As explored in a previous post, rack engineers must consider multiple factors when designing an industrial steel storage rack—including the facility’s floor and the soil underneath the building. In certain areas of the country that are more prone to earthquakes, rack engineers must also design a structure capable of withstanding these forces. ANSI MH16.1-2023: Design, Testing, and Utilization of Industrial Steel Storage Racks includes guidelines for how to perform design calculations that consider soils, slabs, and other factors in applicable seismic rack designs.
Seismic Rack Design and Building Codes
Jurisdictions in the U.S. have building codes that require the design of non-building structures—including storage rack systems—to accommodate earthquake loads. Many use the International Code Council’s International Building Code (IBC), which governs the safe design and installation of steel storage racks. As detailed in a previous post, the IBC references ANSI MH16.1-2023, published by RMI.
Rack engineers incorporate several data points when performing seismic rack design calculations. They include:
- The total weight of the loaded structure.
- The degree of seismicity at the installation location.
- The thickness of the supporting concrete slab.
- The classification of the soils under the slab.
The degree of seismicity at the facility’s location and the soil classifications determines the structure’s seismic design classification. That designation is based on the risk category and the severity of the design earthquake ground motion at the site. The rack design engineer uses this information to perform a seismic analysis of the racking system. This ensures that the racking has sufficient strength and rigidity to resist the local earthquake requirements.
Soil and Slab Provide Critical Seismic Support
Understanding the relationship of these four points is key to creating safe seismic rack designs, explained Arlin Keck, Principal Engineer at Steel King Industries, a member of the Rack Manufacturers Institute (RMI).
“The number of anchor bolts and size of the base plates used to secure the rack to the slab is affected by the interplay of all these factors,” he said. “As a simplified example, if an earthquake generates 5,000 pounds of uplift force, a 111-inch-diameter by 6-inch-thick section of the slab-on-grade—at an assumed weight of 150 pounds per square foot—would be required to resist the uplift. This assumes no other factors, such as a nearby equivalent compressive force, are in play. However, it is important to have a qualified rack engineer verify the specific calculations relevant to a given location.”
Enhanced Components Hallmark of Seismic Rack Design
Depending on soil, slab, and seismic factors, the resulting rack design may feature enhanced components and additional safety features. Among them are stronger beam-to-column connections, increased bracing, heavier welding, and stronger beams and frames.
Additionally, heavy-duty base plates that are larger and thicker allow for additional anchors. “They also improve the distribution of the column’s loading to the floor slab, which is particularly important in areas with softer soils,” Keck added. “This helps the rack resist uplift.”
Building design often incorporates some form of seismic dampening system, which rack manufacturers might emulate in their racking systems. Also, in high seismic areas, another safety best practice is for operations to wrap—or otherwise contain—the load in a manner that keeps it secured to the pallet. This minimizes the risk of stored inventory falling off during an earthquake event.
Looking for Additional Seismic Rack Design Information?
RMI publishes a broad range of rack design resources, including frequently asked questions and answers specific to seismic considerations. 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 the newest seismic maps. Further, RMI also offers a video, Seismic Considerations for Rack Designs, on its website.
