Wind pressure on octogonal POLE for a transmission line

[rubenpezu]

Hi,

I did a research in internet looking for the wind pressure you have to apply in a transmission line pole. I have found that the formula is qz=0,00256 (psf) * Vw^2. 25,6 psf for 100 mph Is this right?

My pole has octogonal shape. Have I apply a coefficient for the shape? Here in Spain we have formulas to calculate the force of the wind against flat surfaces or cilindrical surfaces. Our standard don't say anything about 8-side, 12-side, 16-side poles.

Thanks.

 

[transmissiontowers]

The reference you want is either ASCE 74 or ASCE 48. I think I have used 1.4 for 8 sided poles back when I was writing FORTRAN programs to analyze and design poles but I will check the 2 references I listed when I get back to work. ASCE 74 is the loading guide for our industry (it is currently in revision) and has the drag factors you will use. It may also be in ASCE 48 which is the pole standard. You generally use 1.0 for CD for round poles or shapes and 1.6 for most AISC rolled shapes like angles. I seem to remember using 2.0 for flat plates with sharp edges. If you use the PLS software, the wind speed in input along with the CD and the program figures out the pressure. The wind on the pole is most likely a small percent of the overall pole stress so if you miss the value a little it will not matter much.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[rubenpezu]

Thanks for your reply.

My problem is that we have a family of old poles. More or less from 90's. They were tested in a full scale test and I was comparing the values We used for the test with the values I obtain now. The formulas to calculate are the same. But when the force due to wind they used 117 Kg (1170 N aprox.), with my calculation I have around 200 Kg (2000 N). I think is a big difference.

I am quite sure that my calculation are right, but it's possible that in the station for the full scale test, they didn't check the values? I am quite sure that in the 90's we use the value of 700 N/m2 (according our standard is for flat surfaces) that is not correct I think.

For cylindrical shapes we have 1000 N/m2. I think that we should use 1000 N/m2 for cylindrical surface * Projected area (m2). We have the punctual force applied in the center of gravity. Am I right?

According our standard for full scale test. We have to apply this force in the top of pole. To do this correctly, I suppose I have to take moments in the base to reduce the force in the center of gravity applied in the top of the pole. Am I right?

If you want I could send my calculations. Thanks for your help.

 

[transmissiontowers]

I am very USA old school and always use pounds and inches for calculations, so calculations in metric are very perplexing to me. The ASCE 74 guide has the drag factor for 8 sided poles as 1.4 and the reference is ASCE 7-2005. If I had to reconcile the test data you have with calculations, I would find the wind pressure on each pole piece and sum moments at the base, then find the equivalent shear to place at the top of the pole. The other option is to find the shear that they used in the test to simulate the wind on the pole and calculate the moment at the pole base (from the height on the pole where the load was applied). You could divide the shear by the pole height and apply that linear load in your pole analysis program.

Unless your wire loads are very small, the wind on the pole shaft will be a minor component of the total load.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[rubenpezu]

Thanks for your help,

I have the problem with a small poles where the normal force is 1600 N (360 lbf) and the force due to wind is around 1300 N (292 lbf) and our standard say that the wind due to force have to be applied in the top of the pole for the full scale test. I suppose that if you apply the value of wind force directly in the top this is not correct. That you have to calculate the equivalent force in the top. Am I right?

 

[transmissiontowers]

Thanks for the conversion (if someone told me the pipe my hand was on had 6000 MPa of pressure inside, I would not know if it was ready to blow up or implode from the vacuum).

I would sum moments about the base from the distributed wind pressure and the point load from the flag or wire or whatever the pole is supporting then divide the moment by the pole length to find the shear load to apply at the tip. I assume you are going to do a full scale test to verify that the pole is good for.
I don't recall if you have PLS-POLE and PLS-CADD, but if you do, you can do the analysis there by modeling the pole and applying the wind as one load case and find the base moment from just the wind load.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[rubenpezu]

Thank you transmissiontowers, you are a source of wisdom (I hope I have written well)!!!

I have calculate with sum moments about the base with an spreadsheet that we had (I only have PLS-TOWER). At this moment we don't have to do a full scale test but It will be possible in the future because this product was tested 15 years ago. The problem is I am quite sure the calculations of the wind for that test was wrong.

I have calculate and I have to strengthen the bottom of the pole.

 

[transmissiontowers]

If you have to do many of these tapered poles, then PLS-POLE is probably the industry standard and pretty easy to use.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[rubenpezu]

Thanks,

I will ask POWERLINE to quote the product.

 

[leo5cn]

1.4 for octagnal in a ASCE manual.

 

[PersonalProfile]

AS1170.2 states 1.4 also.

Regards,
Lyle