With our minds turned to efficiency and cost considerations, the increasing density of warehouse storage space brings its own set of challenges. To increase and maximize storage, warehouse owners and operators can choose to install tall pallet racks to accommodate more beam levels, throughout the racking system. However, this set-up should not be chosen without considering a significant factor affecting the stability of racking installations: the height-to-depth ratio. After all, a tall rack serves no purpose if it is going to topple over.
RMI (Rack Manufacturers Institute) outlines the height-to-depth ratio for a single row of pallet rack to be the ratio of the distance from the floor to the top surface of the highest load-supporting beam level divided by the depth of the frame. Section 8.1 of RMI’s ANSI MH16.1: Specification for the Design, Testing and Utilization of Industrial Steel Storage Racks advises evaluating the height-to-depth (HTD) ratio to determine the stability of a single row of standard steel storage rack.
As a rule of thumb, use this formula to avoid the instability of your racking system:
H = Height – measured from floor to the top surface of the highest-supporting beam level
D = Depth – measured along the floor, from the outside of the front column to the outside of the back column.
The height (H) of the racking system divided by the depth (D) of the frame should not exceed six (6) for optimum stability.
For instance, if a pallet rack is 240” high (height calculated from floor to top surface of the highest load-supporting beam level) is divided by 42″ (depth of a standard upright), the ratio is 5.714. It does not exceed 6, therefore it’s considered an acceptable ratio, within 6 to 1, needing only normal anchoring. If two systems are placed back-to-back and are not attached with a row spacer, their depths should be calculated individually as a single row.
If the same 42″ deep frame is 24′ (or 288”) tall, this would result in a ratio of (7), which exceeds (6 to 1) the allowable limit. In such a case, additional measures should be incorporated to further secure the rack and avoid possible overturning.
Adding anchors will stabilize the rack and diminish the risk of upright displacement. Ideally, the choice of anchors should be approved or certified by an engineer. Installing larger baseplates will also allow better load distribution to the ground, with more space to anchor the frame. Preferably, the anchors and baseplates should be designed to resist an overturning force of 350 pounds applied to the uppermost beam level.
Furthermore, adding cross-aisle or overhead ties to connect the topmost sections of the upright frames that span across an aisle, will lower the odds of overturning, which is recommended by ANSI MH16.1 if the height-to-depth ratio exceeds 8 to 1.
Another option is to add row spacers to connect rows of uprights and maintain even spacing between racks. Placing rows of racks in a back-to-back configuration and connecting them with row spacers will greatly increase stability.
This rule of thumb mentioned above is meant for standard rack frames arranged in a single row. It does not apply to racks with cantilevered uprights, or columns that are set back at their base. Whenever you are seeking to make any changes or additions to the arrangements of your storage pallets, or the racks that are storing and supporting them, you should consult an engineer knowledgeable of pallet racking systems. If you would like to book a warehouse inspection or simply speak with one of Damotech's rack safety experts, click here. Remember that space saving can sometimes compromise safety. Following this formula as an easy rule of thumb could prevent your rack from overturning and ensure your rack's stability.