Simplified Wall Design to AS3600-2018

[S020]

Hi All,

I am yet to see a thread addressing these formalised changes yet so I thought I would put some thoughts to paper and see if anyone has further comments.

One of the limitations of the simplified method is that axial stress be kept to 3MPa max unless reinforcement is provided on both faces. So if I spec. equally distributed reinforcement on two faces of a wall I am permitted to push wall stresses beyond 3MPa. Is there a hard limit? When do I get forced to start designing walls as a column with this method? Does my wall become impractically thick to compete against the contribution of compression steel that is restrained? But then again, proposing to lig long runs of walls for 20 storeys will never win you a job ever again.

If a wall sees any tension through its section it needs to be designed using strut tie if H/L<=2? Surely not. Every wall in a highrise tower wall (or even worse - a small 4 storey walk-up) requires its own deep beam model, when summated would result in hours of work to simply check if sizes are correct let alone reinforcement. This seems taxing and a little too punishing balanced against the professional fees a consultancy will price for a low-rise building. Is there no other way around this? Perhaps compute the compressive stresses P/A + M/Z and use the simplified method to calculate the compressive resistance? Shear walls used to be permitted to be designed per section 8 i.e. flexural beams in the 2009 code. What happened?!

Restraint of vertical reinforcement - for walls exceeding 50MPa we need to detail to 14.5.4 which by my own interpretation reads: provide ligatures to all longitudinal reinforcement for a distance the greater of the length of the wall or 1/6 the clear height. Take a 2800 long shear wall that has a clear height of 2800 between connecting slabs - I have to provide ligatures over its entire height i.e. I am designing what looks like a column if my concrete grade exceeds 50MPa. Is my interpretation correct?

There appears to be a complete lack of reference to the new detailing requirements of section 14.6 limited ductile walls in section 11. Surely there needs to be some wording added to the scope of section 11 to reference the requirements of 14.6? Someone has / is going to produce a non-compliant design. It looks like unrestrained ends of shear walls will be forced to include ligatures within termed boundary elements where the compressive stresses exceed 0.15F'c. Easy enough.

To summarise:
Max compressive stress for two sided reinforced wall (no tension) = ?
F'c > 50MPa = ligs required. it's pretty much a column if you're designing shear walls.
any tension = non-flexural
P/A + M/Z compressive zone stress > 0.15F'c = provide ligs (earthquake provision)

Anything I've missed or misinterpreted?

S020

 

[rapt]

S020

The 3MPa limit is to force 2 layers of reinforcement in more heavily loaded walls, over about 3-4 stories in an apartment building. The simplified design clause 11.5 still controls the wall capacity if you want to design by the simplified method. The 3MPa is not defining the capacity.

If you have a highrise, H/L will be > 2 so you can design it as a column. H is the height, not the effective height.

If you read the old code, it is stupid. How can you design a wall with relatively high axial load as a beam?

If you are using >50MPa concrete you have a heavily loaded wall and earthquake design will be required. This rule will change slightly to refer to 10.7 if earthquake is not a consideration. Concrete >50MPa is very brittle. Under earthquake sway/action it will not last long without confinement, or with a single central layer of reinforcement as designers have been doing to the old code (I have heard of 120mm thick with 80MPa concrete. Technically the old code allowed it, But it also required designers to use their brains, and that is not using your brain.

Section 11 does not need to reference section 14. If you are designing for earthquake, section 14 controls detailing and sends you back to 11 to determine strength.

 

[S020]

rapt

Good talking points.

Looks like I've missed is the H/L ratio. H being the continuous height over the building and L length of wall? Is this formally defined somewhere in the code?

If designing a wall as a column and N*/phiNu <=0.5 and f'c <=50MPa (cl 11.7.4) we don't need ligs in the wall, which is great and all, but the capacity generated by the column is dependent on the steel above the neutral axis to resist compression (above & below the decompression point) which in turn is typically granted by ligatures. Is the code / testing saying that ligatures add no benefit below a stress level half that of capacity of the column? What exactly is driving this N*/phiNu <= 0.5 provision and why is the designer permitted to use the capacity generated by compression steel without the need for their restraint?

 

[RayGon]

Would anyone know where the 3MPa limit is from and what is the logic to it?
It appears that this limit is not dependent on the concrete strength used in the wall because if it does this limit would be a formula with a factored f'c.
I suspect it has something to do with the mechanical actions at the base of a wall but what is it?