Always In-Rack for Solid Loads in Open Rack?

5/27/2021

We have a project where the tenant intends to store Class I to Class III commodities that have a typical footprint of 12’ x 3.5’ (42 SF) on open-wire mesh racking.

Between the Definition 3.3.172 (NFPA 13-2019) Rack Shelf Area which specifically calls out “by the placement of loads” and the definition “Open Rack” (3.3.140) it appears that this condition has to be viewed an Solid Rack and would require In-Rack sprinklers no matter what roof level system (CMDA, ESFR, K-25) is installed or the Commodity Class stored.

I know there are some special designs related to “big box retail” stores (Mercantile) for this, but our application is Warehouse (Storage). There are also additional conditions if the footprint is greater than 64 SF.

Is there a way to read the above definitions that would not require in-rack sprinklers when Loads with a footprint over 20 SF and less than 64 SF are placed on otherwise Open Rack?

Requiring in-rack sprinklers is a potentially major expense for an owner with 75+ operable facilities like this.

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5 Comments

Franck

5/27/2021 09:32:55 am

I am afraid you're right and it may cost a lot for your owner with his 75+ locations...

To be considered as open rack arrangement, you need longitudinal and transversal flue spaces (the 6 in. nominal flues on all 4 sides around your maximum pallet footprint of 20 sq ft).
The transversal flue space is even more important than the longitudinal flue space (the longitudinal flue is a "must" requirement for storage above 25 ft, and a good advice below).
The purpose is to have the fire spreading vertically to activate the ceiling sprinklers without spreading horizonrtally too far...
This is the vertical spread of the fire toward the ceiling that will activate the sprinklers. The sooner, the better, and on a smaller area, the better.
With large pallet footprints, this means that the vertical fire spread will occur at longer intervals, will activate more heads at the ceiling, and will spread horizontally within the rack as the water will be prevented to reach the fire by the large pallet loads above...
This is why, when you are considering "solid shelves" in racks (as in your case), you need intermediate in-rack sprinklers to be as close as possible to the fire and prevent the opening of too many heads at the ceiling level.

If you consider the vertical fire spread through the transversal flue space between pallet loads, you will better understand the design requirement for ESFR and CMSA.
If the fire is directly below one sprinkler (ESFR), you will open one head. And then the fire/heat plume will spread in all directions and you "may" activate the second row around that sprinkler. This means 8 additional ESFR, for a total of 9 (if you make a drawing, it is easier to understand... I can't do it in this text box).
But if the fire starts between 2 sprinklers, you will first activate 2 ESFR at the ceiling, and then the second row will involve 10 more ESFR, for a total of 12.
As ESFR are very efficient (and most tests demonstarates fire extinguishments with 1-3 heads), the final design is generally with 12 ESFR (sometimes more, but for higher storage height/more hazardous commodities).
With CMSA, this is the same principle. But as we are on the control mode, a 25% safery margin is added (12 + 25% = 15). You end up with a general design with 15 CMSA sprinklers :)

But if you have solid shelves, the design will not work anymore as you are not considering 2 heads for the initial stage, but 3, 4 or 5... and end up with much more (unknown) operating sprinklers at the end, thus overtaxing your sprinkler protection.

I had once this particular issue in the automotive industry where they stored large plastic parts (bumpers....) in racks, and it became a nightmare for the sprinkler protection.
The better option, if you have a limited amount of apllet loads, is to store them on pile storage arrangement and to keep the rack for smaller footprint pallet loads.

Robert Anderson

5/27/2021 07:18:42 pm

I agree with everything you said related to horizontal fire spread. The concept of Placement of Loads creating solid shelving has been in the Code for several years now, but I visit so many facilities that have racks with oversized pallets, door slabs, lumber, pipe, sofas, hand stacked loads without transverse flues, etc. where this issue has not been address. If there was any gray area in the language it appears NFPA doubled down on the concept in the 2019 Code.

5/28/2021 02:36:55 am

Note that for solid shelves between 20 sq ft and 64 sq ft – as measured between aisles and flue spaces – in rack sprinklers must be added, but not necessarily below every shelf (depending on arrangement).
They should be installed below every shelves when the solid shelves are above 64 sq ft.

franck

5/27/2021 10:09:48 am

Just to add a personnal view (to be discussed with the AHJ / Insurance company) : maybe the class I commodity storage is not much of an issue, in particular if you design your entire storage area for a class III commodity storage, as you will have safety margin from the class III commodity design to cope with the adverse conditions on your class I commodity storage.

You can also add some safety factors by using a larger design area (i.e. not the point for 2000 sq ft, but the one for 3000 sq ft on your design curves from chapter 21.4.1.2) and / or a larger design density (such as the one for the 4 ft aisles, even though you have 8+ ft aisles, for example...).

For the class III commodity, I would better stick with additional in-racks.

5/28/2021 02:35:37 am

Some additional information:

Tests performed with or without solid shelving demonstrated that fire travel was more horizontal than vertical during the early stages of a fire with solid shelves.

This created a broader base and fast spreading fire, making it extremely difficult for sprinklers to work effectively.
Ceiling sprinklers were delayed in operation and did not reach the base of the fire because of the shielding of the shelves. In all tests, the fire was not controlled when solid shelves were present and a large amount of sprinklers operated, whatever the design density at the ceiling level.

In-rack sprinklers were not as effective as when installed in an open rack (no shelves) because the fire spread was faster than the sprinklers could operate.
Following is a further explanation:
• In-rack sprinklers that are situated in open flues do not normally operate until they are engulfed in flames.
o With open racks (no shelves), this is normally fast enough as flames tend to travel faster vertically than horizontally.
o With in-rack sprinklers in open longitudinal flues and with solid shelves covering the transverse flue spaces, this is normally not fast enough as flames tend to travel faster horizontally than vertically and race ahead of the operating sprinklers.
• In-rack sprinklers that are situated directly under a shelf will operate very fast and ahead of the flames as the heat banks of the shelves to the sprinklers.

Always In-Rack for Solid Loads in Open Rack?

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