Wind Load on Light Poles: AASHTO vs NESC

[bridgebuster]

When calculating wind load on a circular pole, I’m hoping someone can explain the difference between using the AASHTO Sign Structure & Luminaire Guide Specs & NESC.

In AASHTO the wind moment on a pole is:

(½ x circumference x wind pressure) x (pole height^2)/2

In the NESC the wind moment is

Wind pressure x (Pole Height^2) x ((Diameter @ ground + 2x diameter @ top)/72) x overload capacity factor

For example, a 30’ light pole on Long Island the moment per AASHTO is about ten times the moment per NESC; the AASHTO wind pressure, including drag coefficient is 28 psf; the NESC wind pressure is 4 psf (even using 9 psf the difference is still large).

We’re working for a contractor on a highway project; designing drilled shaft foundations for light poles. The contractor told our geotechs the embedment is too much - 10'. They showed us the foundation design for an adjacent section of road with identical lighting, and the drilled shafts are shorter about 7'. The design shows the wind moment - about one-third of what I calculated – but not how it was obtained. They said they can’t us the wind calculation.

My thought is that the other designer didn’t use AASHTO; it doesn’t look like he used NESC. Perhaps if someone could explain why two codes yield such different results it might help us refine our design.

Thanks

 

[bacon4life]

The 4 & 9 psf wind loads are for the combined wind and ice loading district. Instead of the district loads, you should be comparing to the speeds in figure 250-2(e).

The NESC is intended for structures carrying wires, and may not be an adequate basis for isolated structures. Typically, more than 60% of the total wind load is due to wind on the wires rather than wind on the poles.

Also, the NESC does not require extreme wind loading on poles less than 60 ft. Historically, skipping the calculations for the extreme wind case on short structures has not led to a significant number of structural failures, but did make distribution design much easier. For distribution circuits, the combined wind/ice loading is typically more severe than the extreme wind case. The 60 ft exclusion does not have a basis in physics, so there have been some code proposals to include extreme wind on shorter structures. For single use structures design with computerized tools, it is definitely a good idea to go beyond the NESC requirement to look at extreme wind. However, typical distribution circuits are quite difficult to accurately computer model because each pole will have a unique combination of medium voltage wires, low voltage wires, transformers and conduit risers along with 3rd party owned communication conductors.

A typical depth rule for wood poles in good soils is 10%+2ft. Thus I would expect to use a 35 ft pole set 5.5ft deep. I am not familiar with lighting poles or AASHTO, so I am interested if the much larger burial depth you mention is typical for lighting poles.

 

[stevenal]

So you are calculating an area presented to the wind to find the force on the structure, then applying that force at the average height to find the moment. Using half the circumference to find the area is a bit on the conservative side, while using the diameter is more realistic. I'm not sure why the top diameter is doubled, or why the divisor is 72 in the second formula. Does it include conversion factors? Where in the NESC did you find this formula?

 

[bridgebuster]

Guys - thanks for your responses and help.

bacon4life - I had a suspicion that the difference between AASHTO & NESC had to do with wires. Typically, we wouldn't design light pole foundations on a DOT project. We'd take them from a standard sheet but these light poles are not the typical DOT arrangement. Usual foundation shaft would be 6 to 10 feet deep depending on soil, slope, groundwater elevation. I'm using AASHTO because that's the standard code for roadway work.

stevenal - the wind load based on 1/2 circumference is modified by the drag factor. I've seen people calculate projected area based on 1/2 circumference & diameter. I did some research and it seems there's no agreement about which is correct. The other formula - the one I called NESC - came from a continuing education class I took a few years back on utility pole design.

 

[stevenal]

NESC says to use the projected area (rule 250C). This area would be the average of the two diameters times the height of the above ground portion of the pole. Of course a utility pole typically has wires connected, but I fail to see this in your formula above. Note that 250C is applicable to metal and concrete poles even below 60.' If the light poles are to be owned by a utility, NESC should apply.

Averaging the diameters puts a 2 in the divisor. Averaging the height above ground gets your divisor up to 4. 72 seems unreasonable.

 

[bridgebuster]

In the USDA Rural Utilities Service Bulletin 1724E-150 they use the same formula. The 72 is called a calculation constant.

The light pole doesn't have any wires attached to it; it's owned by NYSDOT.

 

[bacon4life]

Thanks for pointing that it is only wood poles are excluded.
The formula is for moments rather than area, so we can't just average the diameters. For a rectangle of the pole top diameter, the load is 1/2 up the pole. For a triangle of base = top - bottom diameter, the equivalent force is applied at 1/3rd the height and the area is 1/2* base*height. Creating a common denominator gives (2*top + bottom)/6. The units for pole diameter is in inches, but the height is in feet, thus the additional factor of 12, giving a calculation constant of 72.

 

[stevenal]

Big difference between 72 and 72PI. RUS is mixing units (diameter in inches, height and wind pressure use feet), so conversion is imbedded in the formula. AASHTO formula appears to assume consistent units are used.

 

[stevenal]

Now I see. Your NESC formula used diameter while RUS used circumference and this explains the PI.

Using the same wind pressure I get somewhat larger moment with the AASHTO formula. Seems your biggest discrepancy lies in the wind pressure.

 

[stevenal]

Upon further consideration, I believe RUS applied the wind force to the centroid of the projected area. Multiply the wind pressure by the area of the trapezoid times the height of the centroid of the trapezoid to determine moment. If similar units are used or a units aware program is used (such as Mathcad), the denominator is 6 rather than 72.

 

[transmissiontowers]

I forgot the NESC rule number (somewhere in 252 I think), but it basically says the\at the structure must withstand the hurricane wind in 250C applied in any direction for a structure without the wires attached. It was generally meant for wood H-Frames framed up but before the wires were strung in order to get extreme wind on them along the line because the cross arms and braces have so much wind area compared to a transverse wind where the arms and braces are neglected because of the wind shielding. The rule applies to all structures even those under 60 feet.

As an aside, the structural people on the NESC committee have been trying for years to get the 60 foot exemption removed from the code but the Electrical types always out vote them. A few more blackouts due to wind storms may cause the NERC and local PUC's to step in and require a change to the code (for new construction).

Back 70 years ago when the code was being written, electricity was nice to have so you could read at night and listen to the radio or watch some early TV stations. Today, people can't go 10 minutes without updating their facebook profile and texting with friends 30 feet away.

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