bridgebuster
Active member
- Jun 27, 1999
- 3,969
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
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