g for a&S recommended for the threshold for very low-seismicity cases is an assumed inherent lateral force capacity of 0.05 x 2.5/1.5 = 0.083g. This is indeed a reasonable assumption.

If a National Annex states that the entire national territory is considered to be a case of very low seismicity, then Eurocode 8 (all six parts) will not apply at all in that country.

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