AS3600:2018 Clause 11.5 Simplified Method for Walls Subject to Vertical Compression Forces

[RayGon]

Hi All.

I am curious where the 3.0MPa limit is from in Clause 11.5.2(b) for walls.

I expected this limit to be a factor of the concrete strength (e.g. 0.03f'c in Clause 11.7.1(a)).

Anybody knows where this limit from?

 

[rscassar]

Just read over the code clause, the 3.0MPa limit is in place unless the reinforcement is provided both faces and equally distributed. Earlier last year, the Australian Concrete Institute ran a national series covering changes to AS3600. I remember it being discussed the need for walls to have reinforcement each face for concrete confinement. It came as a result of Wellington Earthquake.

Don't know why the number 3.0MPa was arrived upon.

 

[RayGon]

Hi rscassar! I hope you had a good break. Looking forward to a good New Year.

First day at work today.

Yes, I figured that it is a limit for the confinement requirement for the horizontal loads.

However, what I fail to understand is the the 3.0MPa is not shown as a factor of the concrete strength used in the wall.
The 3Mpa represent on 25MPa concrete the stress level is 12%f'c while if a 45MPa the stress is only 6.6%f'c. All other limiting compression stresses in the code are a function of the strength of the concrete used (i.e. Design for Fatigue Section 18 Clause 18.1 and 18.2, Plain Concrete Section 20 Clause 20.3 and other sections). The range of compression stress levels used in these sections averages somewhere at 0.4f'c.

I read in a thread that the 3.0Mpa was placed to ensure that the heavily stressed walls will be provided with at least some degree of confinement even when the wall is not part of the horizontal sway prevention system (e.g. lift shafts etc.). However, with a the stress level for walls with different concrete strength will not be the same (e.i. 12% for 25Mpa and 6.6% for 45MPa). This does not make any sense to me. Does this mean the at 3Mpa all concrete element in compression (no matter its strength grade) will behave the same under compression when subjected to earthquake and wind load?.