AS3600 wall design

[Adam_2020]

Hello.
I am working on walls that are 4.4m in height. I try 200mm thick and designed them as columns and some of them fail in buckling already. However, if I design it as simplified wall, the slenderness is less than 30 so I work out the PhiNu and it pass easily.
May I ask why there is such a big difference? No moment is involved so far. Purely talking about the axial compression (buckling).
Thank you.

 

[human909]

There have been various past discussions specifically related to AS3600 wall design in this forum and also here:

https://www.eng-tips.com/threadminder.cfm?pid=744

I'm sure other can chime in with better answers, my concrete knowledge does not go too much beyond it being the stuff my steelwork attaches too..

 

[mrlm]

Be wary if this wall is designed for lateral EQ loads (which unless very specifically detailed, all walls are), section 14 of AS 3600-2018 requires a slenderness ratio of less than 20, Cl 14.6.5.

 

[Adam_2020]

Thank you for your help.
-Human909, I had tried to search related topic but so far had no luck finding buckling related posts.
-mrlm, Put seismic aside, why does 'simplified wall' method in section 11 have so much higher axial compression capacity when it fails in buckling easily already when it is treated as column? In regards to effective height, 4.4m is FFL to FFL. So if I have 400mm thick slab, does that mean I can say the effective height is 4m (instead of 4.4m) so 200mm thick should pass the slenderness ratio check? It's a typical apartment building with walls connect to RC slabs.

Thank you.

 

[Enhineyero]

Hi Adam,

The standard does allow a higher slenderness ratio for walls than columns (if I'm not mistaken, under compression there is a slight difference in buckling behaviour between columns and walls and this might be the reason behind this).

I suspect that when you calculate the capacity of your wall as a column it considers it as a slender column and when you analyse it as a wall it considers it as a non-slender wall.

If you try and reduce the height such that the slenderness is around 20 or lower, then the column and wall capacity wouldn't be too different.

Hope this answers your question.

 

[Adam_2020]

Thanks. What height do you normally adopt for designing columns or walls? The clean height or FFL-FFL? like the pic above, is it 4m or 4.4m?

 

[mrlm]

Columns and walls should be designed using Lu, "unsupported length of a column, taken as the clear distance between the faces of members capable of providing lateral support to the column." In short, 4m would be the correct height for your wall.
Apologies if I am assuming too much, but if you are just starting out in design, (not that I have much more experience) be very wary of the earthquake requirements of AS3600 for walls. It is full of traps and extra detailing requirements. I'd ensure you fully understand them or ask a more experienced engineer to guide you. I believe many people in the industry are grappling to come to terms with the new requirements and it can be a nightmare if missed.

 

[Retrograde]

I think it is just one of the idiosyncrasies of the code. You can only design using the simplified design method for non-ductile walls.

 

[rapt]

Is the wall centrally reinforced or reinforced with separate layers on each face?

We have found that centrally reinforced walls fail badly when checked for slenderness as columns.

 

[Enhineyero]

Thanks. What height do you normally adopt for designing columns or walls? The clean height or FFL-FFL? like the pic above, is it 4m or 4.4m?

I would use 4m, refer to the post of mrlm, I concur with what he said. Not sure what exactly you are doing but if you are fresh in the industry it would be best to ask your immediate supervisor if in doubt of anything, they expect new engineers to ask a lot of questions.