For earthquake design purposes, KTA 2201.1 [37] divides components and building structures into three classes, as follows:

— Class I

Components and building structures that are required to fulfil the protective goals (control radioactivity, cool fuel elements and contain radioactive substances) and limiting radiation exposure (safety-related system components and building structures)

— Class Ila

Components and building structures that do not belong to Class I, but which, due to their own damage and the sequential effects, possibly caused by an earthquake, could detrimentally affect the safety-related functions of Class I components and building structures

— Class Ilb

All other components and building structures

The only components and building structures for which seismic safety is required are those in Classes I and IIa. Components and building structures of Class I must be verified in terms of load-carrying capacity, integrity and functional capability, i. e. deformation or crack widths in reinforced concrete must be limited in some cases. For components and building structures of Class IIa, generally verification of load-carrying capacity will be sufficient.

To verify earthquake safety, structural analyses are required reflecting the design basis earthquake and its possible consequences. Possible consequences could include the failure of high-energy containers, not designed to withstand earthquakes, such as feed water tanks in the turbine building of a PWR plant. Combined effects of earthquakes and other extraordinary actions are not generally taken into account as they are extremely rare.

For structural analysis purposes, earthquake effects are to be set as the ground response spectra for the reference earthquake or compatible recorded acceleration over time curves in each case, recording the simultaneous excitation in both horizontal and the vertical direction. The subsequent superposition of parallel stress variables can be taken either as the root of the sum of the quadratics or the superposition rules as in DIN 4149 [39] or DIN EN 1998 [40].

Structural modelling is subject to particular requirements, due to the dynamic effects and to the influence of the subsoil at the site in particular. Precise details of structural modelling, including details of structural damping and subsoil modelling can be found in KTA 2201.1 [37], KTA 2201.2 [41], KTA 2201.3 [42] and KTA 2201.4 [43].

In principle, the structural models to be used for the building structure, including the subsoil for the plant components with their support structures are those which record how the structures behave in the governing frequency range of an earthquake. Depending on the purpose of verification involved, it must be decided whether structural modelling requires a level beam model or a spatial beam model or even a spatial surface structure model, allowing for possible decouplings between the building structure as a whole and part structures or decoupling criteria between the building structure as a whole and components.

As far as the dynamic behaviour of the structure is concerned, the influence of the interaction between structure and subsoil (subsoil-structure interaction) must be taken
into account, varying the soil characteristics to give a lower, medium and upper subsoil strength. The results of the calculations at different subsoil strengths must then be included.

The structural analyses can be carried out using the usual dynamic calculation methods, including in particular the response spectrum method, frequency range method, time history method and the quasi-static method as a simplified method. These are generally used as linear methods. Non-linear methods such as non-linear time history methods are also used in exceptional cases.

The result of the dynamic structural analyses, as well as eigenfrequencies, is to give the internal forces and deformation variables required to assess the strength and deformation behaviour of the structure studied. Response spectra can also be calculated at the intersections with other building structures or components to use these to analyse the building structures or components meeting at these nodes. The resulting method to be used in conducting structural analyses of building structures and components with a view to using response spectra is therefore as follows (cf. Figure 5.5):

— Specify the site excitation as ground response spectra or time history (primary response/primary spectra)

— Calculate the response over time or response spectra of the structure (secondary response/secondary spectra)

— Calculate the response over time or response spectra for system components (tertiary response/spectra)

Ground response spectrum

Frequency

Tertiary spectrum

Fig. 5.5 Response spectrum method (building structure/components)

5.3.2 Floods