Step between accommodation and basement car park area.

[BCO]

Looking for a little assistance with the theory behind the step between areas where vehicles may be present and accommodation. I understand it is to stop fuel from running along the floor and entering the general accommodation. Also that a slope can be provided as an alternative. However is fuel leakage such problem in modern vehicles that these types of passive provision are still required?
Thanks in advance.

[Phoenix]

It's not just fuel, it's the fumes as well. Petrol fumes are heavier than air and a small spill can produce a lot of fumes that will move across the floor a bit like a liquid will.

These modern cars, does everyone have one? Or do some people still have older cars?

Stu

[BCO]

Good point about the cars. But taking fuel leakage as a general issue, is it a problem that is encountered operationally. Are there any cases we know of?

[kurnal]

I would imagine leakage is much less of a problem than in the past, when cars universally had steel tanks and fuel lines. The step between house and integral garage has been a requirement since the earliest building regs as far as I remember. A sill will do instead of a step.

On the other hand and to some extent as a possible balance, unleaded petrol is supposed to be harder to extinguish than leaded petrol due to benzene additives, dont know if this is still the case.

Would you also consider a review of the bund tray in a flamms cabinet now winchester jars are usually not made of glass? I think its the same principle.

[BCO]

Do bunds have additional height for a vapour layer? HSE suggests 10% additional capacity for a layer of foam and turbulence in large bunds but no mention of a vapour layer height. Would be intresting to understand how the figure of 100mm in ADB or 150mm in section 20 for the step was arrived at.

[kurnal]

I was always told it was just to stop the risk of liquid fuel flowing under threshold of the fire resisting door. After all theres invariably a gap- max 8mm in a BS8214  door if I recall.

[Tom Sutton]

After all theres invariably a gap- max 8mm in a BS8214  door if I recall.

Not anymore Kurnal BS8214 has revised its guidance.

9.5.3. Under-door (threshold) gaps

Under-door (threshold) gaps should be in accordance with the manufacturer’s installation instructions for the particular doorset design.
When fitted, smoke seals should give an even contact with the floor but should not exhibit significant increased frictional forces that could
interfere with the closing action of the door (see BS 5588-11).

[Davo]

Tom is right, BS8214 has been dumbed down a la HSE guidance


davo

[Tom Sutton]

Agreed not nearly as informative as the previous one.

[kurnal]

Bottom line is liquid will always be able to run under a fire door. Hence the step.

[wee brian]

In answer to your question - how big a problem is it - The answer is that we probably don't know.

When integral garages first became common this was one of the precautions that they thought was a good idea. whether it has ever made any difference or whether miising steps have resulted in fires etc. I don't think we can be sure.

[BCO]

Thanks for the posts. Looks like it may be one of those dimensions that seemed like a good idea at the time. Or it may be because it generally equates to one course of brick. Either way it’s not clear whether a vapour layer depth is the driver for the 100mm step.

[kurnal]

Just to try and get a handle this I will offer the easy bit, the average garage is 15 sq m and the average car has a 60 litre tank equates to a 4mm depth of liquid in the pool assuming a level floor.

Now who can:
1- Quantify the top of the vapour at normal temperatures and pressure?
2-Whether the vapour pressure is likely to push the vapour through the gap at the bottom of the door?
3- If there was an ignition source on the other side of the door would it be likely to ignite the vapour cloud in the garage assuming there was not an explosive atmosphere in the house at the time? Or would the crack act like a Davey Lamp?

Todays trivia challenge.

[wee brian]

http://www.bre.co.uk/filelibrary/pdf/rpts/partb/effectiveness_of_100mm_upstand_1.pdf

[kurnal]

Wow Thanks Wee B. A mine of information as always.

Now what about LPG vehicles and gas leaks?