Methods of restraining progressive collapse in rack structures

Using a five storey, five bay typical pallet rack it is shown that progressive collapse of the structure will often occur if the rack is loaded to its ultimate limit state and a single leg removed by impact. The results of dynamic analyses were that the speed of impact has less effect on the behaviour of the structure than the magnitude of the impacting force. The paper demonstrates that a non-linear pushover analysis is capable of predicting collapse. The results of a parametric study indicated that ultimate loads were largely independent of bay depths and only slightly dependent on bay width. The paper shows that under service design loads a cost-effective way of reducing the occurrence of collapse is by the insertion of cross-braced plan bracing inserted at the lowest loaded level. To ensure optimum results the pinned joints in the diagonal cross-bracing members and the column base-plate to ground connection must be able to absorb 1.5 mm plastic deformation along the direction of the member. It is also recommended that bolts in the column base-plate to ground connections should be placed in symmetric patterns about the column centroidal axis.