Seismologists in Croatia and some other Mediterranean countries have been of the opinion from the beginning that the response spectra of Type 1 and Type 2 of the first generation EN 1998-1 (Eurocode 8 2004) are not entirely suitable for application (Fig. 1). Since this is a generally accepted conclusion in Europe, completely new spectra have been developed in the second-generation Eurocode EN 1998-1-1 (Eurocode 8 2022).

Here are some other reasons for this statement:

The dilemmas of the Technical Committee CEN/TC 250/SC 8 when the final text of the second generation of the EN 1998-1-1 standard (Eurocode 8 2022) is accepted are discussed here. If UHS spectra are used, in which all points of the spectrum are defined probabilistically, lower values of seismic action for tall buildings would be obtained compared with the elastic spectrum of Type 1 of the first generation. This statement is illustrated by comparing the UHS spectrum with the Type 1 and Type 2 spectra for the city of Zagreb, Croatia (Čaušević et al. 2023), which was hit by a strong earthquake in 2020 (Atalić et al. 2021) (Fig. 2). That earthquake was a test for buildings designed according to the spectra of the first generation. The good behaviour of these buildings was noticed, and the conclusion is that the seismic loading while designing buildings according to the spectra of the second generation should not be less than the seismic loading according to the spectra of the first generation. For this reason, EN 1998-1-1 (Eurocode 8 2022) prescribes two ways of defining spectra.

One principle is proposed for most EU countries: to adopt such spectra for application on the basis of which the seismic loading on buildings will be determined, which will not be less than the loading obtained by applying the spectra of the first generation that have been valid thus far.

Here are some reasons to apply the proposed principle:

There are 9 return periods according to the second generation of the Eurocode (Eurocode 8 2022, Čaušević and Bulić 2020), as shown in Table 1, and they are determined depending on the selected limit state (limit state, LS) and the consequence class (consequence class, CC). For example, for the limit state named the significance damage (SD) and CC2, the return period is 475 years (1), as well as in Eurocode 8 2004:

𝑇𝑟𝑒𝑓 = 𝑇𝑆𝐷,2 = 475 godina (1)

Seismic hazard maps should be prescribed by the National Committee of each EU country. The European Seismic Hazard Maps are derived from the deliverable ESHM20 of the SERA research project (Seismology and Earthquake Engineering Research Infrastructure Alliance for Europe), which received funding from the EU Horizon 2020 research and innovation program to provide updated information on seismic hazards in Europe (Danciu et al. 2021) (Fig. 3).

In the spectrum of the second generation, a compromise is adopted, and a classic deterministic spectrum is prescribed, in which several of its points are determined in a probabilistic way (Fig. 4). It has already been stated that the seismic forces for the design of buildings according to the second generation of the standard should not be less than the forces obtained according to the first generation standard.

The second-generation spectrum is defined deterministically, with several of its points determined in a probabilistic way (Fig. 4). The curve in Fig. 4 for "falls" too steeply. To meet the abovementioned principle that seismic forces should not be less than those obtained according to the first-generation standard (Eurocode 8 2004), the value should be increased in the manner prescribed in Article 5.2.1. (Eurocode 8 2022).

In Fig. 4, represents the ratio of with respect to the zero-period spectral acceleration ().

The provision of Art. 5.2.1 in EN 1998-1-1(Eurocode 8 2022) reads as follows:

(4) For the derivation of , a) or b) may be used:

(a) Map the territory in terms of and obtain by applying Formula (5.3):

is the value of for site category A and for the return period , where

for low and very low seismicity levels;

for moderate seismicity levels;

for high seismicity levels.

The values of may increase in the National Annex.

b) Obtain from a seismic hazard study of the territory under consideration and map it concurrently with .

In accordance with the second generation of Eurocode 8, nine earthquake return periods are introduced (there have been only 2), which means that as a rule, 18 maps should be made (9 return periods and each with two spectral values, and ), as well as at least one map, the one for . The number of these maps can therefore be reduced according to the needs of designers of buildings and allowed by the National Annex.

Therefore, civil engineers object to the acceleration value obtained for the probabilistically defined point of the spectrum for . If it remained like this, we would obtain much less seismic force for tall buildings than for the Type 1 elastic spectrum from the first-generation standard, which should be avoided. The second-generation standard EN 1998-1-1 (Eurocode 8 2022) also provides a way to avoid this.

From (5.3), it can be concluded that for high seismicity levels:

from Fig. 5 (2)

Having in mind that

where

- Topography amplification factor

- Intermediate period () site amplificationfactor

The values for can even increase when and are greater than 1.

If the hazard mapping takes the form of seismic zones, these zones should be delimited, and their seismicity levels should be qualified according to Table 2, where is the reference spectral acceleration calculated for a return period of 475 years.

If a different reference return period is used and is not available, values may be obtained by applying the following equation: