Floor Response Spectra Considering Influence of Higher Modes and Dissipative Behaviour

Seismic design forces of nonstructural components are commonly obtained by application of floor response spectra. This method is usually applied using estimated modal shapes and periods of the main structure; it allows for a separated design of components and their anchorages by the producers of equipment. Simplified formulas for determination of floor response spectra are provided by current codes such as Eurocode 8. All of them follow the assumption of the first fundamental elastic mode governing the acceleration values at the floors. These approaches do not take into account effects of higher modes, topology, ground response spectrum and plastification of supporting structures. Floor response spectra of four different building frames, one typical for an industrial 5-storey steel supporting structure and other three representing 5-, 10- and 15-storey regular steel buildings, were investigated using nonlinear incremental dynamic analyses. The results were compared to current code provisions revealing large discrepancies which have impact on safety as well as on economy of the design. Three aspects were identified and qualified: Application of ground response spectrum values instead of peak ground acceleration as basic input variable; Importance of higher modes; Impact of plastification of the main structure and the components. It could be shown that all three parameters have a significant influence on the acceleration values, on the dimensioning of the anchorages and on the ductility demand for components designed to dissipate energy.