Is this 350mm long wall to short? 2

[UpsideDown]

Due to the architectural layout, there are many short walls (150mm thick, around 350mm long, precast) on the external, between these short walls are floor-to-ceiling curtain walls/doors so no beams are used, just flat slab on top.
I have run the analysis and the results are fine including punching shear (Max. design axial force are around 100kN) However, I feel like these precast walls are too short and there might be some potential structural issue that I miss?
Below is the part plan & 3D view.

 

[r13]

But isn't 150mm thick wall quite normal?

You need to start with a basic check on code definition of "wall" and "column".

 

[Trenno]

Awaiting RAPT & Agent666 to chime in regarding single layered 150mm walls...

The 1:4 isn't codified anymore I believe, but it's a rule of thumb. In years gone by, I believe AS3600 defined a wall for fire purposes being anything greater than 1:4.

 

[KootK]

I just have this feeling that it is too short but I cannot explain.

1) I think that one of your considerations should be the practicality of such a narrow element for the precaster. As others have mentioned, you won't be able to get many vertical bars into this panel and, depending on their setup, your precaster may not want to prestress such a narrow piece. To keep clients happy, precasters usually want their panels to not crack under lifting, transportation, and service stresses. That will be challenge with such a narrow panel, particularly if it's not prestressed as the prestress helps a great deal with crack control.

2) There's no code limit on wall narrowness that I'm am aware of, other than the column aspect ratio business, but I've been limiting wall panel widths to 450 mm minimum in my own work.

3) Another consideration with such a thin wall panel is making a robust, fire rated connection between the column and the slab such that your ETABS/SAFE punching shear calculations are valid.

4) With regard to the designation of column vs wall, I'd be inclined to take it back to the fundamentals. Fundamentally, a column is usually dominated by axial load and a wall is usually dominated by flexural load or, at the least, not heavily taxed by its axial load. I'd want to call these wall panels columns if:

a) Code definitions required it or;

b) Vertical reinforcing required for axial load was in excess of 1% x Ag.

c) The design axial load case occurred much higher than the balanced point on the P-M diagram.

 

[rapt]

Clause 5.6.2 of AS3600-2018 still has the 4:1 rule (it was reintroduced in 2009 I think) to differentiate between columns and walls for fire rating (as do the Eurocode Fire provisions).

As others have mentioned, centrally reinforced walls are not allowed if any level of ductility is assumed in seismic situations.

 

[Tomfh]

Fire rating issues aside, what are the fundamental structural reasons for differentiating between “walls” and “columns”?

 

[r13]

Tomfh,

I feel the question is reversible as well. Joke aside, there are fundamental behavioral and structural response differences among column, wall, and beams, though we can argue all share similar functions, but some response is more dominant in certain member than the other due to its unique function, such as shear wall.

 

[UpsideDown]

Thanks for your explanation.
For 'L' shape precast walls with short return (such as lift core), can I still design them as walls if I increase the size to say 200mm or do I still need to make them columns?

 

[UpsideDown]

Yeah, the only thing that I can find is in the fire rating section.

 

[UpsideDown]

Not sure if I fully understand. So put strength & deflection check aside, double layers of reinforcement is only required for boundary elements? i.e. when a wall is not designed as shear wall or designed as shear wall but boundary elements are not required, we can still use 150mm wall with single layer?

 

[human909]

Yeah, the only thing that I can find is in the fire rating section.

Which on first glance is a bit amiss. But it makes some sense. Fire codes are possibly even more prescriptive than structural codes.

Be sensible. Be an engineer. a 150x350 vertical concrete member sounds like a column to me. Design it as such. Otherwise be very confident you can justify your choices.

 

[UpsideDown]

Okay. Thanks. I will make these walls to be 200x350 with double layers. For 'normal' walls with 600mm+ length, is 150mm with single layer still an issue if the strength/deflection/fire pass? I have seen people using 180mm precast panel (mostly internal ones) with single layer instead of 150mm, don't know what is the consideration for increasing to 180mm.

 

[Tomfh]

Be sensible. Be an engineer. a 150x350 vertical concrete member sounds like a column to me. Design it as such.

If it’s so lightly loaded it works as a plain masonry pier why the need to design as a full blown RC column? Seems a bit heavy handed to me.

 

[KootK]

For 'L' shape precast walls with short return (such as lift core), can I still design them as walls if I increase the size to say 200mm or do I still need to make them columns?

I couldn't say as I don't know the geometry, the loads, or where you land on the P-M diagram etc. For all I know, maybe they could be 150 walls.

One thing to keep in mind for precast is that, in that world "wall" generally means an element with no return because that suits prestressing bed fabrication. It's not that a return can't be done but it probably either involves custom formwork with no prestressing or a second pour addition to a normally cast wall panel, both of which are much more expensive than typical, return-less wall castings. If you have any masonry on the project, I'd consider leaving out the return walls until the elevators are installed and then building them back in with masonry. This eliminates the returns from the precast scope and allows for more flexibility in the installation of the elevator and it's operation panels etc.

I have seen people using 180mm precast panel (mostly internal ones) with single layer instead of 150mm, don't know what is the consideration for increasing to 180mm.

I've seen a lot of 7" walls, single mat reinforced walls too. But, then, they generally have more panel-ish dimensions except at the sides of overhead doors etc. I don't know that that there's any code magic to it. Rather, it seems to just be a reasonable balance between and aggressive design and a design that is practical for fabrication, handling etc. For a solid wall panel that is truly functioning as a wall panel, fire rating isn't a problem for most 6" walls under IBC/ACI. Six inches will give you 2HR for all aggregate types and 3HR for all but Siliceous. Add some drywall on there and the limits are ridiculously thin.

 

[Retrograde]

I have seen people using 180mm precast panel (mostly internal ones) with single layer instead of 150mm, don't know what is the consideration for increasing to 180mm.

If you look at Table 5.7.2 of AS3600-2009 you need at least a 170mm thick panel to achieve 90 mins fire rating, which is typical for a residential building (I think the requirements are the same in AS3600-2018). If the wall is lightly loaded N*f/phiNu < 0.35 then you can go thinner.

 

[UpsideDown]

Okay. Many thanks guys. I will use 150mm for 'normal' walls as they are lightly loaded.

 

[rapt]

I think the 4:1 rule for fire is due to corner bars being more effected by fire than other bars between the ends. So there is a logic for a different rule compared to normal flexure calculations.

The earthquake rule in AS3600 is not only if boundary elements are required. It is for any Limited Ductile or Moderately Ductile wall (14.6.1). If you want to do Fully Ductile you then have to go to NZS and it will force double reinforced as well but probably require class E reinforcement. Agent666 will know more.

 

[Agent666]

I know AS3600 had some recent updates that heavily discourage single layers of rebar in vertical elements.

Similar in New Zealand, amendments to NZS3101 in the last few years have heavily penalised the use of singly reinforced walls. They did not perform well in our recent major earthquakes. Essentially the intent of the code writers was to ensure that because of the relatively poor performance observed in past real world earthquakes that under the mother of all earthquakes (Maximum Credible Earthquake, MCE) that the walls remained elastic. To achieve this, they introduced a few provisions that cover the following:-

1 - applying an in-plane strength reduction factor of 0.7 (vs our normal 0.85)
2 - limiting the maximum longitudinal reinforcement to that which achieves satisfying balanced strain conditions for out of plane loads. This effectively limits the axial load that can be carried as compression block depth increases due to axial load, all but eliminating their use for 2-3+ storey structures where you have narrow piers.
3 - limited curvatures to 0.8 times the curvature at which the outermost bars reach yield. Essentially ensuring the walls remain elastic with no plastic deformation allowed.
4 - limited the maximum ductility factor to a value of 1.25 (in combination with the 0.7 factor above, it essentially means they remain elastic as 0.7/0.85*1.25 = 1.03 equivalent ductility factor (i.e. approx 1.00 elastic)
5 - limits on maximum axial load and slenderness

Notwithstanding all this, your configuration is quite poor. Once you consider foundation flexibility the torsional nature of your floorplate will become even more evident amplifying torsional deformations.

Agree with others it is a column to me, consider it as a column if you exceed the axial load limits in AS3600 (I assume they exist?). Otherwise if you are resistant to treating it as a column, I'd try at the very least to adopt 2 layers of reinforcement.

The downsides of a wall vs a column is the real lack of confinement, hence maximum axial loads that can be resisted are much lower.

ACI318 has some good advice around when something is a wall and when it is not, it is the best codified summary I've seen. Table below:-

ACI also has some advice related to the use of a single layer of reinforcement that is shear based if your shear is higher than just the concrete then you need two layers (there is also a good explanation of their reasoning as to why they is the case:-

I like these ACI provisions, they make sense and provide a much more black and white division between, it is ok, and it is not ok.

 

[Agent666]

If you want to do Fully Ductile you then have to go to NZS and it will force double reinforced as well but probably require class E reinforcement. Agent666 will know more.

Yeah any 'ductile' design to NZS3101 will require a double layer of reinforcement and a considerable amount of confinement. To comply will require class E reinforcement, we don't have any N or L class reinforcement provisions any more. Not that I've ever seen a class N or E bar in NZ ever.... Even when it was allowed for some secondary things, no body ever specified it.

 

[IDS]

Not that I've ever seen a class N or E bar in NZ ever

I'm hoping that's a typo.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Agent666]

Shoot, I meant L!