Deflection Limits on Tall Sculpture

[Contraflexure74]

Hi,

I have a 11m high by 600mm wide boat sculpture standing on a foundation, effectively a cantilever. The spine structure is a CHS of sorts but I'm finding large deflections at the top of the boat (the tip). Anyone recommend a deflection limit for something like this? L/180...L/150? I know the boat structure itself which is 3mm thick polished stainless steel will give additional stiffness to the structural spine but its very hard to model.

Anyone know what that the deflection limit for highway light standards?

John.

 

[KootK]

Much like a light standard, I think that your primary concerns should be vibration related, particularly fatigue. For raw displacement, I think that it would just come down to whatever displacement would scare folks visually. I've gone as low as L/90. I imagine that much of your movement here will be torsional.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JStephen]

Can't help on that, but if you have a lot of deflection, you have the potential for wind vibration problems as well.
8" carbon steel pipe (as opposed to tubing) should be readily available in heavier wall thicknesses if needed.

 

[BAretired]

Pay particular attention to vortex shedding for a boat shape.

BA

 

[Contraflexure74]

Hi guys,

My understanding of vibration due to vortex shedding will only happen if the structure shape is uniform along it's height. The boat shape I have changes cross section along it's height.

Any more takers on the allowable deflection limit for such a structure.

John.

 

[KootK]

That's my understanding as well. At least, in general terms. The worst case for flag pole design is often with the flag down for this very reason. That said, relative to a building, your structure will be:

1) Much more torsionally flexible.
2) Probably more torsionally loaded by wind.

In my head, I can see how it might twist and shed, twist and shed, twist and shed... hopefully not in resonance.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Contraflexure74]

I'm not sure I can see the push pull effect happening due to wind inducing torsional effects on the structure. This would mean that wind would only hit one side of the structure face. Not sure this would happen in reality.

 

[KootK]

So you're hanging your hat on the wind force being equal on both sides of your post? In my head I'm imagining this like a teetertotter with six fat kids on each end and one kid falling off each time that the tettertotter touches down.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Contraflexure74]

Ah, I'm with you now. I was considering changing spine structure to 250x250x10 and fill with concrete. I would hope this would help in torsion and deflection?

 

[KootK]

It would certainly stiffen things torsionally. Frankly, I have no idea whether or not you even have a vibration problem to be worried about. Just kicking around ideas.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[SkiisAndBikes]

I have designed perhaps a dozen large complex sculptures and have had the same question re deflection as well. In the end, my thought process determined that deflection limits are not important, as long as the sculpture is appropriately designed for strength. However, I do agree that large deflections could potentially lead to vibration issues and I have on more than one occasion engaged wind specialists to comment and/or provide analysis. Not one of the sculptures I have worked on would meet the deflection criteria for buildings. One sculpture was installed within the interior of a building, i.e. no significant live/wind loads, and the initial deflection under dead load was in the order of 1.0m. Very unusual sculpture and the engineering was heavily peer reviewed at several levels. In your case with a vertical column I would lobby to get as big a column in as possible to reduce deflection.

There appears to be a flood elevation note on your drawing. Does this mean that the sculpture may be underwater at some point in it's life ... possibly subject to pressure from a flowing river?

 

[Contraflexure74]

Hi,

Wow 1m deflection, now that's impressive. Yes the bottom 4m of the sculpture could be under water during a flood period but the sculpture location is well away from the main flow of the river, the flood water in question is overspill flood water from the river.

The sculpture wall thickness is 3mm stainless, could this distort under the water pressure at 4m depth?
Should I reinforce the infill concrete?
Should I increase the height of the spine internally to 3/4 the height of the actual structure?
As I am in a flood zone I guess I need to design the connection of the boat to the structural spine for the uplift forces due to buoyancy?

John.