Rack Codes & StandardsRack Design & Installation

How Point Loads And Uniformly Distributed Loads Affect Safe Rack Design

When placed in industrial steel storage racks, a uniformly distributed load is one whose weight is evenly spread over the entire surface of the rack’s beams or deck. Conversely, a point load is one with its weight significantly concentrated in one or more places on the rack’s beams or decks. Placing a point load within a steel storage rack that has been designed solely to support uniformly distributed loads could cause excessive beam or deck deflection (bowing) and/or failure. Therefore, when it comes to rack safety, it is important to provide your professional rack design engineer with the full details about the types of products, pallets, and loads to be stored within the system.

That’s because two loads of equal mass may not be equal in their construction or weight distribution. A pallet containing several layers of evenly cross-stacked cases bears weight differently from one supporting an unusually shaped load of steel coils or rolls of paper. Further, there are a variety of pallet types: two-way entry, four-way entry, skids, slip-sheets, nine-block, and more. Some have multiple boards, or stringers, spanning the bottom surface; others have a foot in each corner. A pallet’s bottom construction greatly impacts the distribution of the load.

By knowing the details about the types of loads and pallets the storage rack will hold, the rack design engineer can create the optimal system to support them. Based on that information, the engineer will calculate the maximum amount of deflection the rack beam or decking can support without failing. Applications where multiple pallet and load types may be stored in the same structure should be engineered to worst-case loading as the safest approach.

The maximum permissible amount of beam deflection is included in the rack design engineer’s calculations as detailed in Section 5.3 of RMI’s ANSI MH16.1: Specification for the Design, Testing and Utilization of Industrial Steel Storage Racks. The deflection limit for selective rack equals the horizontal length of the beam divided by 180 (L/180). If a point load is placed on a beam that has been engineered to only support the weight of uniformly distributed loads, however, the risk of a failure is increased. The concentration could cause the beam to deflect beyond its maximum allowable amount, potentially causing the load to fall.

Similarly, placing a concentrated point load on rack decking that has been engineered to accommodate uniformly distributed loads could likewise cause a deck failure and falling load. Although decking provides additional support for a pallet load, as specified in RMI’s ANSI MH26.2: Design, Fabrication, Testing and Utilization of Welded Wire Rack Decking it must be engineered for the specific type of load and pallet to be placed upon it.

Need more information about the safe use of industrial steel storage rack? RMI publishes several specifications and guidelines for the design, planning, use, repair and replacement of industrial steel storage racking and welded wire rack decking.