We have discussed what constitutes solid shelves within a rack structure but it can get a bit confusing when we consider shelf storage itself. NFPA 13 makes a distinction between shelf storage and rack storage. Shelf storage is defined in the standard in Section 22.214.171.124 as:
Storage on structures up to and including 30 in (750 mm) deep and separated by aisles at least 30 in (750 mm) wide.
The standard also acknowledges that it is possible to push shelves together and create Back-to-Back Shelf Storage which is defined in Section 126.96.36.199.1 as:
Two solid or perforated shelves up to 30 in (750 mm) in depth each, not exceeding a total depth of 60 in (1.5 m) separated by a longitudinal vertical barrier such as plywood, particleboard, sheet metal, or equivalent, with a maximum 0.25 in (6 mm) diameter penetrations and no longitudinal flue space and a maximum storage height of 15 ft (4.6 m).
The definition for a single row rack states that it has no longitudinal flue space and a rack depth up to 6 feet (1.8 m) so the difference between a rack and a shelf is based on the overall dimensions. If a storage aid is less than (or equal to) 30-inches (750 mm) deep, then it shall be considered a shelf. If greater than 30-inches (750 m) and not falling into the back-to-back definition, then it is a rack. It seems pretty clear up to this point, like the expression “I know it when I see it”. Photo 1 is an example of a shelf while Photo 2 is an example of a rack:
Photo 1: Shelf Storage
Photo 2: Rack Storage
The first photo is not more than 30-inches (750 mm) deep and is clearly categorized as a shelf because of its depth, assuming a minimum 30 inch aisle is provided to the next shelf unit. The second photo has dimensions that exceed the 30 inch depth and is designed to accept a much different storage configuration. The shelf storage arrangement is not as substantial as the second photo which is obviously designed for heavier loads. It is worth noting that NFPA 13 does not address the weight that either configuration is expected to support. Up to this point, the distinction is easy to understand and seems pretty straight forward. However, there are some configurations that are not as clear.
Photo 3: Shelf or Rack?
Consider Photo 3 which illustrates two shelves, each around 30-inches (750 mm) wide pushed together which now creates a back to back shelf. The unit is designed to hold a lesser load than what racks can support, but again the problem is that by definition this must be classified as a rack. Let’s look at the definition of shelf storage again — “…not exceeding a total depth of 60-inches (1.5 m) separated by a longitudinal vertical barrier such as…” In this scenario, there is no barrier provided and therefore; it cannot be shelf storage and the only other possible classification for this arrangement would be to call it a rack. It is important to note that since there are no flues present, not only does this fall into a racking classification, but it also becomes classified as (back to the last three blogs) “solid shelving in a rack” and would require sprinklers below every shelf (In the current 2016 edition of NFPA 13, sprinkler protection is required under solid shelving for Class I through IV storage less than 12 feet high or 5 feet high (1.5 m) if a Class A commodity).
One can question NFPA for requiring a vertical barrier within a back to back shelf structure as part of the definition, but one cannot question the wording. If the shelf is wider than 30” (750 mm) and does not have a solid barrier in the middle, then it is a rack. As a rack, one has to evaluate if solid shelving is provided recognizing that as we just went over in the previous three blogs, either the shelving material or the load itself defines the shelf area and determines if solid shelving is present.
Understanding this separation between a rack and a shelf, there is another requirement for shelf storage that has to be understood. Chapter 14 (Class I-IV commodity) and Chapter 15 (Plastic and Rubber Commodity) apply to shelf storage. Both of these Chapters limit the maximum height of shelf storage to 15 feet (4.6 m). If we exceed this height, then we must, once again, treat it as a rack structure.
For years, many occupancies (I came across this in the back room of many department stores) provided shelf storage in their back rooms and would limit the height to 15 ft (4.6 m) or less because this was what could easily be handpicked with rolling ladders. There were several stores that would provide a catwalk level (or multiple catwalk levels) of open grating to allow handpicking at increased elevations. It was common to provide in-rack sprinklers under the grating, consider the area sprinklered and “restart” the maximum height limitation. As an example, place a catwalk level at 10 feet, sprinkler underneath and provide an additional 15 feet (4.6 m) of storage from that point on so that one now had 25 feet (7.6 m) of shelf storage. Basically, the rationale was that by providing sprinklers under the catwalk level, one was raising the floor and could restart the count.
The 2016 edition of NFPA 13 clearly stated that this concept of storage height was not allowed by adding wording to Section 188.8.131.52 which states:
Bin box and shelf storage that is over 12 ft. (3.7 m) but not in excess of the height limits of 14.2.1 and that is provided with walkways at vertical intervals of not over 12 ft. (3.7 m) shall be protected with automatic sprinklers under the walkway(s).
Therefore, one could provide a walkway but still can only go 15 ft. (4.6 m) high. The design scenario discussed above is not allowed. It should be noted that this requirement is not in Chapter 15 yet and it would be worth asking the committee, if I have plastic storage can I exceed the 15 foot (4.6 m) limitation by placing sprinklers under the catwalks?
In summary, a shelf is limited in width to 30 inches (750 mm) or 60 inches (1.5 m) if a vertical longitudinal barrier is provided and it shall be a maximum height of 15 feet. If exceeding these values, one had better look at the rack requirements. When looking at the racking requirements, carefully review the solid shelving requirements because in-rack sprinklers could be required.
As always, I welcome your comments: firstname.lastname@example.org
Jerry Schultz, P.E.