There is an important factor in pallet rack specification that is often misunderstood and overlooked: the height-to-depth ratio. While many of the measurements taken into consideration in pallet rack design are simple, straightforward math (such as beam capacity), the height to depth ratio is a little more complex – and it’s something that many non-engineers or non-rack designers either miss, or perhaps even dismiss altogether because they don’t understand the significance. But get this number wrong (or ignore it), and you risk your rack overturning.
Height-to-Depth Ratio Defined
ANSI MH16.1, under section 8 for Special Rack Design Provisions, 8.1 “Overturning” defines the height-to-depth ratio for a single rack row as:
The height-to-depth ratio of a storage rack shall not exceed 6 to 1 measured to the top loaded shelf level, unless the rack is anchored or braced externally to resist all forces. The height is measured from the floor to the top loaded shelf level and the depth from face to face of the upright column.
We place the emphasis on the height measurement because this seems to the part about the height-to-depth ratio that many get wrong.
(NOTE: This ratio is designed for a single rack row. Double rows with frame ties may be calculated differently.)
Example of Height-to-Depth Ratio
As an example, let’s say the the top loaded shelf level of your of your rack is 17 feet (204 inches), and the depth is 42 inches.
204 ÷ 42 = 4.86
Now, you know your height-to-depth ratio is 4.86 to 1. Since that is less than 6 to 1, you should be good to go with normal anchoring.
But let’s say your rack (top loaded shelf level) is 24 feet tall (288 inches) and your width is 42 inches. Here’s the formula:
288 ÷ 42 = 6.85
In this example, your ratio is 6.85 to 1, which is over the 6 to 1 limit. Now what? If the height to depth ratio exceeds 6 to 1, that’s considered dangerous, so now the anchors and the base plates should be designed to resist overturning. This means that the anchorage must resist an overturning force of 350 pounds applied at the topmost shelf level (to an empty rack). If the LRFD method (Load and Resistance Factor Design) of design is used, this force should be treated as a live load and multiplied by 1.6.
If your height-to-depth ratio is over 6 to 1, you should have a rack engineer review the design and specify the appropriate anchors and baseplates to prevent tipping.
That’s not the end of the height-to-depth ratio story
If the height to depth ratio exceeds 8 to 1, the racks should be stabilized using overhead ties. Other ways to deal with ratios such as 8 to 1 and 10 to 1 can include other alternatives such as as bigger baseplates, more anchors, or a combination of ties (examples: cross-aisle, ties to building structure plus base plate and anchors).
If anchoring is used in this case, the design of the anchors must be certified by an engineer. Keep in mind that all these ratios and requirements are for a typical rack frame. ANSI MH16.1. 8.1 states:
Unless it can be shown to be unnecessary because of such factors as soil, slab and frame stiffness, single rows of rack exceeding a height-to-depth ratio of 8 to 1 must be tied externally to the building or cross-aisle to another rack. Stabilizing a single rack with a height-to-depth ratio over 8 to 1 with anchoring alone is not recommended, unless designed and certified by an engineer.
These guidelines are for informational purposes only; you should ALWAYS consult with your rack design professional.
Next Level can provide professional design and installation services for your pallet racks and storage systems. We’ll ensure that your rack system is not only efficient for product flow, but safe, as well. Learn more about our warehousing services here.