FRP for Sway Column to AS3600 (or any other standard)

[blihpandgeorge]

Hi

Looking at a concrete flat slab sway frame (2 storey / 550 columns / 325 flat slab / 9m grid) and note that AS3600-2009 and 2018 Cl 5.6.2 is for braced columns only, it then refers you to Eurocode or other approaches.

Looking at Eurocode EN-1992-1-2, Methods A & B both say braced structures. Appendix C also says braced structures, but has some higher slenderness values that my frame (using ke=2.2 effective length) does fall into.

I thought i would look at NZ codes as I understand concrete frames are more common across there, and NZ3101-2006 is silent on braced/un-braced when it comes to columns and fire design is left to a very simple table.


My questions are:
1) if I sit within the slenderness values of EC2 using Ke=2.2, even if they are for a braced structure, is this valid? My thinking for this is:

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- the slenderness value accounts for the braced/not braced.

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- When designing a column for strength via an interaction curve, sway effect is accounted for in the slenderness - ie there isnt a

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seperate braced and unbraced design approach in determining the interaction curve.
2) any other suggested codes / approaches?
3) general comments?

Thanks

 

[Agent666]

The slenderness stuff in NZS3101 is based on braced columns I believe, ACI has effective length values for unbraced columns if you have access to that.

For example these are the alignment charts from ACI:-

 

[rapt]

Eurocode EN-1992-1-2 with Amendment 1 (released later 2019) has tables in Annex C for sway columns.

 

[blihpandgeorge]

thanks for replies
- Agent666 these seem to give the same k values as AS3600
- RAPT perfect! i will now get a copy of A1, i was using a pre-A1 version of EC2

 

[blihpandgeorge]

hi rapt

i have purchased BS EN 1992-1-2 with amendment 1 as per your comment - thanks for heads up. Annex C looks relatively straightforward and essentially gives a limiting slenderness based on col size / FRL / loading ratio and eccentricity.

My question is to the design a rectangular unbraced column in fire, do i only need to use annex C? Or do you still need to use either the isotherm or zone method to check moment and axial. (Annex C is so straightforward i feel i a missing something using it alone!) I accept that it has limitations such as rectangular reinforced columns of certain sizes only, whereas method A and B allow greater flexibililty.

A few things that make me think it is Annex C only are:
- 4.2.1 note 2 clearly refers to Annex C for unbraced systems.
- 5.3.1 note says Method A & B are for braced structures only


thanks

 

[rapt]

You are reading the same document as me. I know no more.

My understanding is that some German Universities have used the zone or isotherm strength calculation methods to determine the capacities of sway columns under fire conditions to produce the tables in Annex C. Presumably as they have investigated the effects of strength reduction from the Annex B strength reduction methods along with slenderness calculations to give the results in Annex C.