When we discuss racks with a large or high aspect ratio, we are referring to pallet rack upright frames that are tall compared to their depth. This is otherwise known as the height-to-depth ratio or the H/D ratio and has specific requirements covered in the RMI specification for allowable tolerance.
For many applications where the aspect ratio (H/D) is not very high, it may be fine to supply the industry-standard, 1/8” shims, for the installer to use to achieve the acceptable cross-aisle plumb. The RMI limit for cross-aisle out-of-plumb is ½” for every 10 feet of frame height. Installers should attempt to make the rack as plumb as possible in the cross-aisle direction, preferably ¼” in 10 feet or better.
The taller the upright frames, the harder it is to maintain cross-aisle plumbness of the rack. If the aspect ratio of the frame is 8 to 1 (example: 336” tall frame that is 42” deep), an 1/8” shim inserted under one column will move the frame 1” at the top in the cross-aisle direction. For frames with this ratio, thinner shims should be supplied to better allow the installer to “fine-tune” the cross-aisle plumb. A 1/32” shim (22 ga.) would allow for more precision because this would move the top of the frame only ¼” when inserted under the baseplate. A good option on the taller applications may be to supply (1) 1/16” and (1) 1/32” shim per frame rather than the (1) 1/8” shim so commonly specified in the pallet rack industry.
A rack system that varies from bay to bay or aisle to aisle out of tolerance, can cause bigger problems with every bay addition. Bay sizes may grow and beams can be harder to install or uninstall in the future. For this reason, it is a good idea to supply an assortment of shims of different thickness to help the accuracy of the installation.
Additional concerns can arise for applications where the floor is not level and more shims and thicker shims may be required. We cannot over-emphasize the importance of the rack being plumb in all directions. While this article only addresses the issue of thinner shims being a better option for “fine tuning” the cross-aisle plumb, unlevel floors may have to start with thicker shims and then be fine-tuned with thinner shims to achieve plumbness.
No two pallet rack installations are the same because every site and slab each bring their own set of issues for plumbing the rack and making sure shelves are level. It is crucial to assess and address these building site conditions during the initial installation process and regularly inspect and maintain the racks afterward. Working with experienced professionals who adhere to industry standards and guidelines can help mitigate these long-term potential issues.