'Structure' becoming dynamic under wind load? 2

[IH1980]

Hi all. I have a problem I need some advice on.

I am designing a thing for a client where they are installing a truck on top of a totem pole outside their truck factory, as a feature at the entrance. So this thing will be a 6000Kg truck tilted at 30 degrees (as if driving down a hill) on top of a single post, about 4m to the lowest point ie front bumper. In the static case the centre of gravity is about concentric with the post. For wind I have worked out the area and treated it as a vertical cantilever from a pad foundation (4m x 4m x 0.8m) using a 400 x 400 SHS post. This gives around 90mm deflection at the top under full wind loads.

I am a bit concerned about it maybe become dynamic under wind loads (no seismic concerns here in the UK). I can't find a lot of info but in the Eurocode for lattice masts and towers, it says that anything with a fundamental frequency of greater than 2hz does not need to have its response checked (BS EN 1993-3-1 2006 B.4.4 (5). I have used the Blevins formula for a single mass on the end of a cantilever and get 1.8hz.

Two questions: 1, is my method appropriate, and 2, am I being unnecessarily concerned here?

All comments welcome!

Ian.

 

[WARose]

What I would check (among several things; more on that in a moment) is vortex shedding. The formula for the frequency this happens at is available in a variety of books (the Shock and Vibration handbook for one IIRC). You would need to look at the possible wind velocities your tube could see and see if the vortex shedding frequency matches any mode (with a significant amount of mass participating).

But the thing that would worry me (as much as anything) in the situation you have described (assuming I am understanding this correctly: i.e. a cantilever post) is the P-Delta effect. That baby will come over......and it might keep coming. You just have to see when/if it converges.

 

[jayrod12]

I wouldn't expect vortex shedding on an irregular object like a truck to be a significant issue. Generally this is an issue on round, or elliptical tall items like smoke stacks. 4m likely isn't tall enough to worry about vortex shedding. As WARose suggested, it's likely that the P-delta effects will have a much larger contribution to the instability.

 

[KootK]

Some additional thoughts:

1) Vortex shedding or no, I think that a vibration check is appropriate. And I agree with what you've done.

2) I did some flag pole work once upon a time and learned some things that surprised me. One of those things was that the worst case for the pole and foundations can be when the flag is removed from the pole. As Jayrod mentioned, cylindrical things vortex shed all over the place. But flags do not. Rather, the presence of the flag serves to disrupt the cyclical motion. I suspect that your truck may act in a similar manner.

3) The P-delta comments are great advice. Further to that, I would not treat your column as fixed at the base. Rather, I would model it as mounted to a rotational spring whose stiffness would reflect both foundation flexibility and base connection flexibility. Your foundation is pretty expansive so I doubt you'll have much trouble with that.



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.

 

[JoshPlumSE]

KootK's 3rd point is excellent advice. Not something a lot of us would think about. But, for a structure like this, foundation stiffness could be a really important contributor to the P-Delta effect.

 

[CANPRO]

is 6000kg the current weight of the truck, or the weight of the truck when they install it on the pole? If they strip the engine/other items out of the truck before installation this will effect your center of gravity and frequency calcs. Not sure if that could be beneficial, but its something to consider.

 

[SAIL3]

as mentioned above, vortex shedding usually occurs on round objects, but, I suspect(no proof) it might also occur on square tubing with wind @ 45 deg to flat surface....

Vcr=3.41xfnxD=3.41x1.8x22.3/12=11.4 mph
Vcr=critical velocity(mph) where vortex shedding may occur
fn=nat freq (cps)
D=outside dia(ft) of pole...(15.7^2x2)^1/2
not sure if above formula is applicable to the case where one has a large object/wind area on top of the pole....the wind on the truck may overcome any driving force from the vortex shedding of the pole.
with all these unknowns, I guess I would try and address it in some manner, unless, I found some credible and pertinent data to suggest otherwise.

 

[KootK]

Fill the bed up with water and call it a mass tuned damper!

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.

 

[IH1980]

Some great advice here guys, it is all appreciated. To just comment on a few points:

P delta checks - agreed. When I take it 'to the computer' I will make sure the analysis is second order. At this stage I am trying to lay it out by hand so that I know that the answers that come out of the pc are about right.

Truck weight. Complicated! The details I have are for gross vehicle weight, and they are indeed removing the engine and box. I have estimated -600Kg for this as the GVW is 3500kG. This will then shift the CG towards the back axle. However, the truck body is to be a 'structural box' which is effectively used to hang the truck off the pole, so think trusses inside the wagon back coming off the central post. This is then clad in 3mm steel so I have estimated other 3000kG in this 'back box'. In my paper workings I have drawn it to scale, and it happens that when you tilt it 30 degrees, the Cg of the truck (estimated) and the back box are concentric with the top of the post. When we have the actual truck in the shop, we will get axle weights to check actual CG.

Vortex shedding - never even thought of this. Will look into it.

Foundation spring stiffness - this has crossed my mind. Likely to be something I check in the PC using a rotational spring as advised.

I'm glad that my worries are obviously justified to some extent!

 

[BUGGAR]

A star to KootK. A tuned mass damper is brilliant.

Your comment about the flag decreasing flutter loads on flagpoles and similar is well taken. This reinforces how critical it can be to add those spiral flutes to tall skinny columns to change the whole wind dynamic.

 

[racookpe1978]

OK, so I'm "solving" things that may not be a problem.

You empty the truck - single bad & frame, right? Not a trailer and truck? - of invisible weights.

If vibration or motion is a problem, pump some concrete into the truck bed, it will run forward and fill the fwd part but will have little tendency to drain out after a few minutes of thickening. Presto - one point tuned damper weight invisible inside the truck.

 

[BUGGAR]

If it vibrates, give rides in it like those mechanical bulls in bars.