AS3600 - Section 5 Fire Design

[Trenno]

Hi all,

Just wanted to raise a point with the current (amended) Section 5 of AS3600.

Currently there is a big discussion in the office, about 600 x 300 columns in the basement of a 5 storey apartment complex.

"Back in the day" this column size was pretty stock standard, however now according to the new Section 5, the minimum dimension now needs to be 350mm for 2 hour fire rating with neta about 0.5, which of course isn't ideal from an architectural and parking space perspective.

The senior engineers think that the fire section just keeps getting more conservative with each amendment and update.

What are your thoughts on Section 5?

Nearly all carparks up until last year would technically fail the current fire section of our code (with 600 x 300 columns).

 

[rowingengineer]

I guess normally you would have a braced column structure rather than a sway frames. Technically you could go to 5.3.1, as this may provide a better solution rather than the tabulated values. We haven't had to many problems with the fire design after they amended by the code committee before this they were unusable.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[Trenno]

So you are saying, try the EC to try and bring the FRP down, and thus allowing smaller minimum dimensions?

 

[rowingengineer]

We also now spend a lot of time making sure we have the right FRL, sometimes a few minor amendments can get the FRL from 120 to 60 or 90.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[Trenno]

Please share your methods of bringing down the FRL - without engaging a fire engineer?

 

[rowingengineer]

In the DTS provisions of the BCA there is generally concessions for carparks depending on the building classification and type of construction, I am assuming your in a type A building with a basement carpark. So in this situation the carpark wouldn't have the fire generating capabilities of the floors above, so you could reduce you columns require FRL's to 60 or 90. But without knowing the building well I am pissing into the wind. Best to talk to the building cert during the preliminary design stage to establish the requirements. The DTS rules can be interesting when combining different classes.

as to the other question:
As3600 Cl5.3.1 will send you to the euro-code where they will show a calculation of amount of ignored concrete which sets the a depending on grade of steel ect. This will enable you to have small sections however you may need larger covers and more steel. Basically it will allow you to have more options than the table allow.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[Trenno]

Thanks for the help RE.

Unfortunately, it's a Class 3 building with more than 3 storeys (Type A), thus the basement carpark doesn't comply with the relevant concession. I don't think it matters much anyway, a fire engineer will probably be engaged anyway.

 

[hokie66]

My favourite size for columns in this situation is 400 x 500, so I would be fine with the new provision, if it is new. I seem to recall in a previous version that 400 was required with 40 cover for a 2 hour rating, so has there really been a big change?

 

[ooox]

If your basement car park is sprinklered there's concession in the BCA to reduce the FRL - check with your BCA consultant.

Also, if your columns are braced and less than 3m you can assume an effective length of 0.5Lu in determining phi Nu which increases the capacity by about 20% (if you have a slender column) and hence will reduce your neta.

 

[asixth]

Where do you get the "assume an effective length factor" of 0.5. That doesn't sound right. I can't see how it can go below 0.7.

 

[ooox]

asixth,

Cl 5.6.3 (AS3600:2009, Amendment 2)

 

[asixth]

Your kidding? It actually says that

 

[ooox]

Asixth,

Sure does, this is the wording in the code:

"The effective length of a column under fire conditions may be assumed to be equal to the effective length at normal temperatures in all cases. For braced building structures where the required FRP is greater than 30 min, the effective length may be taken as 0.5Lu for all cases"

I'd be interested to understand the rationale when/if the Commentary is released.

 

[rapt]

It comes from Eurocode. You will have to ask them the rationale!

 

[Trenno]

Probably a stupid question guys - but what exactly is the Phi Nu in Clause 5.6.3?

From my understanding Nu varies with ku and thus isn't a single figure like Nuo or Nub?

 

[rapt]

Trenno,

Plot your interaction curve for the column under standard temperature conditions. Then draw a line from 0.0 through your design M*, N* point until it crosses the Interaction curve. It is the value of phi Nu at the intersection point

 

[Trenno]

Or in other words RAPT's little pink line!

Thank you for clearing that up.

 

[Trenno]

Can anyone suggest where I can get my hands on the Eurocode 2, Part 1-2: structural fire design ?

 

[asixth]

Is this it? Remember to add me as a connection on linkedin.

https://www.dropbox.com/s/gyw1dilnp...General Rules Structural Fire Design.pdf?dl=0

 

[Trenno]

Thanks Asixth, that's it!

Just quickly perusing the document, seems as though alot of our fire code is an adaption of EC2...

I'm looking for methods to justify more slender elements (particularly walls + columns), will EC2 methods help achieve that?

I notice AS3600 formulas look similar but there are some additional multipliers/factors in there that push it to being more conservative?

(Also - the link only provides the PDF, I can't find anything that helps me add you on Linkedin!)