Some limitations of modal analysis in seismic design
In the normal-mode, response-spectrum approach to earthquake resistant design of multistorey buildings the extended elastic seismic design loads are frequently calculated as the square root of the sum of the squares of the modal responses. The individual member forces are then determined using these seismic design loads. Previous research workers have examined the limitations of this technique and it is accepted as being generally applicable in practical design procedures.
Recent computer analyses of projected New Zealand high-rise buildings have illustrated two conditions in which the “square root of the sum of the modal responses squared” rule is inapplicable.
In this note these situations are described and suggestions are made of an alternative approach which may be adopted when deriving design loads in such cases.
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Copyright (c) 1969 R. Shepherd
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