Fire resistance of building structure over 18m high

[hammer1]

Fire resistance should be 90 minutes for buildings over 18 m high. However what if the structure is only 19.7m high is 60 minutes sufficient.

Would fire engineering structural analysis solution be required to back up the 60 minute fire resistance and therefore pass AI/Building Control.

[CivvyFSO]

19.7 > 18 so I would say it is over 18m high, therefore make it 90mins as stipulated.

There is good reasoning behind the increase in FR at 18m. The same reason firefighting shafts are provided. A fire would be predominantly fought from inside the building, (As the standard 135 ladder will not be of any use) the 90 mins is really to protect firefighters who will be inside the building. Also bear in mind that the presence of a FF shaft means that there is often more of a delay in the crews setting up, so it could easily be 30-40 minutes from the time of the call until the crews are in place and ready to start squirting some water. 20-30mins is not much time for dealing with the fire, performing searches, damping down etc.

If a sprinkler system is provided (Or a SHEVS) then you can quite reasonably go down the time equivalency route. It is an area of engineering not many people go near for some reason. (I think it is because it looking at heat transfer and it is not easy, and not much work has been done on measuring the actual cooling effect that a sprinkler spray has on a fire) What you are basically trying to prove is, for instance; With a sprinkler system provided (life safety would be necessary) and just 60min worth of protection the structure would react to fire no worse than if there was no sprinkler system and 90mins protection.

Also remember that similar to doors, the 90 mins is not something that will stay up for 90 minutes and falls down, it is something that survives the BS476 test for 90 minutes.

[CivvyFSO]

And that 18m is to finished floor level above fire service access. Is the 19.7 to finished floor?

[hammer1]

SHEVS are located on the firefighting shaft provided, as are 3 dry risers to the 3 staircases.

I have been informed that they will be working in line with Eurocode EN 1991-1-2:2002

I quote

The Eurocode approach may be used where the design of members is based on tabulated data or other simplified rules, related to the standard fire exposure as is the case with ADB.

My response

Come again???

[CivvyFSO]

The eurocode addresses it from the angle of an actual growing fire, and its effect on the structure. It is many many equations and it takes account of ventilation, fire loads, wind etc etc etc.

Here is a small excerpt of the type of stuff it deals with:



OK? 

It is an acceptable and underused method. (Probably because in the average project the amount of money an engineering company would charge for this will often negate any savings made by being able to buy 60min FR instead of 90min FR). Be prepared to have the wool pulled over your eyes though. (I am not assuming all Fire Engineers talk twaddle, this is 'just be prepared' of course) If you are uncomfortable with it then the best thing you could do is to insist that a third party engineering company verifies it, after all with something so complex it is easy to make a (potentially costly) mistake.

[hammer1]

The eurocode addresses it from the angle of an actual growing fire, and its effect on the structure. It is many many equations and it takes account of ventilation, fire loads, wind etc etc etc.

Here is a small excerpt of the type of stuff it deals with:



OK?  

It is an acceptable and underused method. (Probably because in the average project the amount of money an engineering company would charge for this will often negate any savings made by being able to buy 60min FR instead of 90min FR). Be prepared to have the wool pulled over your eyes though. (I am not assuming all Fire Engineers talk twaddle, this is 'just be prepared' of course) If you are uncomfortable with it then the best thing you could do is to insist that a third party engineering company verifies it, after all with something so complex it is easy to make a (potentially costly) mistake.

Hence my come again   comment

I have in front of me a couple of pages of the equations you have exampled. Surely it will need to be verify by building control before passing it. The thing is I cannot understand why they did not go for 90 mins FS and be done with it, it is not exactly a over complex building with a wow factor.

Would the construction/architect or fire engineer company be liable if it was twaddle and it has disastrous effects and not the person taking over the property ??

Think I stick to the management side of Fire Safety thank you very much.

[deenee]

The thing is I cannot understand why they did not go for 90 mins FS and be done with it

Becasue probably they have the calcs on spreadsheet anyway, so all they need do is type input data and, pronto, got the result.
.....and the BS 9999 table 26 is based on derivations from the said BS EN.
I would have thot if they quoted BS 9999 instead of that long complication, it will be favourably received. Offcourse they cant cherry pick 9999, so going all the way.

[wee brian]

It depends on what the extra 30 mins costs.

If we are talking intumescent coatings then it may be worth making the effort.

It may also be because they cant fit the thicker boards (if they are using boards) inside whatever pretty cladding they a putting on top.

[CivvyFSO]

IMO Building control would be essentially liable, as they are the ones who ultimately issue a certificate to say it is safe and complies with the regs. (I am sure the Fire Service would come under media scrutiny too) From law the engineer would have a clear duty of care due to their position, so anybody who suffers a loss due to 'mistakes' made could potentially make a civil claim against the engineering company (Or even building control again). They would then have to prove on balance of probabilities that they performed their professional duty properly. (Toddy or someone might clarify this, or shoot it down in flames. I am no legal expert)

Re: Why?... Fire engineers have to justify their fees somehow. I often see problems 'solved' by fire engineers with a 40 page strategy, where a 'normal' consultant (the likes of which we see here) could have come up with the same final solutions over a cup of coffee and a biscuit without going anywhere near a spreadsheet or CFD. Reading off a table in BS9999 is far too easy, and doesn't justify the cost, and it wouldn't bamboozle the authorities.