This paper is concerned with the equivalent static force method for the seismic design of selective storage racks with uplifting baseplates. It proposes a new procedure for determining the effective natural period of such racks in the cross-aisle direction for use in the NZS 1170.5 equivalent static method. The procedure is derived by comparing the base shear results of trialled Rayleigh methods against nonlinear time history analysis results involving 15 upright frame configurations, comprising 5 baseplate types for three, five and seven level racks. The time history analyses use a suite of 44 ground motion records. It is recommended that the effective natural period be computed in the Rayleigh method using nonlinear static analysis with the storage loads included, where the Rayleigh lateral loads create an overturning moment equal to the restoring moment of the storage loads. The proposed procedure leads to more efficient designs of storage racks with uplifting baseplates compared to the conventional procedure based on the use of linear analysis in the Rayleigh method for determining the natural period, but results in more conservative designs compared to the use of nonlinear time history analysis.
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