Flagpole / Stack Deflection Criteria

[TTUengr51]

I'm currently analyzing a very large flagpole. The overall height is over 200 ft. Due to the size, I realize it will behave similar in nature to a stack/chimney structure with the addition of the flag drag force.

My concern is deflection. I have been looking for a standard limit for wind induced deflection, but have yet to find anything to "hang my hat" on. People I have consulted within the sign industry have suggested a range from H/60 to H/75 (1.3% to 1.7% of height). Has anyone come across a code reference that can back this up.

The only place in which I have found similar criteria is AASHTO. In reviewing the AASHTO manual for highway signs, luminaries, etc., they recommend 2.5% of height (H/40) for dead load conditions and 15% of height (H/6.67) for dead plus wind load conditions.

In my opinion, the AASHTO criteria for wind is too flexible. An H/6.67 ratio at 200' would induce 30' of deflection at design wind load. This would seem to cause a lot of vibrational issues at lower wind speeds. I realize roadway type signs, lightpoles, etc. are typically shorter, so this may not be as much of a concern as my condition.

Any thoughts would be greatly appreciated.



Nick Deal, P.E.
Michael Brady Inc.

http://www.michaelbradyinc.com

 

[graybeach]

I paged through the NAAMM Flagpole Guide Specifications and did not see any discussion of deflection. I would think that stresses would control before you reached 15% feet deflection.

 

[TTUengr51]

I agree, the stresses would definitely govern over the 15% limit that AASHTO suggests. Based upon stress limits only, my ratio is in the H/20 (5%) range. My gut tells me this is still too flexible but would like some type of backup to convince my client that I am not over-designing the pole.

Nick Deal, P.E.
Michael Brady Inc.

http://www.michaelbradyinc.com

 

[graybeach]

I took a look at an existing stainless steel flagpole I recently analyzed because it was having some foundation problems. This pole is 90 feet high, pinned at the bottom and has a support 13 feet up from the bottom. Under 90 mph wind, it deflected 4 feet at the top. This would be a ratio of (90-13)/4 = H/19.5. Maybe H/20 is pretty typical. The flagpole manual has charts showing pole properties for various heights, but it does not go up to 200 feet. Maybe a pole manufacturer could help you.

 

[kslee1000]

The deflection may be huge on the paper, but at 200' above (not 30' though), who is going to notice, or even detect it?
For large deflection, the foundation and anchorage could be areas of headaches.

 

[cvg]

under 90 mph wind, I believe either a) the flag has been removed or b) the flag has blown off. Most flags are removed prior to the notice of gale force winds...

 

[graybeach]

My analysis had the flag in place, and the manual recommends analyzing this way. It is a bit of an upper bound, but it avoids some worry.

 

[miecz]

TTUengr51-

If it's vibration at lower speeds that worry you, the AASHTO Guide that you mention has criteria for that. It may be located in the Appendix (again, I'm at home).

 

[JStephen]

There are limitations on stiffness for stacks due to vortex shedding. Have you checked that without a flag on the pole?

Keep in mind that flag and flagpole standards were probably not written with 200-300' flagpoles in mind, and could be inadequate for the design.

Something else to keep in mind is that replacement of a conventional flagpole is fairly minor work. When you get a flagpole several hundred feet high, it's a very high-profile project that you don't want to be replacing because the wind hit 101 mph.