When placed in a pallet rack, the weight of uniformly distributed loads rests evenly over the full surface of the rack’s beams. This means that the stored or pallet load must cover the entire storage area evenly from side-to-side and front-to-back. Because a storage rack’s capacity rating depends on even weight dispersal, uniformly distributed loads are crucial to the structural integrity and safety of the system.
A point load, on the other hand, is one whose weight is concentrated on the rack’s beams or decks in one or more locations. Point load causes include load legs that rest on beams, pallet runners, fork spacers, or cradle supports.
“Placing a point load within a rack designed to support uniformly distributed loads could result in excessive bowing, bending, or deflection of the beams,” explained Andrew Bishop, P.E., a Research & Development Engineer at UNARCO Material Handling. The company is a member of the Rack Manufacturers Institute (RMI).
“Point loads can result in a more critical maximum bending moment and exceed the maximum allowable deflection of a beam. That could cause a beam to fail and the stored loads to fall,” he continued.
Loads of Equal Mass May Not Have Equal Weight Distribution
To ensure the safest rack design, therefore, it’s important to share details about the loads, product types, and pallets intended for use with the rack engineer. A pallet load of consistently and evenly cross-stacked cases will have a different weight distribution than a one holding coils of wire, noted Bishop.
“For that reason, different loads require different rack designs. They may also require different beam lengths, steel types, or thicknesses,” he continued. “Additionally, the beam connectors, upright strength, and bracing can affect the racking structure’s load bearing capacity.”
The type of pallet used beneath the loads is also an important factor in load distribution. Pallet construction varies widely, with models including two-way entry, four-way entry, skids, nine-block, or other styles. Further, some pallets have a foot in each corner and the middle. Others have stringers — or multiple boards — arranged across the bottom. The configuration of the bottom of the pallet can significantly affect the distribution of the load’s weight on the rack.
Calculation Tables for Uniformly Distributed Loads
When quoting a rack project, a manufacturer may refer to a table of calculations for uniformly distributed loads. This table presents common values that help engineers determine the reactions, bending moments, and deflections of different beam lengths bearing uniformly distributed loads.
These tables offer a close approximation to a real-life application, making them effective for a quick cost estimate. However, Bishop cautioned, there are some caveats with the use of these calculation tables.
“Uniformly distributed load tables assume that the load is spread evenly across the beam or shelf. If a rack engineer is unaware that point loads may be present, they may select a beam from the table that isn’t strong enough,” he said.
Additionally, most beam tables assume the front and back beams bear the load equally. However, depending on the placement accuracy of the forklift operator or intended loading of the system, that may not always be the case.
“Another common mistake is to use a uniformly distributed load table when the width of the storage bay is wider than necessary to accommodate the actual pallet widths,” Bishop added. “Two 40-inch-wide pallets in a 108-inch-wide bay results in a more critical load than a uniformly distributed load. That’s because the placement of the pallets can favor the center of the bay and aggravate stress and deflection.”
Rack Design Can Compensate for Multiple Load, Pallet Types
With details about the types of loads and pallets intended for the rack, the design engineer can create the optimal system to support them. Based on the information, the engineer will calculate the maximum amount of deflection the rack beam or decking can support without failing. For rack structures that may hold multiple pallet and load types, an engineer can design for the worst-case loading scenario for the safest system.
Section 9.3 of RMI’s ANSI MH16.1: Design, Testing, and Utilization of Industrial Steel Storage Racks details the maximum amount of beam deflection used in a rack design engineer’s calculations. At design working loads, beam design typically limits vertical deflections to not exceed L/180 of the horizontal beam length, measured end-to-end.
“Some users may request less permissible deflection so the beams look less bowed when loaded or because their racks integrate with automation, and precision is essential,” concluded Bishop.
Learn More About Safe Rack Design and Use
RMI offers a comprehensive list of Frequently Asked Questions on its website. Among the topics are sections on load beams, loading, design standards, and more. The association also publishes several standards and guidelines for the design, planning, use, repair and replacement of industrial steel storage rack.
