When it comes to designing a safe and durable storage rack system, it’s important that a rack engineer consider the strength of the concrete slab and the soil beneath it. The stability and longevity of rack systems rely heavily on proper support from the ground up. To ensure proper rack support, it’s crucial to understand the relationship between the soil and concrete slab-on-grade.
The Dirt on Soil and Concrete Slab-on-Grade
When designing an industrial steel storage rack system, the rack engineer needs to know if the floor beneath it will safely support the loads placed on it. Known as concrete slab-on-grade, the slab’s weight bears directly on the ground or on a layer of granular fill. As the rack sits on top of the floor, the weight of the steel structure—and the loads stored in it—exert force on the concrete slab.
A concrete slab-on-grade is a lot stiffer than the soil underneath it, explained Arlin Keck, Principal Engineer at Steel King Industries, a member of the Rack Manufacturers Institute (RMI).
“The weight of the racking plus its contents bears down on the concrete floor which in turn bears down on the soil below,” he said. “The concrete slab-on-grade is typically much stronger than the allowable bearing pressure of the soil. That means the point loading from the upright posts will lead not only to a punch-through force on the floor, but also to a bending force on the floor slab.”
When these two materials have significant enough differences in strength, the weight of the loaded rack can cause the concrete slab to crack and/or settle, Keck continued.
How Rack Design Compensates for Insufficient Ground Support
The type of soil at the site therefore drives the composition, thickness, and resulting strength of the concrete slab-on-grade. To compensate for insufficient slab thickness or poor soil, the rack design must incorporate oversized base plates to further distribute the force of the load. That, however, can pose operational issues, Keck noted.
“Most operations prefer to place pallets directly on the ground in the lowest level of racking,” he said. “But large base plates and the anchor bolts that secure them to the floor can obstruct pallets.”
Rack engineers have a couple of alternatives, continued Keck.
“One option is to design continuous, narrower-but-deeper, front-to-rear column base plates. The second option is to add a level of beams just above the base plates,” he explained. “In a system that was originally planned to have four levels, that adds a fifth level, which makes it roughly 10% more expensive because there are more beams.”
In situations where the concrete slab-on-grade’s thickness calls for such rack enhancements, the additional rack costs are typically far more budget friendly than any attempt to remediate the floor, said Keck. “Retrofitting an existing floor in a facility is an expense most companies prefer to avoid.”
Soil Classification Key Detail for Rack Engineers
Because the strength and stability of the soil directly impact the slab’s ability to support a rack system and the loads stored within it, the rack engineer needs to know its classification.
Across the United States a broad range of soil types exist. The characteristics of the ground in a given location—and its ability to withstand the loads placed upon it—contribute to a site’s soil classification. The American Society of Civil Engineers lists nine separate site classes in its ASCE 7-22 Standard: Minimum Design Loads and Associated Criteria for Buildings and Other Structures. They include:
- Site Class A – Hard rock
- Site Class B – Medium hard rock
- Site Class BC – Soft rock
- Site Class C – Very dense sand or hard clay
- Site Class CD – Dense sand or very stiff clay
- Site Class D – Medium dense sand or stiff clay
- Site Class DE – Loose sand or medium stiff clay
- Site Class E – Very loose sand or soft clay
- Class F Soil – Soils that requiring site response analysis in accordance with Section 21.1
Concrete Slab-on-Grade Recommendations for Spec Warehouses
“If a company is looking to buy a spec warehouse—that is, one built without a specific buyer or user in mind—they should get the data on the soil and on the floor,” said Keck. “Most spec warehouses have a 5- to 6-inch floor slab. That may or may not be strong enough to support the higher density rack designs.”
For that reason, RMI recommends that developers constructing a spec warehouse invest in a thicker concrete slab-on-grade at the outset. Doing so increases the potential occupancy and usefulness of the building. It is also more economical to engineer a thicker slab-on-grade initially than to try to rectify an inadequate floor at a later date.
If the soil type is unknown, a geotechnical assessment may be required. This determines the soil’s properties and allowable bearing capacity. It is then up to the rack engineer to design the racking base plates that will accommodate the racking load, the slab-on-grade strength, and the allowable soil bearing capacity. “Likewise, if the thickness of the slab is unknown, a boring test can reveal both the slab’s thickness and its compressive strength,” added Keck.
Have Other Concrete Slab-on-Grade Questions?
RMI publishes a broad range of rack design resources, including frequently asked questions and answers. These cover ANSI MHI16.1-2023: Design, Testing, and Utilization of Industrial Storage Racks, rack design reviews, seismic design categories, seismic factors, and site coefficients. They also address soil classifications, seismic separation, redundancy, and other seismic rack design information.
