ooooooh Richard that's a hard one!
lots of 'ifs' and 'buts'
My hero, The Kurnal, has automatically started looking at this this from his risk assessment viewpoint i.e do you actually need detection anyway? I'm assuming that this is not the question!
Clause 22.2.d of BS 5839 Part 1 2002 recommends that where a room requiring automatic detection also contains a horizontal void of over 800mm in depth, then automatic detection should also be provided in that void. However if the fire risk in the void is low then it can be omitted under a Variation (Note 4)
You ask what type of detection you require. The answer to this is always smoke detection. However, if this likely to cause unwanted alarms in normal use in that environment, then use a rate-of-rise type of heat detector. If there are quick fluctuations of temperature then install a fixed temperature heat detector instead.
Now for the question of how many detectors you require and where they should be installed, and assuming that smoke detectors are o.k., then referring to BS5839 part 1 2002;
Clause 22.3.j recommends that where the beams hang down more than 10% of the room height, each beam would be treated as a wall and therefore a detector would be required between each beam (Note 7 confirms this equally applies to a horizontal void)
However, Clause 22.3. k has specific recommendations that apply in rooms or voids where there are closely spaced beams. Table 2 that relates to this clause and recommends that adjacent smoke detectors should be spaced no more than 2.3m apart (assuming the void is less than 3m deep - if the void is deeper the spacing can be greater!). Therefore you wouldn't need a detector in each space between beams!
Please note that the above figures apply to smoke detectors only. The 8m length of the beams is too long for the recommendations applicable to heat detectors.
Hang on, I've just read your post again and I think you are intimating that you can't install the smoke detectors on the slab because of other services! Clause 22.3.f states that in unventilated voids the detectors must be sited in the top 10% of the void or top 125mm (whichever is the greater). You've got a problem.
I told you it was a difficult one.
I'm going home - my head hurts.
I'll leave it to my hero, The Kurnal, to sort this one out! Hopefully he'll tell you that you don't need detectors anyway! Problem solved?
