How to Calculate OH Room in Light Hazard Area?

5/3/2021

When you have a common office building that is classified as a Light Hazard but you have some rooms that may be Ordinary Hazard such as storage rooms and others.

When you have your layout and one or more of these rooms ends up in your design area, how do you proceed in the calculation?

1.- Do you calculate 0.10 gpm/sqft over 1,500 sqft?

2.- Do you calculate according to Chapter 23 Sections 23.4.4.2.4 and 23.4.4.2.5 specifically for that room even if it is one or two sprinklers?

3.- Do you calculate everything with the density and area for the hazard of that room even though everything else is Light Hazard?

4.- Do you calculate 0.10 gpm/sqft over 1,500 sqft and ensure that the sprinklers inside the room discharge at the minimum density for the hazard they have?

This may be very general and definitely every project is different but every time I run into this I have this same question. I was told to just calculate everything as a Light Hazard but my curiosity always keeps me thinking about it.

Moderator Note: This question is different, but similar to: www.meyerfire.com/daily/ordinary-hazard-within-light-hazard-building

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4 Comments

Matt

5/3/2021 08:08:01 am

If a sprinkler in your design area falls within an OH room or space, simply calculate those affected sprinklers at the revised density (0.20 gpm/sqft).

James

5/3/2021 08:28:21 am

Right, I'm with Matt. Option #4 with a caveat:

One important clarification - "buildings" are not classified as a hazard, "areas" are. A building can include a collection and mixture of Light, Ordinary, Extra Hazard, etc. NFPA 13-2019 Handbook "Designer's Corner" 4.3.1.3 backs this up.

In this case, calculate the portions of the remote area which is considered Ordinary Hazard with its appropriate density, such as 0.15 gpm/sqft or 0.20 gpm/sqft. If the two hazards have no barrier in-between, then extend the higher-hazard by a minimum of 15-feet into the lower hazard area, offset ceilings, or provide a draft curtain, per NFPA 13-2019 Section 19.2.2.

Franck

5/3/2021 08:42:03 am

Much as been said at: www.meyerfire.com/daily/ordinary-hazard-within-light-hazard-building

As a practical approach:

1. If the area is small (1-2 heads) and is not located in the most remote area of the system, I would just design the sprinkler protection in the room with the spacing limitations for sprinklers for OH (or EH) and the pipe sizing accoring to the pipe schedule method (for EH, you have it on the Annex section of NFPA 13).
Regarding the water supply sizing (as the duration and densities are slightly higher for this room than the rest of the building), I would not worry that much (if the room is not in the most remote area, the additional demand for water/pressure will be compensated by the friction losses to reach the most remote area that has been calculated).

2. If the area is located in the most remote area, I would design the sprinklers in the room as above in 1. (for spacing, pipe sizing...), and make my hydraulic calculation with the correct pressure at the sprinkler to have the required density in the OH/EH room (which should be a higher pressure, i.e. a higher flow, for these sprinklers) and as a light hazard for the remaining of the remote area (note that if some heads in the LH area are located on the same pipe feeding the OH area, these heads would result in a much higher flow - this will not be the case if you feed your OH/EH area directly from the cross main).
Regarding the sizing of the water supply, the hydraulic calculation will determine how much additional water you need for that area. The difference in the duration (for a small room) should not have a large impact (could be considered as negligible) on the final volume.

3. If the room is not in the most remote area and is quite large (or if multiple rooms are located within a same area of 1500 sq ft), I would make 2 hydraulic calculations. One for the most remote area (which could be LH only) and one for this area (as per point 2 above), just to be sure. But only if I have a doubt that the room may be large enough to have a negative impact on the final results.

Dan Wilder

5/3/2021 10:54:27 am

Big reason why there needs to be a push for a distinction between "Hydraulically Remote Design Area" vs. "Remote Area". I'm with the #4 option as well and Franck sums it up well.

As the overall building occupancy and the room names determine what occupancy we would design to, there is still the determination that what is physically remote vs what is hydraulically remote that needs to be accounted for.

Small ancillary rooms within the remote area (especially when there is no wall rated separation) just need to be verified that the sprinkler(s) protecting that room are both spaced and are being provided with the correct (or minimum) flow & pressure for the hazard but should not change an overall design area approach (small storage rooms within an office area as an example).

However, if a larger, higher hazard room (still within the applicable building code allowances) is not within the initially physically remote area, an internal "check calc" would be a preferred first step to verify it is not the hydraulically remote system. Best example I can think of would be a science lab within a school building (we approach this room as being ordinary hazard as many AHJ's have redlined our submittals) or a larger IT/server room within an office building.

How to Calculate OH Room in Light Hazard Area?

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