Detection, Structural Beams & BS 5839

[Tadees]

A beam is say 300mm in depth.   The depth of the beam is less than 10% of the ceiling height.  11.2 k) 1) suggests that the detector should be sited on the underside of the beam, because the depth of the beam is less than 600mm. 

However, the next paragraph suggests a different siting methodology, i.e. should not be sited ?closer to the beam than twice the depth of the beam or 500mm, whichever is less.?

Can someone explain the apparent contradiction.

[colin todd]

Yes, I can.

[Tadees]

Hmmm.

[wee brian]

Its probably the difference between being fixed onto something rather than being next to it.

As I recall, you can get a buffer of cleaner air adjacent to obstructions (such as a downstand beam). so smoke might bypass a detector mounted close to a beam. this wont happen to a detector mounted on the underside of a beam. but you could get a delay if the beam is very deep (smoke filling down from the ceiling before it flows over the beam) so that probably why there's a restriction on the beam depth.

Of course, I wasn't on the committee so I don't know for sure.

This question has come up before - read this
https://www.crisis-response.com/forum/index.php?topic=4884.msg52092#msg52092

[Tom Sutton]

What version of BS 5839 are you using I cannot find your reference?

[colin todd]

Old CivvyFSO did a nice job in 2010, so he knows too.  And now he works for me and knows even more than he did in 2010.

[Tadees]

What version of BS 5839 are you using I cannot find your reference?

Hi Tom,

Page 30, 2013 version, although it won't surprise me if another version has come out since I last checked, which changes 3 lines and we all have to pay for the update.

[Tom Sutton]

Sorry Tadees I was looking at Part 1 not part 6, the way I read it you have to abide by both clauses. Clause 1) if the beam is vertically 10% greater depth than ceiling height, then you need the detectors each side of the beam if it is vertically less than 600mm the you can fix it to the underside of the beam  Clause 2) If you are fixing the detectors to each side of the beam they must be no closer than 500mm horizontal or twice the depth of the beam with ever the smaller. CivveyFSO did an excellent submission why.

In your case with the 300 mm beam fix it to the underside of the beam, job done.