With more operations investing in automation to enhance throughput and productivity, it is becoming increasingly important for the floor surfaces to be smooth and level. These factors impact not only autonomous mobile robots (AMRs) and automatic guided vehicles (AGVs) navigating throughout a facility, but also rack-based semi-automated pallet handling systems. A concrete floor’s flatness is critical to ensure a level rack system and successful travel of semi-automated pallet handling devices—also called shuttles or carriers—through it.
“Motorized pallet rack shuttles achieve optimal travel on level rails within a rack structure,” explained Arlin Keck, Principal Engineer at Steel King Industries. The company is a member of the Rack Manufacturers Institute (RMI). “Any variation that creates an incline could cause the pallet handling device to struggle to advance. Likewise, a decline could cause the shuttle to lose control and crash into another load. That makes having as level a floor as possible under the rack essential.”
Floor Flatness Characteristics
What most facility owners refer to as floor flatness actually encompasses two different profile specifications. They include:
- Surface flatness characteristics, such as roughness, wash boarding, or bumpiness.
- Surface levelness properties, including tilt, pitch, or slope.
Historically, measurements of floor flatness were determined with a method that used straight edge. This approach looked for deviations in excess of ⅛-inch across a 10-foot span. Today, however, the American Concrete Institute (ACI) uses the F-number system to profile a concrete floor’s surface. This method includes measurements of both floor flatness (FF) and floor levelness (FL). Higher FF numbers mean flatter floors; higher FL numbers indicate a more level concrete slab.
FF25 flatness is roughly equivalent to a single ±¼-inch defect in a 10-foot span. FF50 flatness corresponds to a ±⅛-inch defect in a 10-foot span. FF100 flatness denotes a ±⅟₁₆-inch defect in a 10-foot span.
“When specifying values for concrete slab-on-grade, it would be written as ‘FF25/FL20.’ The values are linear, meaning that an FL20 would be twice as flat as an FL10,” explained Keck. “Prior to the F-numbering system, most floors had a flatness rating of FF15 to FF35. The average industrial floor has a flatness/levelness reading of FF20/FL15.”
Application Impacts Floor Profile Requirements
Additionally, the target profile specifications for a new facility’s concrete slab-on-grade floor depend on the building’s use, Keck continued.
“A general use building with random travel aisles will typically have only a local and an overall flatness and levelness requirement, with the local being within two-thirds of the overall,” he explained. “For specific use floors with defined traffic aisles—such as those that will be supporting some degree of automation—tighter tolerances will likely be necessary.”
To specify defined traffic aisles with tight tolerances that call for super-flat floors requires use of the Fmin system, which is different from the F-number method. Fmin denotes the minimally acceptable (or worse) flatness and levelness conditions for a given application.
“For the Fmin system, an Fmin100 corresponds to a floor flatness of ⅛-inch across a 10-foot span. That’s equivalent to FF140/FL100, or 0.045-inch/0.080-inch,” added Keck.
Options for Correcting Floor Flatness Issues
As warehouses and distribution centers (DCs) get larger and taller, maintaining floor flatness when pouring a concrete slab-on-grade has become more difficult. Keck added that finding open expanses of land that are completely flat is getting harder. That further adds to the challenge of pouring a perfectly flat floor.
Also, the rack design standards outlined in RMI’s ANSI MHI16.1: Design, Testing, and Utilization of Industrial Storage Racks are based on the loaded racks experiencing no more than a 0.5-inch in 10-foot height deviation. Likewise, ANSI MHI16.1 presumes that the warehouse floors are level.
For that reason, it’s important for the building owner to alert the rack designer to any slope or floor flatness issues, added Keck. In a rack installation perpendicular to the floor, the rack design should include any notional loads and additional forces due to the floor’s slope.
“If there are just a couple of isolated spots that don’t meet the floor flatness requirements of the semi-automated rack system, it may be possible to grind those areas down,” he said. “But if there are several areas or large expanses where grinding does not achieve desired floor flatness, there are other options.”
Shimming underneath the rack system’s base plates to level the rack is one solution, Keck noted. Alternately, adding a layer of grout underneath each affected base plate is another approach to leveling the rack.
“It’s similar to building a little concrete platform underneath each base plate to a height that matches a level laser line set above the floor,” he explained. “Sometimes the floor is so sloped that it may take two to three inches of grout to level up the entire system. But if that isn’t done, the semi-automated pallet handling devices won’t work as expected.”
Learn More About Racks and Floor Flatness
RMI offers a variety of best practice floor flatness recommendations in a collection of frequently asked questions and answers on its website. Also, more details about this topic are in RMI’s Considerations for the Planning and Use of Industrial Steel Storage Racks publication.
