AS 3600 Cl 14.6.3 - Confinement of the Wall Core 2

[tmac7285]

This clause states that "For structural walls where f'c > 50MPa confinement of the wall core shall be provided throughout by fitments in accordance with Clause 14.5.4". Clause 14.5.4 then goes on to tell the designer what spacing of the fitments will be required.

I'm interested in understanding what happens when a relatively tall and slender building (say 35 stories) is dominated by the wind forces and 65MPa concrete or greater is required for the local wall axial compression. The earthquake forces for an assumed ductility (say for Mu = 2.0 and Sp = 0.77) only require 50MPa concrete. The aforementioned clause would then appear to force the designer to use fitments as if the wall were a column where the use of high strength concrete is specified on the drawings, even though the earthquake design taken in isolation would not require high strength concrete for the walls. This might make sense when considering that there is concern regarding the reduced ductility of high strength concrete in general.

Can the fitments for high strength concrete walls be ignored in this example? The code does not address this case.

 

[IDS]

Why do you think the clause may not be applicable?

If you need 65 MPa concrete for the critical loading condition, then the confinement requirements for 65 MPa concrete are applicable.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[tmac7285]

The clause relating to confining a wall for concrete greater than 50MPa in strength is in the Earthquake section of the new concrete code. If I don't require 50MPa concrete for the earthquake design (and would then not need to confine the wall with fitments to this part of the code based required concrete strength), but I do required high strength concrete for the wind design, would I then still have to confine the wall as if it were a column to the earthquake section just because I'm using higher strength concrete.

 

[Agent666]

Can the fitments for high strength concrete walls be ignored in this example? The code does not address this case.

No you cannot ignore this. It does cover it in the code, you detail for the governing case/conditions, in this case if you require 65MPa concrete for one load case then you are required to adoot/take all other concrete strength related provisions into account. You cannot pick and choose.

If you provide the high strength concrete its there for the earthquake even if you didn't need it. If the wall design is governed by wind, then it still has more or less the same strength under earthquake, so your ductility is not going to be 2. The actual ductility will be whatever it is to reach the actual strength provided.

Evaluating the actual ductility based on a strength provided may give you the opportunity to adopt less onerous detailing provisions if AS3600 allows it (not fully familiar with AS3600) if your wall remains elastic or less than or equal to a ductility of 1.25?

 

[human909]

I don't see the ambiguity and I don't see any suggestion that this requirement pertains only to earthquake loading.

To quote:
11.7.4 Restraint of vertical reinforcement
For walls designed as columns in accordance with Section 10, the restraint provisions of
Clause 10.7.4 are not required if either one of the following conditions is met:
(a) N* ≤ 0.5φNu. . . . 11.7.4
(b) The concrete strength is ≤ 50 MPa and either—
(i) the vertical reinforcement is not used as compressive reinforcement; or
(ii) the vertical reinforcement ratio is not greater than 0.01 and a minimum
horizontal reinforcement ratio of 0.0025 is provided

For walls with a concrete strength exceeding 50 MPa the vertical reinforcement shall be
restrained in accordance with Clause 14.5.4 of this Standard.


Sure 14.5.4 is part of the earthquake design requirements but nothing in the above statement suggests seismic loads need to be present to meet the requirements of 11.7.4.

 

[hokie66]

Thanks, human909. That is quite clear.

 

[tmac7285]

human909/Agent666,

I agree with you and the general approach. The reason for my question is that several other engineers in my office don't see it this way. They want to avoid column cages in the walls wherever possible due to the increase in reo and difficulty to build. Between Cl 14.6.3 and Cl 11.7.4, the code is clearly indicating that we should be putting fitments in high strength concrete walls.

human909,

To play the devil's advocate for a moment, the section you are quoting is for walls designed as columns, which we are not necessarily doing here i.e. plain concrete wall design. The code is clear that we can no longer use the simplified method of wall design to Chapter 11 for earthquake wall design, but where does wind design fit into this now? Previously, we would check stresses based on the simplified method for both wind and earthquake and now we check earthquake in accordance with Chapter 14, and I haven't gotten through what happens with the wind. Intuitively, I wouldn't expect to use different capacities of the wall to the new code either, but perhaps due to a lack of ductility demand and consequent deformation for wind load cases, the simplified wall design method can be used. What are your thoughts?

Thanks for your responses.

 

[human909]

What are your thoughts?

Most of my thoughts on this are dictated by code semantics. I'm will claim no expertise regarding concrete engineering apart from knowing it is the grey stuff that anchors my steel. On that note it would seem I managed to undeservedly claim two stars for my post above when as you point out that particular provision is for walls designed as columns.

To play the devil's advocate for a moment, the section you are quoting is for walls designed as columns, which we are not necessarily doing here i.e. plain concrete wall design. The code is clear that we can no longer use the simplified method of wall design to Chapter 11 for earthquake wall design, but where does wind design fit into this now?

Wind doesn't fit into this at all. You've chosen a certain level of ductility and you need to adhere to those requirements. They are in place to ensure ductile behavior.

Here is a bit more background, that might help explain the logic.

https://www.sria.com.au/pdfs/Reinforcement Detailing EA presentation EL.pdf

For those playing at home without access to AS3600, this is the clause (repeated from above):
14.6.3 For structural walls where f'c > 50MPa confinement of the wall core shall be provided throughout by fitments in accordance with Clause 14.5.4

and it applies for structures subject to earthquake loads except if one designs for non-ductility eg μ=1 or full ductility of μ=4 (the latter has a whole other code set)

 

[Enhineyero]

To play the devil's advocate for a moment, the section you are quoting is for walls designed as columns, which we are not necessarily doing here i.e. plain concrete wall design. The code is clear that we can no longer use the simplified method of wall design to Chapter 11 for earthquake wall design, but where does wind design fit into this now? Previously, we would check stresses based on the simplified method for both wind and earthquake and now we check earthquake in accordance with Chapter 14, and I haven't gotten through what happens with the wind. Intuitively, I wouldn't expect to use different capacities of the wall to the new code either, but perhaps due to a lack of ductility demand and consequent deformation for wind load cases, the simplified wall design method can be used. What are your thoughts?

If you read cl 11.1(b) walls can be designed as a column or slab depending if the stress at mid height of the column does exceeds 0.03fc'. As far as I know most walls will have a relatively small bending stress at mid height, hence stress at this location is mainly axial compression. This suggest that for you to design the wall as a slab it meant that compression is not a considerable, which means that the wall is not really acting as a wall in the convetional sense but more like a slab. In my experience, most structural walls are designed as a column (Bending and Axial interaction) or pure compression.