Combination for global effects

Eurocode 8 reduces the value of variable actions to be combined with the design seismic action beyond the level of a single member ('global' effects, such as the overall seismic shear or overturning moment in a storey, etc.) below that used locally for the verification of members and sections. This is to take into account the reduced likelihood that the live loads ijj2 ¡Qkj may not be present over the entire structure during the design earthquake. The reduction is effected in the calculation of masses, as these affect the inertia forces. This reduction of live loads may also account for a reduced participation of masses in the motion due to possibly non-rigid connection to the structure (in other words, some masses may not vibrate in full phase with their support, or at full amplitude).

The reduction factor to be applied on live loads i¡)x ¡QK¡ for buildings is defined in Section 4 of EN 1998-1 as an NDP. The recommended value is 0.5 for all storeys - other than the roof-used for residential or office purposes or for public gathering (except shopping areas) which are considered as independently occupied, or 0.8 in those storeys of the above uses which may be considered to have correlated occupancies. No reduction in live loads is recommended for any other use or on roofs.

If the same value of ip2,; applies to all storeys, the facility of reducing live loads in some storeys below the value ^.¡Qk,/to be used for the verification of members and sections is inconvenient for the design, if masses are determined from the results of the analysis for live loads. There are two ways to implement this facility:

• The masses will be assigned without an analysis for live loads, which is convenient if masses are lumped at the mass centre of rigid diaphragms along with their rotational mass moment of inertia, but inconvenient if masses are to be assigned to nodes in proportion to their tributary area, to automatically account for rotational mass moment of inertia or when the diaphragm is not considered as rigid.

• The masses will be assigned to nodes on the basis of separate analyses for live loads on groups of storeys with different reductions of live loads; this option may be unavoidable if different ip2 i values are used in different storeys, but depends on the options available in the analysis program used.

Given that at the storey level the resultant of the nominal value of live loads, Qk, normally does not exceed 25-30% that of permanent loads, Gk, and that percentage is multiplied further by the value of the tp2 i factor (0.3 usually, 0.6 or 0.8 rarely), the designer may have to consider whether the overall economy effected by the further reduction of live loads in some storeys is worth the additional design effort.



4.5. Methods of analysis

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