Separate In-Rack & Overhead in Same Calc?

1/23/2025

If I have an in-rack system that needs to be calculated with the ceiling system, does the ceiling system need to be positioned above the most demanding racking?

This might result in neither individual component area used being the most demanding of their type.

Or do I calculate the most demanding rack along with the most demanding ceiling?

I drew a quick-and-dirty sketch of a warehouse with the most demanding rack in red, the most demanding ceiling area in green, and the water source in blue.

If the two areas are not lined up, this goes against the assumption that only one fire is happening in the building at a time. I'm thinking that the two need to be averaged over each other to come up with the most demanding single area, even if the most demanding ceiling or rack are not involved.

Thanks in advance for your input.

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11 Comments

Brett

1/23/2025 06:52:57 am

When the in-racks are required to be balanced with the ceiling sprinklers, NFPA 13 requires one remote area for both the ceiling sprinklers and the in-rack sprinklers. See 25.12.1.4 from the 2019 edition.

As far as where that remote area is, what I recommend is to figure out which of the in-racks or ceiling sprinklers requires the highest minimum pressure and then use the most remote area for whichever one does. For example, if the ceiling sprinklers require 30 psi and the in-racks require 20 psi then I pick the most hydraulically remote area for the ceiling sprinklers since the in-racks will over flow more water there.

I have never iterated in between the two areas like you have shown, but maybe I've been over simplifying it.

Anthony

1/23/2025 07:37:24 am

I'd agree it should be the same "fire area"

1/23/2025 08:09:16 am

This makes a lot of sense. Thanks!

Dan Wilder

1/23/2025 07:47:32 am

When this has occurred, I have always performed the calcs over top of each other as this is the situation that a fire scenario would occur in.

As the rack system is typically its own riser, the combination of flows occurs back at the riser room anyway so it's not technically wrong to run a calc in this fashion either as it would prove worst case for both systems (Furthest rack location=most feed main to location and the hydraulically most remote of the overhead) but I have run into far more questions from AHJ's about not having the two calc's over each other than the alternate, even with a narrative.

I will check with owners to see how far they expect to extending racking in this configuration and do run a check calc to check the hydraulic remote area of the overhead system and add the rack calc does indeed work and detail that info out.

1/23/2025 08:36:11 am

The visual shown is the most hydraulically demanding scenario that would allow for future flexibility if the owner/tenant ever wanted to move their racks, but what Dan said about the fire operating sprinklers in the racks and at the roof directly above is a reasonable remote area for proving a fire can be controlled for that particular rack.

Michael W Russell

1/23/2025 08:55:46 am

Tbh, the green remote area is not correct to begin with. It needs to be rotated 90 deg. and pushed to the far right, assuming the riser is in the blue, lower-left corner.

When shown properly, the overhead remote area should fall nearly right above the red, in-rack area. Seems pretty straight forward.

Glenn Berger

1/23/2025 08:58:45 am

The issue here is that both systems may be in operation at the same time and as such you need to calc the remote area that is above the racks. This is still considered one event, but requiring both systems to operate to control the fire.

Jesse

1/23/2025 12:39:44 pm

While realistically, the fire sceanrio will involve the ceiling level AS and in-rack automatic sprinklers (IRAS) in the same area, I actually provide several calcs balanced to the point of connection. I'll calc ceiling and IRAS in the same area.

But I will also calc ceiling and IRAS in each of their hydraulically remote areas, and then balance that to the point of connection.

The reasoning being, the IRAS area often their own autonomous riser. The hydraulically remte area for the IRAS may not coincide with the remote area of the ceiling AS

Pete H

1/23/2025 01:28:16 pm

Congrats, you have multiple design areas. One should be the in racks with the ceiling sprinklers above it as per the in rack design requirements.

One should be the most demanding area of the ceiling grid, proving the ceiling system works.

That said, is that really the most demanding area of the ceiling? It's not auto peaked in the middle of the grid? How many heads do you have on your outriggers?

Franck

1/23/2025 02:55:33 pm

It reminds me the good old time when we were still making hydraulic calculation by hand…
I highly recommend it to everybody as it helps to understand how it works…
Of course, this is mostly feasible with tree systems, as iterations calculations for looped/gridded systems are a nightmare!

If you take the most demanding point at the ceiling and calculate the iras below, you will normally end up with the most demanding point (total flow / same pressure) as you then need to balance your system to the same pressure.
The flow demand from the iras should be the same in all racks, but the pressure demand at BOR will be smaller for the rack below your green area compared to the rack in red area.
When balancing both systems, you will end up with a higher combined flow demand where both systems meet if you take iras below the ceiling demand, and then more friction losses from the BOR to pump house / city water supply.

All comes from Q = K sq root (P)
Q1 = K1 sq root (P1) for ceiling
Q2 = K2 sq root (P2) for iras
Calcs give you Q1, Q2, P1 & P2.
Then you can calculate K1 representative of ceiling system and K2 representative of iras.
If P1>P2 (most cases), your total demand will be :
Flow = (K1+K2) x sq root (P1)
Pressure = P1

Note that this works only if the ceiling demand effectively has a higher pressure requirement at BOR than iras.
If it is not the case, it means that your system is not well designed (too small diameter to feed the IRAS).

Best balanced systems are when both pressures at BOR have quite similar pressure. This is the best way to optimize your calc results.

Jack G

1/23/2025 06:17:47 pm

Basically perform each calculation separately and try to get each close as possible at point of connection so as to limit the overflow when you balance.
I would perform the calculation where the remotes are shown. I would question the client if he intends to add more racks and would he like to purchase an evaluation of his future plans

Separate In-Rack & Overhead in Same Calc?

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