Variable Beam Height For Smoke Detectors

[JoeH78]

Dear ALL,

How we are supposed to determine the beam height if ve have the beam which has variable height in longitidual direction. This directly will affect the D/H ratio which is one of the governing case for smoke detection installations, but I couldn't found in elaboration on that in NFPA 72 Chapter 17, your guidance will be appreciated.

 

[cdafd]

Install per::??


For ceilings with beam depths equal to or greater than 10 percent of the ceiling height (0.1 H), the following shall apply:
Where beam spacing is equal to or greater than 40 percent of the ceiling height (0.4 H), spot-type detectors shall be located on the ceiling in each beam pocket.
Where beam spacing is less than 40 percent of the ceiling height (0.4 H), the following shall be permitted for spot detectors:
Smooth ceiling spacing in the direction parallel to the beams and at one-half smooth ceiling spacing in the direction perpendicular to the beams
Location of detectors either on the ceiling or on the bottom of the beams
*For beam pockets formed by intersecting beams, including waffle or pan-type ceilings, the following shall apply:
For beam depths less than 10 percent of the ceiling height (0.1 H), spacing shall be in accordance with 17.7.3.2.4.2(1).
For beam depths greater than or equal to 10 percent of the ceiling height (0.1 H), spacing shall be in accordance with 17.7.3.2.4.2(2).


Can you post a simple diagram of what you have

 

[JoeH78]

Please check the precacst girders marked with blue arrow below, they have variable height near the column they are 46cm and at the mid span 165cm, in order to decide the beam height how about if I take the average of variable height of that precast girders as (46+165 ) = 105cm ( beam height )

Bottom level of precast girder is: 800cm
Ceiling Height at mid span is: 1050cm

 

[cdafd]

I am conceptually challenged

But seems you would put them at or near the peak

And than space according

Not near 72

https://ams1.startpage.com/av/anon-...ee60001ddf4d66486e4be000a6a444e5951377ba3f014

 

[JoeH78]

ok thanks for helping,

then each trapezoidal corridior in transversial maner will be treated as pocket and smoke detectors need to be provided the per pocket, total length for each bay is 120mt.

Or If we can use the air sampling which I believe is very expensive,

which one would you use if you were in such situation ?

 

[cdafd]

1. In general what will they be doing in the building//// what process??

2. Fire sprinkler system?? If so wet?

3. I am thinking no on air sampling

4. Are the smoke detectors required by code or contract or owner wants them?

5. Can I ask how you are involved? Engineer ,, fire alarm company, other???

 

[JoeH78]

> 1. In general what will they be doing in the building//// what process??
Generally assmbly, copper welding, storage of refrigerators

> 2. Fire sprinkler system?? If so wet?
yes

> 3. I am thinking no on air sampling
Mee to because I've done air sampling on very specific occasions which requires determining the very rare gas or something

>4. Are the smoke detectors required by code or contract or owner wants them?
Requested by mechanical engineer but not clear justificiation why simply they choose only the highest point ?


> 5. Can I ask how you are involved? Engineer fire alarm company, other???
yes I'm consultant engineer actually structural

 

[cdafd]

Hummm seems like might get false alarms with smoke detectors???

They don’t want duct detectors???

Or

Are there HVLS fans involved and need to shut them down??

Maybe need a discussion on why wanted and where?
If installed should meet NFPA 72


Have you done beam detectors before?

Do you have a fire alarm buddy to consulte?

 

[JoeH78]

Hummm seems like might get false alarms with smoke detectors???

They don’t want duct detectors???

Actually we don't have the duct system in the plant except the office area but that is very small when compared to whole factory area

Are there HVLS fans involved and need to shut them down??
Maybe need a discussion on why wanted and where?
If installed should meet NFPA 72

They are kinda like "we are more experienced whatever we say has to be done" manner, and I'm trying to prove them to be wrong or at least I'm trying to learn which is correct and cost efficient way

> Have you done beam detectors before?

yes this is very common in this region and I believe is very cost effective I really wonder why they suggested to use air sampling cause I really have not done it for last 10 years in nearby.

> Do you have a fire alarm buddy to consulte?

For the time being no