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Title: Restraining Progressive Collapse of Pallet Rack Structures
Author: Ying, Adeline Ng Ling
Awarding Body: Oxford Brookes University
Current Institution: Oxford Brookes University
Date of Award: 2008
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Abstract:
Steel rack structures have been widely used in the warehouse industry. They are an effective means of storing goods - strong, light and flexible. A few major and minor collapses of these structures have been reported but a literature review showed that most of the previous work has concentrated on the static behaviour of the structure. Therefore, it is the aim of this research to study the collapse behaviour of rack structures and to propose a solution that is simple and cost effective. Steel rack structures are different from conventional structures. They use perforated columns and the beams are hooked to the columns using end-plates with pressed-out lugs. This end-plate is welded at the beam-ends. A simple pull-out test was conducted as the pull-out strength of such connectors is not readily available but was thought to be of importance to the collapse behaviour of rack structures. Four samples were tested and the characteristic mean strength of the connector was calculated. The pull-out strength of a four hooked connector was 27kN. Analysis conducted on the model frame showed that this limit would not be reached at collapse. Hence, the beams would not pull-out and failure would be dominated by other mechanisms. LUSAS finite element software was used to build a numerical model which had 5 bays and 5 lifts; the front and rear frames laced by D-bracing. This type of bracing is unsymmetrical and this configuration was found to influence the collapse behaviour of rack structures. Buckling analyses showed that the presence of cross-aisle side loads caused the frame to buckle in shear where the front and rear frame swayed in the opposite directions. Besides, the buckling load factor of the structure was reduced by 10% when 1% cross-aisle side loads were applied. The analysis also showed that the removal of a column at the bottom of the structure was more critical than at any other level. The buckling capacity of the structure was reduced by 60% when an external bottom column was removed and 40% when an internal bottom column was removed. Dynamic collapse analyses established that the collapse of a rack structure was independent of the momentum applied. Pushover analysis was found to be sufficient to assess the performance of the structure subjected to a lateral force. The analyses were conducted first with the assumption that the frame was fully ductile. They were then repeated with the assumption that the frame was brittle. Results from the analyses show that the two frames behaved differently and had different collapse mechanisms. When a lateral force acted on a ductile frame, the bracing system started to fail followed by the formation of a plastic hinge at the impact location. A mechanism was formed and the structure collapsed. However, when a lateral force acted on a brittle frame, the bracing members pulled-out of the structure and increased the unrestrained length of the column. The column buckled and the structure collapsed. The pushover force of a brittle frame was only 50% of the corresponding force of a ductile frame with the same symmetry and loading. Analyses of a brittle frame also showed that the strength of the baseplate had a significant effect on the pushover cap'acity of rack structures. The current baseplate is weaker in the downaisle direction because it has to resist a moment as well as a shear force. Thus, it is concluded that baseplates should be designed so that the centroid of the bolt group coincides with the centroid of the column. Adding plan cross-bracing to the rack structures was found to be a simple and effective way to increase the pushover force of the structure against collapse, especially when the impact load acted in the cross-aisle direction. For a ductile structure, it is more effective to place plan cross-bracing at the first level of the structure. It would increase the pushover capacity of the structure by 18%. For a brittle structure, it is more effective to place the plan bracing at the top of the structure. It would increase the pushover capacity of the structure by 25%. The plan bracing must be placed on all bays on a particular level as the direction and location of the impact load is unpredictable. This study recognized the importance of structures to have sufficient ductility. Analyses were conducted on a few examples of structure with limited ductility. Plan bracing was placed at different levels of the structure and it was concluded that adding plan bracing at the first level is more effective than at the top of the structure. Only Imm ductility of the bracing joint connections is required for the structure to behave like a 'fully ductile frame whereas a 1.5mm ductility is required if plan bracing was placed at the top of the structure. Parametric studies showed that this method work for all sizes of frames and is more essential for deeper frames.
Supervisor: Not available Sponsor: Not available
Qualification Name: Oxford Brookes University, 2008 Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.490505  DOI: Not available
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