Keywords: pallet rack, progressive collapse, semi-rigid connections, three-dimensional analyses, cold-formed steel, storage structures.

A standard pallet racking system is made up of columns, beams, bracing members and connections. These lightweight, cold-formed steel structures normally fail due to elastic instability with plasticity occurring after the maximum loads have been achieved. Research has concentrated on the static performance of the structures [1] and limited studies have been conducted to investigate the dynamic collapse behaviour of these structures. The first research reported was undertaken by McConnel et al. [2,3]. They recommended that to prevent progressive collapse racks should be designed so that the connections between the beams and uprights should have low pull-out strengths. This is not feasible as the longitudinal stability of the structure depends on the rotational stiffness of the beam-column joints; low pull-out strength leads to a low stiffness which would cause premature elastic failure.

The current study has developed a numerical model which predicts the types of collapse which can occur and which produces cost effective methods of reducing their occurrence. Previous research by the authors concentrated on investigating the buckling load of the model [4] and in particular the effects of removing legs. It was shown that the buckling load of a rack could be reduced by 50% by removing an end (external) leg below the first pallet level. As a first stage the improvement in buckling performance by inserting cross-braced plan bracing at the bottom level, top level and all levels, was investigated. This showed that if buckling was the sole criteria that the buckling loads of the model frame were improved by 11% if plan bracing was introduced at any level. The reduction in buckling load was only 12% when an external leg was removed and 14% when an internal leg was removed. As impacts are not static, dynamic analyses were undertaken. The dynamics of removing a leg were modelled. It was found that the maximum impulsive force that could be carried by the frame was the same as that found from a static push-over analysis; impacts below the maximum static pushover force simply caused the structure to oscillate without failing.

In the push-over analyses the vertical load was kept at the design load and horizontal loads corresponding to impacts increased until collapse occurred. Adding plan bracing at the lowest level improved performance by impacts parallel to the down-aisle direction by 5% and in the cross-aisle direction by 14% for frames where full-cross-aisle bracing used at the lowest level assuming that the joints had at least 1.5mm ductility against failure. Frames with ductility below this limit failed as soon as the joints reached their maximum loads and the redistribution of internal forces caused cross-aisle frames to fail prematurely with maximum loads approximately 35% of the static failure loads. This result was verified for a limited parametric analysis varying bay width/bay depths.

1

A.T. Sarawit, "Cold-Formed Steel Frame and Column Design", PhD Thesis Cornell University, USA, 2003.

2

R.E. McConnel, S.J. Kelly, "Structural aspects of progressive collapse of warehouse racking", in "Proceedings of the International Speciality Conference on Space Structures", Guildford, 630-635, 1984.

3

K.M. Bajoria, R.E. McConnel, "Progressive Collapse of Warehouse Racking", The Structural Engineer, 61(11), 343-347, 1984.

4

A.L.Y. Ng, R.G. Beale, M.H.R. Godley, "Dynamic Collapse of Rack Structures", in "Proceedings of the Eighth International Conference on Computations Structures Technology", B.H.V. Topping, G. Montero and R. Montenegro, (Editors), Civil-Comp Press, Stirlingshire, United Kingdom, paper 121, 2006. doi:10.4203/ccp.83.121

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