When building a new warehouse or distribution center that will house pallet rack, it’s optimal for the designer of the facility’s floor to work with the rack designer. Doing so ensures that the concrete slab can support the anticipated loading of the rack. As examined in a previous post, if the concrete slab-on-grade is not strong enough to support the rack system or compensate for the softness of the soil, it is more likely to fail. For that reason, the Rack Manufacturers Institute (RMI) offers several best practice recommendations for new concrete slab-on-grade design.
The American Concrete Institute (ACI) also publishes a number of slab-on-grade design specifications applicable to a range of different uses of concrete. For applications utilizing industrial steel storage rack, the provisions of ACI 318 apply to the design of the floor slab and the anchor bolts used to attach the rack to the floor.
Importance of Concrete Slab-on-Grade Design Increasing
“The strength of the concrete slab-on-grade has become increasingly important in recent years,” noted Arlin Keck, Principal Engineer at Steel King Industries, a member of the Rack Manufacturers Institute (RMI).
“Operations are doing everything they can to maximize their storage capacity,” he said. “Loads are getting heavier, racks and buildings are getting taller, and storage requirements are getting denser.”
For that reason, RMI recommends the building owner share as much information as possible with both the foundation engineer and the rack engineer about:
- The intended loading on the racks.
- The existing or new slab-on-grade.
- The allowable soil bearing pressure.
In new construction, having these details could reduce the chance of problems with the concrete slab and the rack structure. This often results in a more economical rack and concrete slab-on-grade design, added Keck.
“All of those factors make it increasingly important for the floor engineer and the rack engineer to work together at the outset of a project,” he said.
When planning a rack-supported building, floor strength becomes even more critical, added Keck. These rack structures incorporate wall girts and roof purlins (or equivalent components) to support the wall and roof cladding. The rack’s design must not only be capable of supporting normal storage rack loads, but also withstanding wind, snow, rain, roof live loads, and seismic loads (when required).
“For a rack-supported building, the rack design becomes primary since everything else is built around it—as opposed to the racks just being ‘fit’ into the building,” Keck explained. “A 5- or 6-inch-thick, specification typical, default floor—depending on where in the country it is—won’t work when potentially placing product that may exceed over 50,000 pounds in each bay.”
When the rack-supported building houses an automated storage and retrieval system (ASRS), the demands on the floor can become even more challenging, he continued. That’s because the higher the ceiling, the thicker the slab needs to be.
“An example would be a 120-foot-tall system that may need to support 150,000 pounds,” Keck explained. “The weight and the height both contribute to the forces that the concrete slab-on-grade design and soil will need to support.”
Coordinating Rack and Concrete Slab-on-Grade Designs
To enhance strength, concrete slab-on-grade designs typically call for the addition of rebar in the floor. Coordinating the racking, baseplate, and anchor bolt layout with the rebar layout will help facilitate installation.
“Doing so will minimize the chance of anchor bolts striking the rebar when ‘standing’ the racks,” Keck said. “This can be time consuming, but having a general contractor coordinate this requirement is important. Otherwise, the foundation engineer would need to approve drilling through the rebar. Also, the installer would incur added expenses and installation schedules may need extensions.”
Best Practices in Concrete Slab Designs
Detailed in RMI’s Considerations for the Planning and Use of Industrial Steel Storage Racks, Section 2.7, there are several best practices for floor slab-on-grade design. These include:
- Stress distribution through the thickness of the concrete slab. This helps in determining if the slab will experience tension stresses at the anchor points securing the rack to the floor.
- Thickness of the concrete slab-on-grade design. This measurement (in inches or millimeters) affects the size and thickness of the base plate. It also factors into determining the length of the anchor bolts used to fasten the rack to the floor.
- Strength of the concrete. Also a consideration when specifying the size and thickness of the base plate and the diameter of the anchors, this measurement is calculated in pounds per square inch (psi) or megapascals (MPa).
- Soil bearing pressure beneath the concrete slab. Measured in pounds per square foot (psf) or kilonewton per square meter (kN/m2), this information affects the size and thickness of the slab and the rack base plates.
- Modulus of subgrade reaction. Determined in pounds per cubic inch (pci) or kilonewton per cubic meter (kN/m3), this calculation affects the size and thickness of both the concrete slab and the rack base plates.
- Reinforcement in the concrete slab-on-grade design. This refers to the number of re-bars integrated into the slab in each direction, which affects the floor’s uplift capability.
- Expected floor slab joint movement, if any.
- Type of floor slab joints. Concrete slab construction often uses different joint types, including doweled, keyed, or interlocked. The style of joint (or other force transferring mechanism) used also impacts the slab’s uplift capability.
- Distance of the base plate from a joint. The proximity of a rack column’s base plate to a floor joint may reduce the rack and/or the slab capacity.
- Distance of the anchor from a joint. Placing an anchor bolt too close to a slab joint may impact the load bearing capacity of the anchor.
Get More Information About Rack and Floor Designs
RMI’s rack and slab-on-grade design resources include frequently asked questions and answers specific to flooring issues and best practice recommendations. More information about this topic is in RMI’s Considerations for the Planning and Use of Industrial Steel Storage Racks.
