Combustible Concealed Space clarification

[dtexter]

Hey All, I have a combustible concealed space (ceiling cavity) that doesn't meet any of the requirements of 8.15.1 THRU 8.15.1.2.17 for omitting sprinkler heads from this area. My question is two part:
- Can I leave the space unsprinklered if I increase the Density Area to 3000 sq.ft for the room area below the ceiling?

- This would essentially over water the room area below the unsprinklered ceiling cavity but would not provide protection if a fire was to start inside the ceiling cavity.​

- Do I have to have to increase the Density Area to 3000 sq.ft for the room area below the ceiling if I sprinkler the interstitial ceiling cavity?

- My thoughts would be, why would I have to increase the Density Area below the ceiling if I'm already sprinklering the cavity space.​

Any clarifying thoughts are greatly appreciated. Thanks!

 

[TravisMack]

You need to read that section carefully. I tell people to ask two questions with concealed spaces.
1 - Do I need sprinklers?
2 - If I am permitted to omit, do I have a penalty for not doing?

The order of those questions is important. You first determine if you can omit. If you can't omit per the criteria, then you provide sprinklers to it. If you can omit, you then have to determine if there is a penalty. Not all allowances to omit require the penalty.

But, you never get to take the penalty in order to omit sprinklers.

You also need to read and understand the penalty criteria. It states all spaces adjacent. Adjacent means left-right-up-down. So, if my intersitial space between floors 2 and 3 omit sprinklers and get a penalty, I have to apply that penalty to sprinklers in the ceiling of Level 2 and Level 3.

Travis Mack, SET, CWBSP, RME-G, CFPS
MFP Design, a Ferguson Enterprise

http://www.mfpdesign.com

 

[UFT12]

Adjacent means left-right-up-down. So, if my intersitial space between floors 2 and 3 omit sprinklers and get a penalty, I have to apply that penalty to sprinklers in the ceiling of Level 2 and Level 3.

...unless the slabs are fire rated with no openings in the area and with a rating of at least equal to the water supply duration. This applies also for vertical separations.

 

[dtexter]

Travis, I am assuming the "penalty" you take is the increase in Density Area? Just trying to make sure I am understanding your verbiage. I noticed in another post you mentioned having to apply the "penalty" to the floors above. I'm assuming this is what you're referring to when you talk about adjacent area? How does this factor into your hydraulic calcs if other parts of the building don't have to operate at the increased Density Area? I guess you'd have to run separate calcs for different parts of the building to get your different pipe sizes since flows would be different?
I appreciate the feedback.

 

[UFT12]

How does this factor into your hydraulic calcs if other parts of the building don't have to operate at the increased Density Area?

Assuming the combustible concealed space is not fire separated to the adjacent areas and assuming these areas are not sized for a higher hazard that surpass the water demand that results from this area increase, these parts of the building do have to operate at the increased density area.
The reason here is that if the concealed space without sprinklers is combustible and fire pass through it or originates in it, by the time it will cross over to the adjacent areas, it may be too large of a fire for the sprinkler system to cop with the standard dicharge criteria (figure 19.3.3.1.1) hence the compensation.

An other alternative would be to design the system with an design area equal to 3000 ft2 (sliding up the curve) thus satisfying the penarty inherently, assuming that you need to apply it. That requires however some consideration about other things involved.

 

[dtexter]

Makes sense UFT12... I am just wondering what the economic fallout would be of just designing the entire system with that larger density area. I would think you'd have some grossly oversized pipes for a building that is just used as office space.

Thanks for the insight. Much appreciated.

 

[UFT12]

just wondering what the economic fallout would be of just designing the entire system with that larger density area. I would think you'd have some grossly oversized pipes for a building that is just used as office space.

True, but it depends. There may be cases with more individual concealed spaces in an area that could justify such an increase.

 

[TravisMack]

Remember, that you can slide up the curve with density. You do 0.07 /3000. So, you will likely use a lot of 4.2k sprinklers in the office area. A 4.2k sprinkler will be 7 psi up to 158 sq ft with a 0.07 density, while a K5.6 is 7 psi up to 211 sq ft. So, depending on your spacing, you may be able to use the 4.2k to cut down you over-discharge. It will still be a lot more water. If you could do a QR reduction (assuming the penalty is not required) you could do as low as 0.1 gpm / sq ft over the most demanding 900 sq ft for a total of 90gpm. A 0.07/3000 is 210gpm at a minimum. So, you will potentially require a lot more water which will mean over-sized piping compared to no penalty.


Travis Mack, SET, CWBSP, RME-G, CFPS
MFP Design, a Ferguson Enterprise

http://www.mfpdesign.com

 

[UFT12]

On a side note, beginning with edition 2022, sliding up the curve is no longer an option for new installations.