Slender Architectural Column 2

[StructuralAddict]

Hi,
I need to design an architectural column that supports only its own weight.
The column length is 9.0 meters and its diameter is 0.3 meters.
The KL/r > 100.
So, the ACI requires that Section 10.10.1 be used to compute forces and moments in the frame by performing second-order analysis.
My question is: do we need to perform the second-order analysis even if the column is supporting only its own weight?
Thank you!

 

[Compositepro]

A column that only supports its own weight should ideally be tapered.

 

[BAretired]

A very slender column will be affected by lateral forces such as wind or seismic events more than normal columns. A second order analysis is recommended.

BA

 

[KootK]

My question is: do we need to perform the second-order analysis even if the column is supporting only its own weight?

I vote no. If the axial load on the column is less than, say, [0.05 Ag f'c], I don't see second order effects coming into play. That said, there are exceptions to things and I know very little about your situation at this point.

 

[BAretired]

And so the answer is a clear yes or no!

BA

 

[MIStructE_IRE]

A sketch and some info would help get some clarity.

Is the column restrained at the top or is it a pure vertical cantilever?

Is it steel/concrete/timber/other?

Is it external and subject to wind/seismic?

Out of interest... What is its purpose?

 

[StructuralAddict]

Thank you for your answers!

Please find below a photo of a similar column:

 

[MIStructE_IRE]

That’s not only carrying its own weight by the looks of it? Unless the soffit is self supporting and you’re providing a deflection head?

 

[oldestguy]

Then some day along comes a parade and they need an anchor for a large sign out some distance from the building. Or a painting contractor has to secure his scaffold some how against strong bending moment.. Wow, that's my support.

 

[StructuralAddict]

That’s not only carrying its own weight by the looks of it? Unless the soffit is self supporting and you’re providing a deflection head?

Please note that this photo shows something similar to what I am designing. I am designing the bigger columns to be exactly similar to the smaller columns in the photo. It's like just upscaling the two small columns in the vicinity of the window. The columns are just supporting their own weight + a small weight from the "slab" extending between them. There are structural columns embedded in the walls that support the building.

I hope this provides clearer description of the current situation.

 

[KootK]

I am designing the bigger columns to be exactly similar to the smaller columns in the photo.

That sounds pretty slender. How tall and wide are these things?

At the least, make sure that your columns don't absorb so much incidental load/deformation from the surrounding structure that they euler buckle.

 

[StructuralAddict]

That sounds pretty slender. How tall and wide are these things?

The length of the bigger columns is 9.0 m and their diameter is 0.3 m.

I considered a pinned-pinned support when calculating the KL/r ratio. Does this seem reasonable? Or should I consider the column having a free end in this case?

Thanks!

 

[KootK]

You're most welcome. By my math, the sketch below is H:W proportionate to your column. Scary slender. Can it handle wind and such? A little vortex shedding? Seriously though:

- KL = 9000
- Pu = Max load you can see this thing drawing from the attached structure, accounting for that structure's inherent tendency to deflect and creep.
- Make sure the thing doesn't Euler buckle.

Nobody's likely to get hurt with this. However, if the column Euler buckles, it'll kick to the side like the goddam letter cee. A tiny bit of axial displacement results in gobs of lateral displacement. Truly though, I think that the only second order effect at play with this is the old fashioned column buckling.

 

[JAE]

Your KL/r is about 222 based on the size and length you state, using a K=2.0 (cantilevered out of the ground) and an r = 0.3 x width (using a square column of equal area to your circular column)

That's very slender and for buildings per ACI, not really smiled upon....or perhaps allowed.



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[BAretired]

The diameter is 300 mm, so r = 300/4 = 75 mm. If hinged each end, Kl/r = 120. It's pushing the limits a bit, but it might be acceptable if it is not carrying any load other than self weight. If one end is fixed and the other free, Kl/r = 240, definitely not recommended.

BA

 

[Settingsun]

If you consider the column unrestrained at the top, you need to fix the base for stability. I'd suggest you make sure it's restrained at the top.

Kl/r = 120 isn't far outside the 100 limit. Try the hand method and see whether slenderness is significant. (Although, since you didn't get a simple answer here, you probably could have done the 2nd order analysis by now.)

The book Reinforced Concrete Mechanics and Design by MacGregor shows a photo of a load bearing column at 50:1 aspect ratio, so this should be OK. As others have pointed out, you need to be confident it's sufficiently robust for 'the real world'. Wind, EQ, creep, the slabs applying off-centre load or causing the heads to rotate etc.

(Not too sure they'll be good looking columns - are they in proportion to the rest of the building?)

 

[Lomarandil]

(OP)
My question is: do we need to perform the second-order analysis even if the column is supporting only its own weight?

I vote no. If the axial load on the column is less than, say, [0.05 Ag f'c], I don't see second order effects coming into play.

Is it that second-order effects don't come into play, or that they're not very significant without another driving force (like BA was getting at)?

I think about the similar situation with a masonry wall -- you tend to not get much P-d effect unless an external load is acting out-of-plane.. but if so, the deflection and P-d stacks up quickly.

Separately: KootK is on the right track thinking about vortex shedding and aeolian vibrations. You'll get some benefit from material damping (which I didn't check), but a solid concrete column of these proportions is likely to hit resonance with some wind speed around 10-15m/s -- enough wind load to be worrysome, and low enough to happen every year.

----
just call me Lo.

 

[TehMightyEngineer]

What about if we design around the failure mode? Let us assume the column deflects and cracks, but ensure that the structure doesn't become unstable or unattractive afterward. Polypropylene / polyethylene macro fiber reinforced concrete should ensure your cracks are kept small and provide some tensile strength in the concrete against fracture. Provides some additional insurance for seismic and high wind events (or contractors hanging stuff off the column).

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)

 

[KootK]

The logistics of constructing such a thing must be interesting. Two pours at least but, if you'll be close enough to primary structure, I suppose that you can brace your formwork off to that. Structurally, and logistically, it would be nice to do something so slender as prestressed concrete columns. I imagine that the articulating geometry would make that infeasible however.

 

[CANPRO]

What is the architectural finish? You could use a steel tube with vertically slotted connections instead of concrete.