Clauses 4.2.2(1), EN 1998-1 recognizes that a certain number of structural elements which are not essential
4.2.2(3) parts of the seismic-resisting structural system of the building may be considered as'secondary seismic', as far as their role and contribution to earthquake resistance of the building is concerned. The main objective of this distinction is to allow for some simplification of the seismic design by not considering such elements in the structural model used for the seismic analysis of the building. Accordingly, only the remaining elements, which are termed 'primary seismic members', should be modelled in the structural analysis and designed and detailed for earthquake resistance in full accordance with the rules of Sections 5-9 in EN 1998-1.
The differentiation between primary and secondary elements is essentially equivalent to the traditional distinction in US seismic design codes for new buildings between members which belong to the lateral-force-resisting system and those that do not. The terminology of primary and secondary elements has also been adopted by the US prestandard for seismic retrofitting of existing buildings.45,46 In EN 1998-1, the term 'seismic' has been added to make it clear that the characterization applies only to the seismic action.
The building structure is taken in design to rely for its earthquake resistance only on its primary seismic elements. Cyclic degradation of the strength and/or stiffness of primary seismic elements is disregarded, provided that their dimensioning and detailing fully follows the rules and requirements given in Sections 5-9 of EN 1998-1 for elements designed for energy dissipation and ductility.
The strength and stiffness of secondary seismic elements against lateral loads is to be neglected in the analysis for the seismic action. However, their contribution in resisting other actions (mainly gravity loads) should be fully accounted for.
The contribution of all secondary seismic elements to the lateral stiffness should be not more than 15% of the lateral stiffness of the system of primary seismic elements. For this requirement to be met, the full structural system, consisting of both primary and secondary seismic elements, should develop lateral drifts less than 1.15 times those developed by the system of primary seismic elements alone. Drifts should be computed for the same system of horizontal forces, acting separately along the two main horizontal axes of the building and having the heightwise distribution of clause 18.104.22.168.3 (for the lateral force method of analysis), and should be compared at least at roof level, but preferably at all storeys.
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