Load Capacity Requirements in the new RMI Storage Rack Standard ANSI MH16.1-2021
The revised American National Standard for storage rack, ANSI MH16-1-2021, published by the Rack Manufacturers Institute (RMI), includes new requirements for calculating the load capacity for a pallet rack application. The previous standard, published in 2012, calculated capacities using only the column unbraced length. Guidance in the 2021 revision uses several additional factors that affect stiffness of the rack structure to calculate capacity.
The capacity of the frame will depend on nine factors beyond just the length of the unbraced column span, including:
- Average load to maximum load ratio;
- Beam-to-column connector stiffness and strength;
- Beam stiffness;
- Column stiffness;
- Base plate and anchorage detail;
- Site seismicity;
- Number of storage levels;
- Aspect ratio of the frame, or height-to-depth ratio; and
- Warehouse or retail environment.
In order to determine if the global stiffness of the rack assembly meets the new requirements, the designer will need to perform several calculations. The stiffness calculation will determine the adequacy of the selected column. A frame capacity calculated in accordance with the 2021 standard may be less than the capacity calculated using the 2012 standard, all things being equal.
However, if the average load data at the facility is available, and connectors with adequate stiffness are selected, then the calculated frame capacity may be close to the capacity calculated under the 2012 requirements. In such cases, the average load data provided will become part of the design assumption and must appear on the load application and rack configuration (LARC) drawings and the load plaques.
While this makes frame selection a bit more complex, there are also advantages. One such advantage can come from identification of an average product load per bay, as opposed to a maximum product loading. Many storage racks utilize an average loading that is less the maximum loading is in all storage locations. This may be due to empty positions with no load, for instance, or just many pallets with lighter product. Failure to identify an average product load will likely require use of stiffer connections or an increase in the column strength to comply with the 2021 revision requirements compared to the 2012 requirements.
Designers would previously use frame tables that complied with the 2012 standard to determine frame capacity. These tables are no longer valid under the 2021 standard, but they can still serve as a starting point in determining the correct frame for the rack application. With a reasonable average load assumption and stiffer connectors at lower beam levels, in most cases the rack design will be similar to capacities calculated under 2012 standard. In some cases, no design changes will be needed.
The consideration of all of the above factors makes working with the rack designer even more important than it was before. With a thorough understanding of your rack application, a rack designer will be able to select the most economical frame that still complies with the 2021 standard.