Compartmentation in shops

[zimmy]

Hello,

I would appreciate any advice or opinion on the following in order for me to formulate a response.

I am an FRS FSO and have received a Building Regs consult from an AI for a shop premises. The proposal is to demolish existing compartment walls in an unoccupied shop premises to create one large compartment of approx 3800 sq m. The AI is claiming that it satisfies the functional requirements of ADB by offering an independant Computer Fluid Dynamics model showing that the compartment performs better in fire conditions than a compliant model of 2000 sq m.

I have a few issues with this. Clearly, it does not satisfy ADB, being almost double the floor area for a single storey shop. Looking for leeway in BS9999 doesn't help as the standard here is the same.

The CFD models used are quoting a t2 fire growth rate as 'medium' and both BS7974 and BS9999 suggest that a shop should be a 'high'  growth rate. This may not matter as the model is comparative between a compliant and non-compliant compartment.

In my opinion, the proposal does not offer any compensating feature, but merely shows the recognised standards to be not applicable. I therefore have a dilemma, to agree the proposal and accept a shop compartment double the size with no compensating features, which may set a precedent and open the floodgates for any shop to do the same, or disagree with the proposal, leaving the FRS to take retrospective action under the FSO which could be problematic and no doubt lead to an appeal.

Does anybody have experience of this, or has anyone allowed increased compartment sizes in similar situations and if so, under what conditions?

I'm grateful for your thoughts.

[John Webb]

The various shop fires I recreated when working at Fire Research showed t² growth generally of 'high', hence that being what is quoted in the standards, I suspect! I would therefore suggest that you should require them to run the CFD model using the faster growth rate.

I wonder if the CFD model appears to give a better result because in the early stages of the fire the gases have more room to spread out and therefore the smoke layer stays at a higher level during the likely evacuation period?

Sprinklers, particularly fast-response ones, should adequately compensate for a larger area.

Is the shop a 'stand-alone' or part of a complex at all? If the latter then the CFD ought to show if any smoke control/dispersal system can cope or not with the potentially bigger fire from a larger floor area.

[zimmy]

John,

I will indeed ask for the models to be run at a higher growth rate but suspect that as they are comparative, it may not matter what rate is used if the oversize compartment performs better than the compliant one.

My understanding of the model is that the better performance is achieved by the fact that the higher than average roof height can accommodate the smoke and heat for the required time before losing bouyancy. Unfortunately I'm not a qualified engineer and can't fully analyse the results, but it would seem that the standards are based on floor space where they should possibly be based on compartment volume.

I suspect the whole issue is being proposed to avoid the installation of sprinklers, which would allow an unlimited floor area.

The building is a single storey detached, with no other buildings in close proximity. No form of additional ventilation is proposed.

Thanks

[CivvyFSO]

Computational Fluid Dynamics will certainly show that a large volume helps in the evacuation phase, but that is not what compartmentation is all about. Compartmentation is there for a few reasons. One reason is to limit the size of the fire that the fire service will have to deal with, another is to limit the number of people exposed to a fire.

The model should definitely be run at a fast fire growth, and it should be run way past the normal evacuation period, if the fire resistance of the compartment wall was 1 hour, then the model should be run for that same period of time. If more than 2,000m2 is damaged, then they have not acheived the same goal.

Regardless of what the AI says you can enforce suitable measures for reducing the risk of spread of fire under article 8. Tell them you will do this, (Keep good records of any such communications) then if it goes ahead as described, reinstate it afterwards through enforcement. They can appeal the notice, and if the secretary of state decides that it is ok as it is, then at least the decision isn't on your shoulders.

PM me if you want, I may be able to assist more.

[CivvyFSO]

They should also take into account the potential for flashover in the extended timeframe.

[Fishy]

Absolutely... define "...the compartment performs better in fire conditions..."?  In the initial stages of the fire, perhaps it does (bigger = greater dispersal volume might = longer to untenable conditions)... but means of escape will take longer and this misses one purpose of compartmentation - which is to limit ultimate fire size - See Section 8 (Compartmentation) in the E&W AD-B - particularly 8.1.  I'd intuitively be very surprised if the conclusion of the model was that the peak fire size in the larger compartment were lower than in the smaller...?

I'd also not assume that a different choice of fire growth rate will have the same effect on results, for comparison purposes.

Would be interesting to hear more details of in what respect the results were better?

[SamFIRT]

Would be interesting to hear more details of in what respect the results were better?

me too 

[zimmy]

Fishy,

As you suggest, the results are based on teneble conditions at 2m height taking longer in the larger compartment - common sense i know, but also claim that as travel distances are compliant and both compartments are smoke logged by anticipated FS arrival time, these are not applicable. The temperature at 2m is also lower. There is no reference to peak fire size in the results, but a t2 medium growth rate reaching a peak of 15mW after 15 mins is quoted in the parameters for the models.

[CivvyFSO]

Part of the functional requirement B3 is:

Where reasonably necessary to inhibit the spread of fire within the building, measures shall be taken, to an extent appropriate to the size and intended use of the building, comprising either or both of the following:

a) Sub-division of the building with fire resisting construction
b) installation of suitable automatic fire suppression systems

Also ADB 8.1 states that the object is twofold:

a) To prevent rapid fire spread which could trap occupants of the building; and

b) to reduce the chance of fires becoming large, on the basis that large fires are more dangerous, not only to occupants and fire and rescue personnel, but also to people in the vicinity of the building.

[CivvyFSO]

And a t² fire with a fast growth rate is at 38MW by 15mins.

Look at what perimeter they have used too. A growing fire has an ever-changing perimeter. (The above calc indicates a perimeter of about 30m) A quick spreadsheet I set up for a 6m high building of this area has the potential to be at a stage conducive to full involvement / flashover at about 8 minutes, so the perimeter given is largely academic as the whole area could be involved. This will clearly be a longer time than the smaller compartment would take, but the difference is the size that the fire can be. Your version is almost 4,000m² of fire, when the code compliant version is 2000m² of fire.

[Phoenix]

I would whole-heartedly throw my weight (which is sadly increasing) behind Civvy's and Fishy's comments.  The AI and the fire engineers here are quite simply talking rubbish.  They have displayed a complete lack of knowledge and understanding of the aims of the building regs and fire safety legislation.  They are buffoons of the first order and you can tell them that I said so.  I have seen so many similar displays of incompetence that this doesn't surprise me but it does dishearten me slightly.  I'll bet I, and some others on here, could have a good guess at who the AI and fire engineers are.

I was just about to post the quotes that Civvy just posted - don't have to now.  The size limitation is nothing to do with protecting means of escape (not directly anyway) or maintaining smoke at a certain height, it's about keeping the fire to a reasonable size for a reasonable period of time.  As Civvy's already said, to demonstrate that their building is as safe as a code compliant one they would have to show that the fire will not be larger than 2,000 sq m in area after an hour of burning.  How will the fire know to stop spreading when it reaches 2,000 sq m?  I have a sneaking suspicion, it won't!

The proposal is nonsense and I would not hesitate to reject it as unsatisfactory.  Frankly, it's reckless, arrogant and even insulting for the proposers to even put this forwards.

I suggest you get a big red ink pad, a big rubber stamp that says, "b[censored]s" and that you stamp each page of the proposal before sending it back.

More seriously though, you should heed Civvy's advice to record all correspondence concerned with this proposal so that your route to enforcement, if it goes ahead as proposed, is supported and eased.  Civvy's said it all really, don't know why i'm bothering - just wanted to support his comments because the affrontery of proposals like this irritate me.

Stu

[BLEVE]

In the case of a fuel controlled fire, based on an assumed 550 kW/m²the difference in heat release rates between the two compartment sizes are considerable:

Compartment of 2,000m² reaches 1.1 GW.

Compartment of 3,800m² reaches 2.09 GW.
 

[CivvyFSO]

And don't forget to complain about the AI. He is either blatantly lying in order to get the work, or he is too stupid to be allowed to make such decisions.

[thebuildinginspector]

And don't forget to complain about the AI. He is either blatantly lying in order to get the work, or he is too stupid to be allowed to make such decisions.

Completely agree - I work for a large AI - speak to ACAI or the Construction Industry Council (the body that issues our licence to operate)

Sadly, as an industry this kind of thing, where people don't know there limitations, have insufficient knowledge or cynically/weakly will accept anything a client proposes to them, is becoming all too commonplace. 

"Depressed" of Altrincham

[Fishy]

A thought - not sure where you are, Zimmy, but some of the larger Metropolitan fire authorities have specialist Fire Engineering departments whose job it is to review/interrogate fire engineered solutions (I'm being generous) such as this.  I know LFB does, for example.  Might be worth a call to see if they'll share their knowledge, over the 'phone or even take a quick look at the proposals for you?