Analysis of sign post! 5

[ksarita]

Hi,

I have to do a structural analysis on a sign post pole.( i mean the vertical rod only). The data i have is the velocity of the air and the area of the sign pad( this pad is welded to the sign pole.)
I dont need to analyse the pad, but i have to do structural analysis on the pole.
Now my question is, how to transfer the velocity effect on the pad into force and how to transfer this load on to the pole.

Please guide me. i need help.
thanks in advance
Sari

 

[jcfoley]

Sari,

One of the aerospace guys can probably answer this better than me. But, I believe that you are looking for the air stagnation pressure for a given wind velocity and temperature. You should be able to verify this in any good compressible fluids textbook.

Hope this helps.

Chris

 

[EricZhao]

One simple solution. Assume the oneside of the pad is vacume and the other side is Standard air pressure. The pressure force can be calculated easily. But this method will exagerate the aero-force by many times.

If you are going to really consider the aero-dyn force. It is going to be difficult. Most fluid textbooks don’t list solutions to this. The aero-force will be proportional to Wind Velocity^2 and Aero of the pad and viscosity. But the drag coefficient is often determined by testing instead of calculation. When the wind past the pad, it generates swirl after the pad, and the swirl is going to fall off periodically, which makes the pad and pole vibrate. So you have to consider the natural frequency of the pole.

So the best solution is use CDF to calculate the aero-force. You can also google to see if you can find something. Or you can check some high way construction standard, maybe they will list some data on this.

 

[ksarita]

thanks for your suggestions.

Eric, suppose if we take the simple solution as you mentioned then how will i calculate the pressure force. and how will i transfer this on to the pole.

Can you guide me in this, i am really new to this and i have a meeting that i have to tell my opinion about this.

Thanks in advance
Sari

 

[Drej]

The dynamic pressure, q, acting on the sign will be given as:

q = 1/2 rho (v**2)

Hence the force, f, acting at the centroid of the signed area will be:

f = 1/2 rho (v**2) A

where:

rho = air density
v = velocity of the air
A = area of the sign

Isn't it this simple?

-- drej --

 

[ksarita]

Thank you for you reply Drej,

So its like a pressure force acting on a vertical plate.
That's so simple.
So can i do this way.
I will calcuate the pressure force and apply it on to the pole surface (near the interface between the pole and the plate from the centroid of the plate.
Do you this is a good way of transferring the loads from the plate.

Sari

 

[Drej]

Yes, it's that simple.

1. Calculate the force acting on the sign due to the wind velocity
2. then find the centroid position of the sign
3. then apply the force to that point and calculate the moments/shears --> bending/shear stresses in the post using f/A and simple bending theory (My/I) etc.
4. Job done (see illustration below)



] = sign
| = post


top of sign

| ] <--
| ] <--
| ]o <-- wind load acting at centroid 'o'
| ] <--
| ] <--
|
|
|
|
|
|
|
|
- <-- calculate the shears and bending stresses here.
-
ground

 

[UcfSE]

You may have a couple of options for wind pressures depending on the building code. The ASCE 7 is the standard typically used or referenced. It will have a specific procedure for determining the wind pressure to be used on the sign. That will take into account shape, height above ground, flexibility (for dynamic effects), and so on. How the frce is transferred to the pole depends on how and where the sign is connected to the pole.

 

[TGS4]

I would also apply some torsion to your post. I have lost count of the number of stop signs that I have driven past (well, actually stopped at, then driven past ;-) that we vibrating in a torsional mode. The cause of the periodic torsion force is the shedding of the von Karmen vortices, which alternate from one side to the other.

 

[ThomasH]

Even if Drej is pointing you in the right direction his method is oversimplifying things.

You have a windspeed and from that you can calculate something sometimes called reference pressure: q = 1/2 rho (v**2), so far so good.

Now you have to consider the dynamics of the situation, the interaction between you signpost and the windflow, for this you can use a building code. Make sure that you work with the relevant windspeed. Typically it is eighter 3 seconds average or 10 min average. Based on this and the signposts natural frequency you get an increase in the windload.

Finally you have to consider the geometry (shape) of the signpost. This gives a so called "shapefactor". I don't know the geomety of your signpost but a typical one would have a shapefactor of about 1.8 depending on it's solidity.

The total windload on the post can be 4 times higher then the "q" mentioned before but it can also be higher depending on the dynamic interaction between signpost and wind.

You REALLY shold check the applicable code before doing the final design.

Regards

Thomas

 

[Qshake]

Not only should you check the code but you should check an applicable code and start with some foundation of structural analysis.

Much of what has been presented in previous posts is first year structural analysis, if not statics work.

The applicable code here in the US would be AASHTO's Guide Specifications for Sign and Luminere Structures. If it is not directly applicable to what you are doing it, at least, will give you direction on what to check for.

In addition to checking static forces and deflections, you should also reactions and displacements resulting from dynamic conditions.

Finally, as most sign and luminere structures are made from thin walled pipes of various metals you should check buckling also.

Most of the above doesn't require a FEA....you can do quite well by using Roarks Stress and Strain book. Or matrix methods using simple beam elements.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[Nildev]

I think as suggested by TGS4, Torque is must. This is because if the area of the pad is more then torque value will be much more dominant. Also the torque value depends on angle of attack. Best thing is not to apply force at centrod, rather model pad and distribute the pressure on the pad. Connect pad and rod rigidly. All calculation will be taken care.
Hope this helps rather than creating confusion.