Flat plate design

[hoshang]

Hi
Please find the attached image. How one use edge beams for the flat plate perimeter considering positions of lift and stair?

 

[milkshakelake]

Apply uniform or triangular load where it connects to the slab. And uniform load of stuff on the beam (like CMU to frame the lift walls).

 

[hoshang]

Hi,
I meant where to add edge beams. Also, should one add a column on right side of the upper lift text?

 

[milkshakelake]

It's kind of a very complicated and very basic question at the same time. Here's a starting point:

Some things to think about:
1. Reentrant corner definitely needs a column.
2. Need a beam for the facade next to lift -> Other beams are just framing this beam.
3. With my suggestion, some of the slab is left "unsupported." It's not technically unsupported, just slightly more discontinuous than I like. You can do long-term deflection analysis for this, or add more beams around the lift. Or add another column at the corner of the lifts (which will also remove some of the beams), but I understand that owners don't like a lot of columns. It's just something you need to solve when you have a lot of reentrant corners like that.
4. Don't really need beams around the stair openings. Think of it like the corner of a building with a cantilever slab for the stair landing.
5. There's a million other ways to do it and think about it...it's just a starting point.

 

[jayrod12]

I don't see a need for additional columns. I'd likely be beaming it out as per the attached.

 

[milkshakelake]

That would work, but I think there's no slab there. It would become an exposed beam.

 

[hoshang]

With my suggestion, some of the slab is left "unsupported." It's not technically unsupported, just slightly more discontinuous than I like. You can do long-term deflection analysis for this,

Why long-term analysis? Edge beams are beneficial for punching shear and unbalanced moment.

or add more beams around the lift.

Do you mean the green one in the attached image below:

Or add another column at the corner of the lifts (which will also remove some of the beams)

Do you mean the ones in the attached image below:

4. Don't really need beams around the stair openings. Think of it like the corner of a building with a cantilever slab for the stair landing.

I can't catch this. Can you elaborate more?

I think there's no slab there. It would become an exposed beam.

Yes, there's no slab there.

 

[milkshakelake]

 

[hoshang]

Hi milkshakelake,
Please mind that there are concrete walls around the cross locations in original post image (lifts).
You wrote (with yellow):
Punching shear possible issue, move col
The adjacent bay to the stair is 7m wide (it's for toilets), and the stair is 4.3m wide. Where the column should be moved?
The red line beam for stair can't be there due to window above landing. If any, it should be under landing.
You wrote:
It's same thing as edge of building.
I can't catch it.

 

[BulbTheBuilder]

Please mind that there are concrete walls around the cross locations in original post image (lifts).

If you have concrete wall around lift then you don't need those beams (over wall) as you have a continuous support but you need that red column to avoid having a cantilevered slab

Punching shear possible issue, move col
The adjacent bay to the stair is 7m wide (it's for toilets), and the stair is 4.3m wide. Where the column should be moved?

You literally have only half of the column supporting the slab which is critical. Preferably, shift inside (to the left) so both edges of slab and columns are aligned.

.

Same thing as the edge of the building. No beams required. You don't need beams (structurally)

 

[hoshang]

No need for beams here

It's same thing as edge of building.

Same thing as the edge of the building. No beams required. You don't need beams (structurally)

If I understand correctly, no beams required at the edge of the building. Aren't edge beams helpful for punching shear resistance?

 

[milkshakelake]

Flat plate design doesn't have beams at the edge or between columns. Flat slab design has edge beams. Where I practice, flat plate is used everywhere and flat slab is never used.

Yes, beams help for punching shear resistance, but they cost a lot of money in terms of labor. It might be different where you practice. If we have a punching shear issue, we:
1) Increase column size
2) Increase slab thickness
3) Add punching shear reinforcement, like stud rails

 

[rapt]

That is a new definition for me Milkshakelake.

Historically most flat plates have still had edge beams.

Your flat slab definition has normally applied to internal columns, not edge. And that is in providing drop panels, not beams.

Lately the edge beams have been dropped as purveyors of formwork systems have argued against their use.

Structurally I consider this to be an extremely backward step and would much prefer to have edge beams around the perimeter of my flat plate, than rely on your other solutions.

Far easier to justify deflections for facade support and also Robustness requirements.

 

[milkshakelake]

You're right, I think my definition is off. Sorry about that; it's something I read years ago and probably got jumbled in my head.

I do prefer edge beams as well. It's definitely better for punching shear and facade deflection, and gives me a warm fuzzy feeling. It's just never used where I practice, and hasn't been used for a long time. I superceded a job that had them because the owner said it cost too much and wanted a proper flat plate design. If I were to ever use it, I think I'd be thrown out of the market. That being said, if it's common where OP practices, it should definitely be used.

 

[BulbTheBuilder]

I think I read an ACI article, and it referred to flat plate as flat throughout and flat slabs having drops... I can't talk about edge beams.

 

[hoshang]

Thanks all,
Do you think edge beams improve punching shear at first interior column locations somehow? My thought is that torsional strength of edge beams may provide resistance to punching shear at first interior column locations through integrity reinforcement if provided sufficiently. Can I think about punching shear at first interior column as if the first interior column were removed and integrity reinforcement would come into play?

 

[milkshakelake]

@rapt I was pondering your point, and I remembered ACI 318 does mitigate the facade deflection problem of flat plates with clear span limits with/without edge beams (unless deflections are calculated). So I don't think it's such a major step backwards to omit them; some of the problems have been accounted for in code.

 

[hoshang]

What about my concerns on 14 Oct 23 06:47?

 

[rapt]

many facade types have strict deflection limits that the ACI rules do not cover. And if you believe L/D deflection limits actually give sensible results, I will say no more.

Hoshang,

How could an edge beam improve punching shear at the 1st interior column? I must be misreading your second sentence!

 

[milkshakelake]

I think hoshang's line of reasoning is that there is additional torsional stiffness at the edge, so the edge columns take more moment (which is true per direct design method), and therefore this helps the punching shear in the 1st interior column? I don't follow exactly, and I'm not sure how integrity reinforcement comes into play. Maybe you can explain again.

I personally wouldn't worry about 1st interior column punching shear. It has 4 sides punching shear perimeter instead of 2 or 3, which outweighs the additional P. Or better yet, run two calculations for 1st interior column: one without the edge beam, one with the edge beam, and see how the punching shear is affected. I bet it's less than 5% difference.