Sprinkler Spacing Measured to Wall or Glass?

1/12/2022

We have a project light hazard occupancy, that is a highrise building, and has glass all along the perimeter of the building.

Is the maximum sprinkler spacing measured off the knee wall, or the glass?

There is a knee wall that rises up from the floor 2'-6" and is 12-inches in depth.

Do we measure the 7'-6" off this knee wall for floor coverage, or do we measure off the glass for the maximum distance?

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10 Comments

Mike L

1/12/2022 07:25:53 am

We typically measure the spacing off the face of the glass. The thought being the top of the kneewall will be covered with people's effects (aka combustible load).

Alex

1/12/2022 08:05:22 am

NFPA 13 (2016) 8.5.3.2.4 states that sprinklers shall be measured to the wall when sprinklers are spaced near windows and no additional floor space is created.

I believe the code section above is to referring to windows with a small soffit (1-6 inches). In your case, where you have a 12 inch soffit, I agree with Mike L that tenants will use that space more like a shelf. I would protect based off of the furthest edge of the soffit (the glass).

Thanks

Jesse

1/12/2022 08:18:42 am

Typically, the measurement would be from the face of the wall. However, in your case it should be from the face of the glass. This will inevitably become a shelf or something.

SCHULMAN

1/12/2022 08:44:10 am

ok, I'll dissent ...
would need to see a detail but the hollow metal frame probably sticks into what is the 12" knee-wall leaving only 6" or 7" of surface for employee "knick-knacks" ...
can't see any AHJ requiring this be measured from the glass ..seems silly not to take the code at its word and measure from the wall.
maybe an AHJ could answer ...

Dan Wilder

1/12/2022 09:02:04 am

This is where a conversation with the AHJ is good to have and followed up with for documentation. The knee wall is likely hiding the floor/window interface & rating (the framing holding the exterior glazing to the building along with the smoke/fire horizontal assembly). This isn't floor area by definition and the recessed window area should not be counted/defined for sprinkler spacing (the NFPA handbook has a section view illustration).

However, because this is likely the entire exterior facade, makes up a majority of one wall and the ability to store on this pony wall it can be implied, the window would be the correct. Had the wall been lower, the argument could be made for the floor area (do the arch floor plans indicate this area as being part of the useable square footage of the floor? If so the glass becomes the measurement point).

I've seen an illustration to this before via tech notes or similar but cannot find anything...I'll link if I find it at lunch.

Casey Milhorn

1/12/2022 09:09:02 am

I would have to agree with Schulman on this one as well. Sometimes it feels like we are getting too legalistic with interpretation in todays time. I would argue the load is going to be very light, the spray pattern of the head (tested to a flat wall in fire tests) will get some additional throw due to the elevated knee wall. Additionally the surface beyond and underneath the fire load is non-combustible. I agree with a discussion with the AHJ is the best approach though.

Jack G

1/12/2022 10:11:36 am

In a high rise building it is always good practice to space off the face of the glass.
There are no longer fire breaks that extend outward to keep fires from jumping floors.
In the meridian building, where I protected 5 Comcast floors , in an existing building , I had a habit of putting a sprinkler in front of the windows in an old building. The building was partially sprinkle red.
The fire kept jumping floors until it hit the comcast floors where 8 sprinklers at the windows put the fire out ( from a gravity feed from the roof tank ). According to the fire report.
So you never know ! .

Anthony

1/12/2022 04:34:04 pm

Some questions to consider:

How much combustible energy (stuff that can burn) could possibly be stored against the glass?

Will the sprinkler only spray is the sprinkler maintaining a fire rating on the glass wall?

Is it an open floor plan where a fire will not be stalled by walls?

Is there an ambulatory concern (ability to get away or exit the area)

Is the structure wood or steel and concrete?

If these questions lead you to think that the sprinkler needs to be closer to the glass then you should be considering increasing the hazard not worrying about 1' ~0-6'' of shelf space. Remember libraries are considered light hazard.

Jonathan Joseph

1/12/2022 04:56:21 pm

9.5.3.2.3 (2019) The distance from the wall to the sprinkler shall be measured to the wall when sprinklers are spaced near windows and no additional floor space is created.

There is an illustration for bay windows in the NFPA 13 handbook

We pretty much have to cover the floor area. NFPA 13D bay window or architectural feature cannot be more than 18sqft and project out more 2 feet and not be more than 9ft. long.

Dwight H Havens

1/13/2022 05:43:33 am

As previously noted the NFPA 13 standard indicates that the spacing is to be measured to the wall, not the glass. I think that it is also important to remember that the spray pattern from a standard spray pendant or upright sprinkler is circular, not a box, extending out to 11.25 feet, so unless the window sill is more than 3.75 feet, it is expected to be within the design coverage of the sprinkler. Such a deep window may require special consideration.

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Sprinkler Spacing Measured to Wall or Glass?

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