Due to changes in the new ANSIMH16.1 (RMI 2021 Specification) the frame tables traditionally used to select an upright frame for a pallet rack application are no longer valid.
The actual capacity will depend on more than the simple unbraced length of the column and will need to account for the following factors:
Average to maximum load ratio (Very Critical)
Beam-to-column connectors chosen (Very Critical)
Base plate and anchorage detail
Number of storage levels
Aspect ratio of the frame (height-to-depth ratio)
Warehouse or Retail environment (Importance Factor Determination)
Frame Tables to be Used as Only a Guideline
The new specification requires a computer analysis where the global stiffness of the rack assembly, considering the factors listed above, are evaluated. The calculated stiffness of the rack assembly is used to check the adequacy of the column that is selected. This means that a frame that worked previously from the tables under the 2012 code may no longer be strong enough unless a reduced average load is provided and/or stronger beam connectors are added. At first, this sounds like bad news, however:
If the end user can state with confidence that the average bay load is less than the maximum (which is almost always the case) and supply that data as well as the beam connectors being used at the lower shelf levels, the frame choice for a given application will most times be the same as it was under the pre-2021 requirements.
”How do I select a frame for an application?”
First step: Engage the end user in a conversation about the storage application and work to look at the load data to arrive at an average load per bay that may be less than the maximum. Remember that at any given time, there may be empty positions with zero load. For users who cannot obtain the average loads or insist that no reduced average is permitted to be used, the result will be both heavier beam connections and an increase in the column required compared to the RMI 2012 frame selection. This will almost always translate to a higher cost. If the end user can give a reduced average load, it must be a value that is based on information that they are confident with. The 2021 RMI does not allow the reduced average to be less than 67% of the max. Values of 75% - 90% are usually fairly easy to justify.
Next step: Look at the old tables for strictly a “ballpark” idea of the frame that will be needed, understanding that larger connectors may be needed at lower beam levels. Use of the larger connectors will generally add enough stiffness to the racks to return the column capacities close to their pre-2021 table values. If the project is in an elevated seismic zone or the frame quantities are high enough that the cost of an increase to the column size or the cost of heavier connectors would be difficult to accept, contact UNARCO for a preliminary design and frame selection.
Frame Tables on the following tabs should only be used as a guide to “ballpark” a frame’s capacity rating. In cases where a reduced average load is provided and more rigid connections are used at the lower levels, the capacities will be close to the values given in these tables. The tables are left on the website to provide a starting point for frame selection. For larger projects, projects in elevated seismic regions, or any projects where changes to column gauge or connection details could be costly, consult Unarco Engineering for the frame selection. Unarco will review all frame selections and designs for compliance to the new provisions in the 2021 RMI Specification.