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RC columns that do not meet the 4:1 section aspect ratio requirements for 5.7.1 design!

RC columns that do not meet the 4:1 section aspect ratio requirements for 5.7.1 design!

(OP)
Hey guys/gals

I'm currently doing my final year engineering thesis on a common problem that I have come across in regards to RC column design for fire resistance. I would like to provide a comparison between industry practice.

The problem is this: due to AS2890.1 requirements for structural member clearance around a parked vehicle within a car parking structure, the firm I have been working at for the past 7 years (started drafting with them) are presented with 1000mm x 300mm columns for design from Architects. Designing this column using section 5, clauses 5.6.3 and 5.6.4, fails the fire resistance requirements (120mins). Clause 5.7.1 can not be applied due to the section not having a 4:1 aspect ratio.

If you reduce the dimensions to say, 1000mm x 250mm (which is inherently weaker), the column can be designed for fire resistance using clause 5.7.1 and passes the fire check!

I am wondering what the industry practice is for designing this type of column (aspect ratio of less than 4:1) for fire resistance when section 5 of AS3600 can not be applied.
I fail to believe that every engineer is designing these types of columns using EN1992-1-2 (European code).

Any info you can give me would be greatly appreciated!

Any

RE: RC columns that do not meet the 4:1 section aspect ratio requirements for 5.7.1 design!

5.6.2 Structural adequacy for columns
The FRP for structural adequacy for braced columns shall be determined using either
Clause 5.6.3 or 5.6.4. Where the ratio of the longer cross-section dimension of the column
is equal to or greater than 4 times the shorter cross-section dimension, Clause 5.7.2 may be
used.

NOTE: Clauses 5.6.3 and 5.6.4 cover only braced columns that comply with a series of
restrictions. For unbraced or sway columns and braced columns outside these restrictions, see
Clause 5.3.1 or the BCA and use an alternative solution.
Where columns are to be designed as walls using Clause 5.7.2, the case of a wall exposed
on two faces shall be adopted and the column shall be reinforced with two layers of
longitudinal reinforcement (one layer located adjacent to each face), and the two layers
shall be structurally restrained together.

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."

RE: RC columns that do not meet the 4:1 section aspect ratio requirements for 5.7.1 design!

I think fire design of RC columns isn't well understood and probably needs a total rework of the existing code section.

I looked into the Euro codes earlier this year attempting to justify a 200x1000 column, but ended up opting against it due to buildability issues.

RE: RC columns that do not meet the 4:1 section aspect ratio requirements for 5.7.1 design!

This is the problem when people introduce rules that have limits like this one.

The 4:1 rule was removed in 1984 because it was not logical and resulted in designers specifically selecting columns to fit the rule, eg 800 * 190 columns were the rage for a period.

It was not in the original Eurocode document that the current AS3600 rules are based on.

Unfortunately, it was decided to add it again based on its presence in the old code by people who did not realise why it was removed in the first place.

RE: RC columns that do not meet the 4:1 section aspect ratio requirements for 5.7.1 design!

Its very frustrating. Especially since architects do not know about the influence of fire in design. A lot of the time you are restricted in basement parking to max 1000x250. This can be quite problematic for large loads.

RE: RC columns that do not meet the 4:1 section aspect ratio requirements for 5.7.1 design!

It now quite essential that you go to the Eurocode if you need to design slender columns or blade columns, designed as walls. This will allow any column size or load to be accurately assessed.

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