The existence of a perfectly flat, level concrete warehouse floor is something of a unicorn. That is, they’re extremely rare (some might even say they don’t exist). Yet, the columns and beams of an industrial steel storage rack system must be straight and plumb to properly transfer the forces of their loads to the concrete floor. Low spots and uneven surfaces in the concrete make that exceptionally difficult. Therefore, to compensate, installers add shims underneath the column base plates.
“Not just any shim will do, however. There are specific guidelines for the size, material composition, and installation of shims used under base plates,” explained 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).
“In some instances, installers use one shim in an isolated application to offset the effect of a low spot in the concrete slab,” he said. “When this happens, the shim allows this column to maintain alignment with the rest of the row of rack frames.”
For integrated, deep lane storage systems, the highest spot in the floor defines the fixed starting point against which all other shims and base plates are set. “This applies to drive-in, pushback, and pallet flow rack. It also affects mezzanines, pick modules, and elevated work platforms,” continued Hirst. “The installer uses the high spot within the system as a datum, then shims all the base plates in the system to that point.”
RMI’s ANSI MH16.1: Specification for the Design, Testing and Utilization of Industrial Steel Storage Racks covers proper shimming of rack base plates in detail. Here, Hirst covered the high points of shims and base plates.
Dimensions of Shims and Base Plates
First, the shim’s size must be identical to — or larger than — that of the base plate. The reason, Hirst said, is that the rack engineer determines the outer dimensions of the base plate based on calculations of the concrete slab’s strength.
“A smaller shim dimension would concentrate the loads across a smaller area,” he explained. “Therefore, the shim must match the size of the base plate, or exceed it, to ensure a uniform transfer of the base plate load to the concrete.”
Material Composition of Shims Important
Because each shim will become an integral part of the overall rack’s design, their material composition must have a higher compressive strength than the concrete slab.
“Typically, shims are steel plates. I’ve also seen certain types of plastic used,” noted Hirst. “Installers should never use wood shims, however. Wood lacks the compressive strength necessary to safely support the rack structure.”
Shim Stack Thickness Requirements
The total thickness of a stack of shims placed under a base plate should be less than six times the diameter of the largest anchor bolt used in that base. Further, the installer must secure the shim stacks together using a method that can transfer all the shear forces at the base.
If excessive shimming is necessary to compensate for uneven floors, the rack engineer may require guarding or welding of the shim stack. This helps protect it from powered mobile vehicle or load impacts, said Hirst.
“Also, shim stacks under base plates in drive tunnels or adjacent to aisles with vehicle traffic should be evaluated for risk of impact and guarded accordingly,” he added. “Alternatively, some use grouting to protect the shims and hardware from impacts.”
While standard shims typically measure 0.125-inch thick, thinner versions exist. Made of 14- to 24-gauge steel, these thinner shims are ideal for specific installation situations, said Hirst.
“Taller columns in excess of 30 feet or frames with recessed front base plates benefit from using thinner shims,” he explained. “That’s due to the large movement at the top of the frame resulting from the insertion of a small shim at slab level.”
Options for Securing Shims and Base Plates
After placement, it is critical that the installer secure the shims such that they remain in alignment with the base plates.
“If the installer does not secure the shims, a forklift or pallet load can knock them out of alignment,” Hirst observed. “Or they simply work themselves out from under the base plates over time due to repeated loading and unloading of the system. Seismic forces in the frame can also dislodge shims.”
Some shims feature a locking mechanism that prevents their rotation out from under a base plate. Alternately, using two anchors to secure the base plate and shims underneath to the floor will lock them in place. Other options include nesting, welding, or interlocking.
Additionally, it’s important to note that adding shims can shorten the embedment depth of the anchors.
“The rack design engineer specifies an anchor length and embedment depth,” Hirst explained. “If the shim stack exceeds 0.25-inch, consult the rack engineer to determine if longer anchors are necessary to achieve the required design embedment into the floor slab.”
Inspect Base Plates, Shims, and Anchors Routinely
To ensure the safest rack system, an operation should implement an ongoing inspection program. Base plates, shims, and anchors are among the different components to examine as part of those inspections, advised Hirst.
“In addition to damage from impacts, missing anchors, or shifted shims, a concrete floor slab can settle over time. Or it can heave in certain circumstances. These issues can knock the rack out of plumb and potentially compromise its safety,” he said. “The system may require re-shimming to return it to plumb tolerance.”
Learn More About Safe Rack Design
RMI offers multiple publications covering the safe design and use of racking. ANSI MH16.1: Specification for the Design, Testing and Utilization of Industrial Steel Storage Racks is available for purchase. Additionally, the association shares safety best practices in “Considerations for the Planning and Use of Industrial Steel Storage Racks.”
