How Height-To-Depth Ratio Contributes To Overall Storage Rack Stability
A key objective of safe rack design is to ensure the finished structure won’t topple over when subjected to horizontal forces such as an earthquake, forklift impact, or high wind. To significantly minimize this risk, 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.
The HTD ratio describes the relationship between how tall the rack is compared to how wide it is at its base. For example, a rack that is 10 feet tall and 10 feet wide (a 1 to 1 HTD Ratio) will be much more stable (less likely to fall over) than a rack that is 10 feet tall and 1 foot wide (a 10 to 1 HTD ratio). To calculate the HTD ratio, the measured height of the pallet rack is divided by the measured depth of the frame. The height should be measured from the floor to the top surface of the highest load-supporting beam level; the depth should be measured at floor level, from the outside of the front column to the outside of the back column.
If the calculated HTD ratio is determined to be 6 to 1 or less, securing the rack base plates to the floor with normal anchoring will provide sufficient stability. Conversely, should the HTD ratio exceed 6 to 1, additional safety measures must be incorporated. Specifically, the anchors and the base plates should be designed to resist an overturning force of 350 pounds applied to the uppermost beam level.
When a rack’s HTD ratio is determined to be greater than 8 to 1, ANSI MH16.1 recommends utilizing overhead (or cross-aisle) ties that stabilize the racks as an additional safeguard. These ties extend across the aisle to connect two frames together at the top for greater support and to minimize the risk of overturning. Additionally, when overhead ties are needed, the frame heights are often further extended in order to avoid being hit by a load during placement or removal from the top pallet position of the rack. The design of any anchoring used for racks this high must be certified by an engineer.
The HTD ratio specifications apply to both roll-formed and structural rack configured in a standard, single-row (not back-to-back). Racks arranged back-to-back require the proper type and quantity of row spacers to secure the two frames together. If unsure, contact an engineer.
Finally, rack systems designed with sloping or offset legs are subject to different engineering calculations and analysis. Slope leg or offset leg frames should not be used in a single row application without an engineer certifying the design.
Looking for more insight into rack specifications? Download a copy of ANSI MH16.1: Specification for the Design, Testing and Utilization of Industrial Steel Storage Racks.