In buildings the vertical component of the seismic action may in general be neglected, because:
8 its effects are normally covered by the design for the persistent and transient design situation, which involves the permanent actions (dead loads) and the imposed ones (live loads), both multiplied by partial factors for actions, which are normally significantly greater than 1.0
• except when a building has beams with long span and significant mass along the span, the fundamental period of vibration in the vertical direction is controlled by the axial stiffness of vertical members and is short, therefore spectral amplification of the vertical ground motion is small.
Eurocode 8 requires taking into account the vertical component of the seismic action only when its effects are likely to be significant, in view of the two arguments above against this likelihood. This is considered to be the case when both of the following conditions are met:
(1) The design peak vertical acceleration of the ground, avg, exceeds 0.25g.
(2) The building or the structural member falls in one of the following categories:
(a) the building is base-isolated
(b) the structural member being designed is (nearly) horizontal (i.e. a beam, a girder or a slab) and spans at least 20 m or cantilevers over more than 5 m or consists of prestressed concrete or supports one or more columns at intermediate points along its span.
In the cases listed in condition 2(b), the dynamic response to the vertical component is often of local nature, e.g. it involves the horizontal elements for which the vertical component needs to be taken into account, as well as their immediately adjacent or supporting elements, but not the structure as a whole. For this reason, Eurocode 8 permits analysis on a partial structural model that captures the important aspects of the response in the vertical direction without irrelevant and unimportant influences that confuse and obscure the important results. The partial model will include fully the elements on which the vertical component is considered to act (those listed above) and their directly associated supporting elements or substructures, while all other adjacent elements (e.g. adjacent spans) may be included only with their stiffness.
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