An advanced design procedure for the safe use of steel storage pallet racks in seismic zones

Industrial steel storage pallets racks represent one of the most economical solutions for storing goods and products when space is limited. This well-recognized convenience is however counter-balanced by a structural response that is generally complex to predict, especially under earthquakes. The design procedures adopted worldwide do not seem to take adequately into account for the key features associated with these structures. From the engineering point of view, racks are designed as moment-resisting steel frames but of an unconventional type because they are characterized by an extensive use of thin-walled cold-formed members. Furthermore, the overall dynamic response is often greatly affected by a non-negligible deterioration of the joint behavior due to large excursions in the plastic range, with the direct consequence that the load carrying capacity is reduced. In this paper, the well-established non-linear time-history (NLTH) method of analysis is combined with the low-cycle fatigue (LCF) damage approach in order to (1) investigate the damage distribution, (2) assess the residual fatigue life and (3) estimate the effective load-carrying capacity after an earthquake. In particular, key open problems related to the seismic design of racks are identified and the NLTH-LCF procedure is introduced and discussed. Finally, attention is focused on a practical case study related to a medium-rise doubly-entry pallet rack. Reference is made to two recent Italian earthquakes and two models have been adopted to reproduce the cyclic joint behavior of beam-to-column joints allowing for a direct appraisal of its influence on the overall rack response.