A Rule of Thumb: Rack Height to Depth Ratio
When designing pallet racks, people tend to pay attention to the easy parts. Beam capacity is straight math; the capacity of a pair of beams, so long as the load is evenly distributed, the load properly positioned, and the safety clips are installed, is a reliable number. Frame capacities are a more complex, as they rely on vertical spacing between levels. Get these factors right, and typically it doesn’t take an engineer to design a safe and reliable rack system.
But there is another factor that many don’t understand and often overlook — the Height-to-Depth ratio.
The RMI (Rack Manufacturers’ Institute) defines the height to depth ratio for a rack row as “the ratio of the distance from the floor to the top beam level divided by the depth of the frame.” This ratio can’t exceed 6-to-1. For a standard 20′ tall, 42″ deep frame with the beam at the top level, the ratio is 5.714 — an acceptable ratio with normal anchoring. If that same 42″ deep frame was 24′ tall, it would have a dangerous 6.857 ratio. Steps such as special anchors and base plates or overhead rack ties may be necessary. Typically at this ratio, professional rack engineers are needed to ensure the rack is safe.
Note that this ratio is designed for a single rack row. Double rows with frame ties may be calculated differently.
If you are buying racks from a catalog, most all of the commercially offered ones are going to be within the 6:1 ratio.
What force overturns a rack? According to RMI & ANSI specifications, anchorages should be able to resist an “overturning force” of 350 pounds applied against the topmost beam level of an unloaded rack (a lift truck striking a rack as it’s loading or unloading pallets certainly qualifies). If the LRFD method of design is used, this force should be treated as a live load and multiplied by 1.6.
Anchoring: You should consult your rack provider/manufacturer for information on what is considered a normal anchor. It’s typically included with installation manuals/drawings for new racks, and those are available online for many rack brands as well. This is usually a 1/2″ diameter anchor with a proper depth, assuming a non-seismic application.
Nonstandard applications: Consult a qualified rack engineer for base plate, ratio, and anchoring standards for seismic zone applications. Another example is outdoor installations, which must be adjusted for wind force. In particular if the load is large and solid, so that it catches wind. This can cause significant stress on the rack, and potential overturns. Also, this ratio can vary, depending on the installation surface, load type, spacing, and even the rack manufacturer. Always consult with your manufacturer to determine a safe operating ratio for your particular rack system.
This guide should be used for informational purposes only. Consult a professional for exceptionally tall or narrow racks. For more specification information, see the RMI page of the Material Handling Industry website.
Scott Stone Cisco-Eagle's Director of Marketing. He has over 25 years of experience in the industry.