Design for Actual or Potential Storage Height?

4/13/2020

Low-piled storage versus design /area method: I notice a quite huge difference about requirements of NFPA 13 for using low-piled storage where where is no limitation for area and the method when the virtual storage height is used to determine according to the clearance the area / density method.

For example we can have a building with a high clearance with Group A plastic storage as low-piled at 5-feet (1.3 m). Using the low-piled method, you need Ordinary Hazard Group 2 0.20 gpm/sqft @ 1,500 sqft (7.5 l/min/m² @ 210 m²) when using ara /density method, but with the virtual height you need more than 0.73 gpm/sqft (30 l/min/m² @ 230 m²). In my example, the area of low piled storage is a very huge area 170,000 sqft (10,000 m²).

What is the actual criteria? Thanks for your comments.

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11 Comments

Dan Wilder

4/13/2020 08:56:27 am

Our AHJ's have taken the approach that the fire sprinkler design must be for a worst case height (or max clearance modification) of Group A Plastics to the highest point in the building bottom of roof.

They modified the IFC Chapter 32 to state:
"Minimum design requirements for speculative warehouses. The design of the automatic sprinkler system for speculative Group M, S or F occupancies shall be based on storage of a cartoned Class A non-expanded plastic to the available storage height. The storage height shall be determined by subtracting 48 inches from the highest point of the roof above each
system for ESFR and 30 inches for area density
applications."

The only exception to this is if the owner signs paperwork that the storage will be kept to a specific height, commodity, and storage configuration and then the address is tagged with the info. We clarify in our bids what the design will be (typically the worst case) unless we get specific info otherwise and then we push to get meetings with the AHJ early to verify their design request before submittal.

Casey Milhorn

4/13/2020 10:04:45 am

Dan, that is very forward thinking of the AHJ. Just curious which AHJ this is, where located?

sean

4/13/2020 02:07:20 pm

I don't know where dan is but I see that in multiple towns in the chicagoland area.

Dan Wilder

4/13/2020 08:01:15 pm

City of Phoenix specifically but the surround jurisdictions have adopted similar measures.

Casey Milhorn

4/14/2020 08:51:35 am

Thanks Dan and Sean. Good to know the trends and different requirements out there. This is definitely an issue in some areas of the Southeast. Spec warehouses with no idea of the tenant, or an idea, but everyone is being secretive about who it is and what they are using the space for.

James

4/14/2020 08:54:01 am

Casey that sounds like a near-impossibility to design for! It's hard enough to get storage information from owners who know what they want to store... let alone a mystery future tenant.

It's not like most owners just walk up and are willing to pay for everyone to design for the worst-case scenario.

Dan Wilder

4/14/2020 11:55:12 am

James - When the FD has an incident with loss of life because owners went the cheap route to build their building (OH2 in a 30' tall building), getting legislation passed to add these kinds of measures becomes really easy.

Unfortunately, it takes a loss like this to get them to be "forward thinking" even though they are just playing catch up.

The design may not capture everything, but it captures most scenarios.

James

4/14/2020 12:03:08 pm

Dan - Understood. Personally I like the concepts you've mentioned of forcing the design to the conservative scenario unless the owner specifically directs otherwise with storage information. When the AHJ writes/enforces it that's even better as I don't have to be the bad guy as an engineer.

FIREPE25

4/13/2020 02:46:07 pm

I am not sure if i am understanding your question correctly, but i will give it a shot. I think you are saying that you cannot meet the definition of miscellaneous storage because your area of storage is too large. This can be a problem in certain instances because if you cannot be classified as miscellaneous storage then NFPA 13 requires that you design to the acceptable clearances established in Chapter 12 (2013ed.). For Group A plastics these clearances are either 10'-0" or 20'-0" (depending on situation). However, in your example you say you only have 5'-0" of Group A plastics. If you look at 15.2.1 (solid piled group A plastics) and read the commentary that goes along with it from the handbook you do not need to obey any area requirements for less than 5'-0" of storage.

Pete D

4/13/2020 03:36:46 pm

I always designed for the highest height achievable with the water supply, neglecting the relatively small cost of larger orifice sprinklers. If there's a pump/ no pump cost consideration, and the design is for a specific occupant, I would put the storage height limitation on the riser placard. This should be satisfactory for that occupant to obtain a U&O permit from the AHJ. I suspect your question is about maximum clearance to storage which comes into play with plastics and tires. Where a maximum clearance to storage is required, you have to design for the storage height that would result in that clearance (varies with building height). It shouldn't come into play for 5 ft storage height.

Franck

4/14/2020 10:16:29 am

As a summary to all above comments (and point of view from Insurance industry), if the storage is always below 5 ft, as already indicated by Firepe25, you have to use the miscelaneous storage requirement for 5 ft of plastic.

But if you go above 5 ft, then you have to take the "clearance" requirement into consideration and you will end up with a huge difference in term of density.

We apply such recommendations in the insurance industry when we have a given density and the storage is "too high", giving alternative such as :
the sprinkler protection in the area should be upgraded and designed to deliver a minimum density of 0.73 gpm/sq ft over xx sq ft (as you suggested in your example) or the storage height of group A plastics should be limited to 5 ft.

We have the same approach with idle pallets (you will see that we have much more flexibility for wooden pallets up to 6 ft or plastic pallets up to 4 ft, as long as you can limit the number of stacks).

Then I understand the conservative approach from some AHJ, especially in the design phase of a project, as, if you can store higher, there will be a time when you will store higher...

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