Use 4-ft Obstruction Rules for 2-ft Obstruction?

6/30/2023

I’m designing a wet system protecting storage utilizing 25.2K ESFR pendents.

I have some continuous obstructions greater than 2-ft wide (less than 4-ft wide) that require protection below the obstructions. It's under the 2018 IBC, and NFPA 13 2019 edition. The obstructions are mostly closely-spaced conduit groups.

I would like to place the ESFR pendents within 3-inches to the side of the group with the deflector 6-inches below and have listed water shields installed for intermediate level rack type protection.

Chapter 9 in NFPA 13 seems to allow this, but it is in a subsequent section dealing with obstructions greater than 4-ft wide. The verbiage and the annex figure do not clearly state the width of the obstruction.

This protection scheme will be clearly denoted and submitted to the AHJ, but I want to get input if there are any potential issues with this design.

Any questions, comments, or concerns will be greatly appreciated!

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8 Comments

Dan Wilder

6/30/2023 08:03:29 am

Section 9.5.1.3 says that the Sections 9.5.2 through 9.5.5 apply to all sprinkler types unless modified to be more restrictive.

9.5.5.3.1 says obstructions over 4' wide.

9.5.5.3.1.2(2) through 9.5.5.3.1.5 are the sections you reference for the adjacent vs under.

9.5.5.3.3 says all sprinklers shall be the same type and specifically mentions ESFR

14.2.11.3 MODIFIES 9.5.5.3.1 for ESFR for width and location requirements, but protection beneath (other than spacing requirements) are not changed.

There is no other location beneath obstruction requirements that are modified within Chapter 14, only the more restrictive width and location of the obstruction requirements for when so I would say you've got a strong case.

It would be more beneficial to either have the conduit moved to be centered between branch lines/sprinklers or possible move the overhead BL/sprinklers to be centered over the obstruction purely out of a cost benefit but I know that is not always possible.

Jesse

6/30/2023 08:10:39 am

Dan is spot-on, but I'd really emphasize trying to get the MEP to move those cable trays.

Glenn Berger

6/30/2023 08:20:04 am

To me the obstruction rules were clear before I started going into the field to view obstructions. With field experience, I have questioned the minimum requirements of NFPA 13 as it relates to obstructions (and other items).

You are the DOR, if you feel that additional sprinklers are required below obstructions then add that requirement to the contract plans and specifications.

I use a basic rule, if I cannot see the sprinkler above an obstruction, then additional sprinklers are required below the obstruction, regardless of size of the obstruction.

Colin Lusher, PE

6/30/2023 08:50:10 am

No offense intended, but this is the reason people dislike AHJ's.... why make up your own rules?

"if I cannot see the sprinkler above an obstruction, then additional sprinklers are required below the obstruction, regardless of size of the obstruction"

This doesn't make much sense; if there is a 3-ft obstruction 6-ft below the sprinkler (standard spray), then you're not going to see the sprinkler, but the sprinkler will spray around the obstruction due to the fully developed spray pattern. Have you ever seen a sprinkler actually discharge? The floor under that duct will get plenty of water on it. The rules of NFPA 13 are fairly conservative and have been developed by extensive testing. No need to make up your own rules in my opinion.

Patrick Drumm

7/9/2023 12:37:54 pm

I agree with Colin. Also, it is very important to understand the legistlation regarding state building codes, fire codes, and their adoption of NFPA standards in your area. In a few states I am familiar with, the State Fire Marshall's Office is identified specifically as the AHJ and only their office can modify code requirements. There are other formal processes for interpretation including reviews by the state fire boards. City and town officials are directed to enforce the codes as they are written. That does not mean that I do not do my best to reach agreements with local officials, but if they were blatantly disregarding codes and making up their own rules it would be an issue that would need to be addressed with the state fire board.

Casey Milhorn

6/30/2023 09:20:42 am

I would also add that there was some fairly extensive recent testing done on obstructions to ESFR sprinklers and there were some very favorable results. I think they had another phase or two planned, and this was 1 or 2 years ago. Hopefully there should be some results coming out soon on this, if they haven't already. It will take some time for NFPA to review and adopt any changes of course.

Franck

7/3/2023 02:35:37 pm

Wasn’t it for obstruction at ceiling level from construction elements (beams, joists…) and position of sprinklers underneath (with possible greater distances than initially allowed), rather than obstruction below sprinklers ?

There was some research results on that topic at the last NFPA conference in Las Vegas (presented by FM global with modelization - later confirmed by actual testing).

Chad

8/15/2023 11:21:29 am

May I add a small twist to this discussion? What if we are talking about elevated conveyors, such as in a large distribution center, where the dimension of the conveyor exceeds 4'?

Does the requirement for a sprinkler(s) to be located under the conveyor apply only if there is or will be storage or some other combustible under the conveyor?

What if the owner says that there will be no storage so no sprinkler is required? NFPA 13 doesn't specify this exception though some believe it is implied.

Use 4-ft Obstruction Rules for 2-ft Obstruction?

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