Flagpole / Stack Deflection Criteria
Flagpole / Stack Deflection Criteria
(OP)
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.
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






RE: Flagpole / Stack Deflection Criteria
RE: Flagpole / Stack Deflection Criteria
Nick Deal, P.E.
Michael Brady Inc.
http://www.michaelbradyinc.com
RE: Flagpole / Stack Deflection Criteria
RE: Flagpole / Stack Deflection Criteria
For large deflection, the foundation and anchorage could be areas of headaches.
RE: Flagpole / Stack Deflection Criteria
RE: Flagpole / Stack Deflection Criteria
RE: Flagpole / Stack Deflection Criteria
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).
RE: Flagpole / Stack Deflection Criteria
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.