AS3600 cl 11.2.1 vs 11.7.3 - Two Layers fo reinforcement in walls

[li0ngalahad]

Hi everyone

I would like to bring to everyone's attention these two apparently contradictory clauses from AS3600.

Clause 11.2.1 says
Braced walls where in-plane horizontal forces, acting in conjunction with the axial forces, are such that where a horizontal cross-section of the wall
[...]
(b) is subject to tension on part of the section, the wall shall be designed for in-plane bending in accordance with one of the following options as appropriate—
[...]
(ii) if H/L ratio >2 design as a column in accordance with Section 10 where vertical reinforcement is provided in each face, except that Clause 11.7.4 may override the requirements of Clause 10.7.4. The provisions of Clause 10.7.1 -(b), Clause 11.4 and Clause 11.7 shall still apply.
[...]
When considering design actions due to earthquake, the determination of whether the horizontal cross-section of a wall is subject to compression over the entire section shall be based on a lateral analysis undertaken using a structural ductility factor (mu) of 1.00 and a structural performance factor (Sp) of 1.0, in accordance with AS 1170.4.


Clause 11.7.3 says
The vertical and horizontal reinforcement shall be provided in two grids, one near each face of the wall under any of the following conditions :
[...]
(b) Any part of a wall structure where tension exceeds the tensile capacity of the concrete under the design ultimate loads. When considering design actions due to earthquake, the determination of whether or not a wall horizontal cross-section is subject to tension shall be based on a lateral analysis undertaken using a structural ductility factor (mu) of 1.00 and a structural performance factor (SP) of 0.77.
[...]


I have always interpredeted 11.2.1 (b) (ii) as, if there any tension in the wall, if aspect ratio is >2 then it must be designed as a column and 2 layers of reinforcement must be provided (one each face).
On the other hand 11.7.3 says it is required to provide 2 layers only if the tensile stress is above f.ct . I always sort of considered this a minor contradition and assumed the more conservative requirement in 11.2.1 to override 11.7.3

However a colleague pointed out to me that 11.2.1 wording can be interpreted as "if there any tension in the wall, and if aspect ratio is >2, then it must be designed as a column only where 2 layers of reinforcement are provided" and 11.7.3 describes where you are required to provide 2 layers. So the column can still be designed as a wall if 1 layer reinforcement is provided, i.e. when tensile stresses are below f.ct and compressive stresses are below 3 MPa, as per Cl 11.5.2 (a)

I must admit I now see the point and probably this interpretation is right. Im just not clear on why the complexity and convolution, why the code just doesnt say "if tension stress exceeds f.ct with mu=1 and Sp=0.77, two layers of reinforcement must be provided and the wall must be designed as a column". Also, why the commentary doesnt explain this more clearly? (sorry for the usual AS3600 frustration rant haha)

I would like to hear what is the consensus among the eng-tips community on these requirements. Thanks in advance.

 

[mrlm]

I tend to agree with your interpretation.

I think the reference to tensile strength in 11.7.3 is poorly worded. When we are designing slabs and beams, 8.1.2 b) states that the concrete has no tensile strength. So I take the meaning of 11.7.3 to be when the tensile strength is exceeded (which is zero) then you need two layers of reinforcement in the wall. With this interpretation, there is no contradiction between clauses.

Also, f.ct is the uniaxial tensile strength. In this design situation, the wall is not subject to uniaxial tension. It is more like flexural bending from an applied shear force.

 

[li0ngalahad]

I see your point however I believe section 8 is not really applicable for shear walls in this case, that's just an assumption taken when calculating the beam flexural strenght. But concrete has a tensile strength per-se, as per clause 3.1.1.3.

Regarding your second comment, generally I consider f'ct for coupled walls and for in-plane bending in walls, as locally the section may be considered subject to uniaxial tension, and use f'ctf only for out of plane bending (slabs or, more rarely, walls).

On another note , thinking about it, clause 11.7.3 talks about "tensile capacity" not "strength". f.ct is a strenght. Capacity would require a capacity facotr phi, which for plain concrete would be 0.6 I would think. so 0.6*0.36*sqrt(0.36) - which would lower the chance of having only 1 layer of reinforcement quite dramatically

 

[mrlm]

I agree that section 8 is not applicable to shear walls but in just about every ultimate design situation, the tensile strength of concrete is ignored. I used that as a reference as in the column or wall section I could not find a comment on it, however, it was my underlying assumption the tensile strength would be ignored.

A cursory look at the text book talking about the design of columns in uniaxial bending, no reference is made to the tensile strength of concrete and all tension force is resisted in the steel.

I believe what I said initially is valid.

 

[li0ngalahad]

Not every ultimate design situation assumes zero tensile capacity of concrete, the most evident example being bending capacity of plain concrete footings and pedestals.

Anyway, 11.7.3 is a clause that defines reinforcement detailing, not ultimate design, in particular prescribes when 2 layers of reinforcement are required. You would still need to provide enough reinforcement to resist whatever tensile design forces the wall has (therefore assuming no tensile capacity of concrete) but this clause says that if cracking is expected, i.e. tensile strength (or should I say capacity) is overcome, reinforcement must be provided in 2 layers (to increase ductility).

The apparent contradiction with 11.2.1 remains, unless of course we accept the (quite convoluted) interpretation I've explained above. The code once again is better at confusing engineers rather than helping them (and the commentary adds further confusion).