When installing pallet rack, the columns — or uprights — must be plumb (not leaning) and straight (not bowed). Plumbness refers to how close the frame is to being perfectly vertical once installed. Bowed indicates a bend or curve in the middle of the column. If one or more columns are leaning (out-of-plumb) or bowed (out-of-straight) beyond a certain point, their maximum load capacity decreases. That creates a potentially unsafe condition, increasing the risk of a rack collapse.

Other Challenges Caused by Leaning and Bowed Columns

Additionally, leaning rack columns can be difficult to load, noted Jonathan Hirst, P.E., Vice President and General Manager of North American Storage. The company is a member of the Rack Manufacturer’s Institute (RMI). He emphasized the importance of an aisle’s clearance space being the same at the top and the bottom of a rack face to enable safe loading and unloading by forklifts or automation.

“Consider a forklift operator lifting a load to a storage position 40 feet up. They first position the load six inches away from the face of the rack, then lift. If the rack is leaning into the aisle, that six-inch gap shrinks as the load rises. That increases the likelihood of the load hitting a beam as it ascends,” he said.

“Operations that deploy automated shuttles or cranes within their rack structure — or use automatic guided vehicles (AGVs) and autonomous mobile robots (AMRs) to interface with the rack — need even tighter tolerances,” Hirst continued. “The software and programming that directs their navigation needs very precise targets for positioning within the rack.”

Further, the human eye is remarkably adept at identifying leaning or bowed columns and beams, Hirst added. “If rack components appear misaligned or crooked, associates can see it. Even if the discrepancy is within the rack design allowances, people working among the rack will likely feel uncomfortable. That can lead to problems with a workforce,” he noted.

Incorporating Notional Loads in Design Calculations

When a column is out-of-plumb, even by a small amount, the pallet weight not only imparts a vertical load on the column, but also a side load. There are often small variances that in the structure or its environment. These include dips or swales in concrete floors, or minor imperfections in the columns’ fabrication or installation.

To compensate, rack engineers use notional load scores in their system design calculations, Hirst explained. “Notional load calculations allow for a limited amount of out-of-plumb or out-of-straight tolerance in a rack design. That accounts for real-world conditions.”

RMI’s ANSI MH16.1: Design, Testing and Utilization of Industrial Steel Storage Racks standard specifies limits for both out-of-plumb and out-of-straight columns. It sets forth a ratio of 1/240 — one inch over each 240 inches (20 feet) of height — as the maximum limit for both conditions. Columns leaning or bowing in excess of this limit should be unloaded and straightened. Since this condition could result from rack damage, the rack owner should repair or replace any compromised components immediately.

Exceptions to Out-of-Plumb and Out-of-Straight Limits

“It is important to note that the 1/240 rule is not always applicable,” continued Hirst. “The International Code Council’s International Building Code (IBC) allows for out-of-plumb rack system installations — as long as the rack engineer designed the frames for the anticipated plumbness.”

An example, he said would be installing a rack on a large concrete slab constructed at a uniform thickness over a plot graded with an unspecified slope.

“Imagine building a multi-level, box storage structure built on this sloped floor. To level the rack, it would require thousands of shims under the columns’ base plates,” he said, noting that would require additional time and labor during the installation. “Instead, it might make more sense to engineer the system to match the floor slope.”

Learn More About Notional Loads and Rack Columns

RMI’s latest edition of ANSI MH16.1, published in 2023, includes a more comprehensive and sophisticated method for analyzing storage rack stability in seismic areas. Called the direct analysis method (DAM), the new calculation methodology includes the application of notional loads. This depicts a more accurate representation of how the rack will respond to seismic forces. Read more here.