Seismic behavior of palletized merchandise in steel storage racks
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During the past few decades, the number of large public warehouse stores, also known as big-box stores, has grown significantly across the United States and Europe. Large steel storage racks are used in such facilities in order to effectively take advantage of the available space and put on display, for the shopping public, the maximum possible amount of goods. Occupant safety in these facilities during earthquake events depends both on the structural performance of the building and on the performance of the storage racks, while the performance of the racks is determined by the performance of the rack frames and the response of the stored goods. Earthquake shaking may cause pallet contents to spill or topple off racks and rack frames to collapse or overturn. Both occurrences pose a significant life-safety hazard to the exposed shopping public and, therefore, the mitigation of this hazard should be a part of the objectives of any seismic design procedure. This thesis focuses on the behavior of contents stored on steel storage racks and proposes measures for the reduction of hazards associated with content shedding. The effects, on the content response, of (i) several coefficients of friction at the interface between merchandise and rack shelves and (ii) different loading conditions of the rack, are investigated, while the concept of inclined shelves as a measure for reduction of the probability of content overturning is introduced. The concept of inclined shelves refers to back-to-back or back-to-wall rack configurations with their shelves slightly tilted towards the inside or towards the wall, respectively, and represents the anticipated tendency of the merchandise to move inwards, when in motion, due to the contribution of the weight component parallel to the inclined shelf. An analytical model describing the dynamic behavior of pallet contents on rack shelves under earthquake excitation is developed based on rigid body dynamics, Coulomb friction and Newtonian impact laws. This model is implemented in a computer code and parametric incremental dynamic analysis (IDA) studies are conducted examining the effects of coefficients of friction, loading conditions and shelf inclination on the merchandise shedding fragility. A two stage experimental study consisting of pull and shake table tests is conducted to verify the response trends identified by the parametric studies and to provide information on the calibration of the analytical model and on the validity of the assumptions made during its development. During the pull tests, loaded pallets of different weights are pulled over a rack shelf, while during the shake table tests, high intensity ground motions are imposed on the base of a single level - single bay back-to-back rack specimen. Taking advantage of the large amount of experimental data, the analytical model is extended to better capture the observed behavior and a comparison of numerically computed and experimentally measured response is performed for all tests. Finally, two parametric IDA studies are conducted using the updated analytical model. The first study investigates the effects of the inclined shelving on the content shedding fragility for the configuration used during dynamic testing, while the second study investigates the effects of the vertical component of the base excitation on the response by introducing vertical motion components in the context of incremental dynamic analysis.