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  • Jerry Schultz, P.E.

Solid Shelving (part 4) - Defining Depth of Multiple Row Rack

Throughout the writing of this blog, one subject that I have gone back to time and time again, is solid shelving. You would think that at some time, the issue would be resolved and the revisions would stop. The 2022 edition of NFPA 13, Standard for the Installation of Sprinkler Systems is proposed to be revised again to address solid shelving, but this time in multiple-row racking. The proposed changes have been approved by the committee and are out for final review by the general public. The only possibility to change the document now is by submitting a Notice of Intent to Make a Motion (NITMAM) and then have a floor vote at the annual meeting. NITMAM’s have to be filed by February 17, 2021.

In 2016, I wrote a blog post (Fire Testing, January 18, 2016) where I noted that there were no new tests presented to the committee as justification for the initial solid shelving changes to the standard that happened in 1999. In a nutshell, those earlier changes defined solid shelving and set an area limit for a solid shelf before requiring in-rack sprinklers. As time progressed and additional revisions took place, it became obvious that the committee was saying that the load itself defines if solid shelving is present and not whether or not shelving material is present. The committee reviewed tests that were conducted in the 1970’s and reinterpreted the results. This cycle, the proposed change to the solid shelving requirements once again is based on interpretation of a test conducted in the 1970’s. At that time, the test protocol only tested 16 foot deep racks. Not being involved with these tests, I do not know why that was what they tested. These racks could have been common in the industry or even more likely, that is the rack they had available. I cannot state why they only tested 16 feet deep racks.

The present committee reviewed that earlier test data and felt that since the test was limited to a given depth there was a need to change the standard. Hence the solid shelving requirements are being modified again. Three options are proposed -- provide transverse and longitudinal flues (longitudinal flues have never been required in multiple-row racking), limit the depth of a multiple-row rack, or provide in-rack sprinklers.

Several cycles ago, it was recognized that since a longitudinal flue was not required, we could have storage abutting each other with no limit on quantity. This was contrary to the direction that the committee was headed on solid shelving. In 2013, (see Solid Shelving Part 3) the standard was modified to state that if longitudinal flues are not provided in multiple-row racks, (remember they are still not required in multiple-row racking), a transverse flue would be required every 5 feet. A concern was expressed that if the rack were 100 feet deep, this could result in a solid shelf area of 500 square feet (5’x100’), but the general consensus appeared to be that transverse flues allowed for sufficient water penetration. Again, it is not just a solid shelf material that defines solid shelving, it’s the actual pallets of product abutting each other.

Seven years later (this cycle), the committee questioned if this was the right approach and went back to the test from the 70’s and proposed a limit on the depth of a multiple-row rack. The following change has been accepted by the committee and will be finalized in June of 2021, pending outcomes of any potential NFPA membership vote (NITMAM) and NFPA Standards Council action. The new requirement states (my editorial comments are in italics): Multiple-Row Racks

Unless the requirements of or are met, multiple row racks shall be considered racks with solid shelves. Note that you must meet one of the sections.

Multiple-row racks without solid shelves shall be considered open racks where both transverse and longitudinal flues are provided at a maximum of 5 ft. (1.5 m) intervals. Interesting that this provides a solid shelf of 25 square feet, yet for single and double-row racks, we are limited to 20 square feet.

Multiple-row racks shall be considered open racks where transverse flue spaces are provided at maximum 5 ft. (1.5 m) intervals and the rack depth does not exceed 20 ft. (6.1 m) between aisles that are a minimum width of 3.5 ft. (1.1 m). This option eliminates the deep 100 foot rack but provides for a solid shelf area without in-rack sprinklers of 100 square feet. Again for single and double-row racks we are limited to 20 square feet?

This section is stating that either a transverse flue and longitudinal flue is provided every 5 feet or the multiple row rack is a maximum of 20 feet deep and separated by a 3.5 foot aisle from your next multiple row rack or in-rack sprinklers are to be provided under every tier of storage.

FM Global has similar requirements but offers more clarity. Their Data Sheet 8-9, which is available right now, states the following: In multiple-row racks, maintain minimum 6 in. (150 mm) wide vertically aligned longitudinal and transverse flue spaces a maximum of every 5 ft. (1.5 m) throughout the height of the rack. For multiple-row racks not in accordance with Section

a) Provide minimum 6 in. (150 mm) wide transverse flue spaces a maximum of every 5 ft. (1.5 m) horizontally, and

b) Limit the depth of the multiple-row rack, as defined by minimum 8 ft. (2.4 m) wide aisles, to 20 ft. (6 m).

The big differences are that FM is mandating an 8 foot aisle, not the 3.5 feet that NFPA requires. Again, one can treat it as solid shelving and add in-rack sprinklers.

It must be recognized that the in-rack sprinklers are required (if one elects to go this route) even if an ESFR overhead system is provided.

These requirements are proposed for the 2022 edition of NFPA 13 and have already been added to FM Global Data Sheet 8-9. If in-rack sprinklers are not provided in a multiple-row rack, the allowable depth of the rack will be limited or transverse and longitudinal flues will be required. The requirement will more than likely be adopted into the 2022 edition. There is typically a delay in municipalities adopting the latest standard, so you probably will not start to see it enter the overall warehouse environment until at least 2022. FM insureds will have to take it into account immediately.

I have written numerous articles on solid shelving and they can be found on our web site (

As always, I welcome your comments:


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