Building height

[lyledunn]

Does anyone know how the requirements for the fire resistance of floors were arrived at? I understand why, but there seems to be a rather arbitrary approach in terms of stated values. A building at 17.99m requires 60min but one at 18.01m requires 90min.

[Fishy]

I'm pretty confident that the choice of 18 m (no prizes for spotting that this is roughly equivalent to 60ft) wasn't 'arbitrary' - there will have been a reason for it, though that reason may not be apparent.

The issue you raise is common to more or less any standard or guidance that quotes a measurement - if you have a benchmark recommendation then at some point a small increase in a dimension may result in a significant change in performance requirements.  It's just the way that standards work.

[lyledunn]

I understand that 18m is about the height at which rescue could be effected from standard ladder arrangements on fire engines. Rescue and firefighting might then involve firefighters going in to a building and up a number of storeys so it is not unreasonable that structural integrity is increased with building height and other measures are required. What I am interested in is the origin of the requirements in minutes. Why 60 or 90 and not 57.6 or 79.7? That is what I mean by arbitrary.

[Fishy]

It's simply because they're round numbers relating to the significant fractions of an hour. 

The important thing to remember about fire resistance ratings is they have precisely nothing to do with predicting how long something will last in a 'real' fire - they're simply how long the element lasts if you subject it to the furnace conditions mandated in the test standard(s).  The furnace time/temperature curve looks very little like any real fire that would be likely to occur - it's pretty much what you get if you pump a constant amount of heat into an insulated metal box.  That's why engineers who try and use fire resistance ratings in their ASET/RSET calculations are simply demonstrating their lack of knowledge of the basis of the tests - despite the fact that almost every fire resistance test report will include a statement that the results don't indicate the element's likely performance in a real fire!

The ratings are therefore a means of ranking the expected performance in a fire - you don't know how long these things will last, but you know with a reasonable degree of certainty that a 60-minute fire resisting structure will last longer than a 30-minute F/R structure (floor, wall, door, whatever).  Our 'code' writers therefore make decision as to how important they think an element is in a fire, and correspond that with a place in the ranking (30, 60, 90, 120, 180 etc...).  Insurers do the same, but generally with higher ratings.

It's a bit more scientific than my trite explanation - there has been research done to relate fire load to expected structural fire resistance performance, for elements with a known test rating, and it has demonstrated that using the above methodology will tend to give you a 'safe' design, for the most common types of building element and fire load density.  There's also the fact that every country in the World who has a building code uses the same methodology (and pretty much the same test regime, with the exception of some specialist applications like offshore & road tunnels), & it seems to work!

[John Webb]

I think much of the linking of height and fire resistance requirements goes back to the "Fire Grading of Buildings, Part 1" published 1946 as Post-war Building Studies No. 20, and followed by the BS Code of Practice CP3 published in various parts in 1948, 1962 and 1968. Read and Morris's "Aspects of fire precautions in buildings" (BR137, ISBN 0 85125 361, 1983) goes some way to explain how things developed.

[colin todd]

wheeled escape plus addtional ladder or hook ladder -you must remember hook ladders Johnny.

[Fishy]

wheeled escape plus addtional ladder or hook ladder -you must remember hook ladders Johnny.

I've heard this attributed before - curiosity caused me to review PWBS a couple of years ago to see if it were written down & I couldn't find this stated anywhere, except when the 'cut-off' screen is provided in the staircase.  42 ft is mentioned (50 ft wheeled escape at its working angle minus 5ft to allow for floor to window cill height and 2ft projection).

[lyledunn]

I will just give you the update on my project. New hotel in refurbished office building, top floor height around 20m. Fitted with 60min ceilings as specified as sprinklers were intended to be fitted. Turned out that the power supply for the pumps was not available so sprinklers did not go in. Build mistakenly continued with 60 min ceiling spec (2 layers of Soundbloc plasterboard on MF system with 100mm mineral insulation below an existing ceiling which comprised circa 250x50mm joists at 300mm centres with 40mm t and g hardwood floorboards over). BC will not approve even though engaged a respected fire engineer who set out the case that the composite make up would achieve 90mins. BC say that the Hackett Report casts doubt on desk top studies. That may be correct for high rise residential buildings but this is only a 5-storey hotel.
So we need test evidence of fire resistance. That is likely to be a delay of at least 6 months. That would be intolerable. So we are left with the only option of tearing down the ceilings in 50 bedrooms and replacing the Soundbloc with Fireline. That is a disaster in a hotel that is almost ready for the soft furnishings!
Perhaps you will understand why I was trying to establish the reason for the 18m rule posed in my original post.
I don't know what Mr Todd is on about with his wheeled escape and hook ladder but it doesn't sound a substantial enough reason to wreck this hotel!
By the way Fishy, many thanks for your comprehensive reply. If I could establish the level of increased risk that exists with our hotel being 20m to top floor and having at least 60 min fire resistance against a code compliant building, I would perhaps have more heart in embarking on this demolition job.
I should point out that all other structural elements are 90min and each of the bedrooms are constructed as 60 min cells.

[Messy]

Sounds like a nightmare job. I bet you will be glad to put this one behind you!
I understand the developer is threatening to pull out of other developments in protest. That is not good news for anyone is it? A little bit of pragmatism goes a long way sometimes

[colin todd]

Davey, you remember hook ladders.  Tell Tate & lyle about how you trained on them as a young recruit in that is now the 2nd finest FRS in the whole of London.

[Fishy]

This reflects our own experiences post-Grenfell - building control bodies can be far pickier about evidence of fire performance in particular, and less flexible generally when it comes to risk-based justifications of 'variations' from codes & standards.  It's all part of the new landscape we find ourselves working in - and the changes have only just started, in my view!

Hackitt's criticism of BS 8414 / BR 135-based 'desktop studies' was probably fair (so far as I understand written technical opinions were being given by people who'd never even seen a BS 8414 test rig), but to regard all 'assessments' (no-one in that part of the industry calls them desktop studies) with the same suspicion shows a lack of knowledge about how the passive fire protection industry works.  They're integral to the product certification system & provided that they're produced by someone with substantial direct experience of the relevant fire tests, there's nothing wrong with them.

[Messy]

Davey, you remember hook ladders.  Tell Tate & lyle about how you trained on them as a young recruit in that is now the 2nd finest FRS in the whole of London.

You really need to go a little easier on those single malts Colin. Sometimes you don't make a lot of sense to others who are sober

[Owain]

So we are left with the only option of tearing down the ceilings in 50 bedrooms and replacing the Soundbloc with Fireline. That is a disaster in a hotel that is almost ready for the soft furnishings!

Would it be possible to retrofit the Fireline over (under) the existing finished ceilings? Still work, but a lot less disruptive and messy.

[lyledunn]

Owain,
Their is no British Gypsum test for that arrangement. In any event, the metal frame has to be set for the weight of the plasterboard. The current frame layout will not support the additional weight. It is not a matter of additional hangers, the actual frame members have to set at maximum distance specifications. Thank you for the suggestion anyway.

[Bill J]

wheeled escape plus addtional ladder or hook ladder -you must remember hook ladders Johnny.

Would this be an example of such ladders?

https://www.facebook.com/BritishFilmInstitute/videos/349802649007733/

Bill