Flag Pole 1

[whyun]

In designing flag poles for the post and the foundation, one must consider wind forces on the pole as well as wind forces on the flag. If the flag dimension is known, we must consider a certain percentage of the flag area to catch wind drag force.

I am not aware of a specific code section (in UBC) that requires a specific percentage but am used to arbitrarily assigning say 25% to 33% of the flag area. (Local jurisdictions and the state of California seems to prefer the 33%)

I would like to know what other engineers have experienced related to this topic and if there are any specific references that talks about wind drag on flags and banners and such, i'd most appreciate it.

 

[Taro]

The industry standard is the National Association of Architectural Metal Manufacturers' Guide Specifications for Design of Metal Flagpoles Manual. Available from

http://www.naamm.org

 

[whyun]

Thank you Taro. I knew of such publications but do not have a copy. Would you share with me on what they recommend to use for wind drag? Thanks again for your info.

 

[vmirat]

There is an old thread that gives the formulas for flags. Do a search on "Drag Forces on Flags." It's thread 507-1001.

 

[jheidt2543]

Why not go to Elder Flag Co. website and ask the question. They must have solved this hundreds of times in the course of selling their flags and poles. No sense in reinventing the wheel.

 

[whyun]

vmirat, thanks for the info. great thread and discussion there.

jheidt2543, i think the flag seller just sells flags. poles are manufactured by others. some pole manufacturers provide foundation as a part of their structural system and some just provide the pole only and the foundation is left to be designed by the engineer of record.

Wind force is straight from the code. Soil capacities vary depending on the site so use UBC minimum or hire a geotech to provide a report. One thing missing in the code is the info on how much area to use for flags. Likewise, what about designing a chainlink fence post? code has no design criteria. do you "anticipate" a solid banner sttached to it and design it for wind against the whole surface or do you take a certain percentage. If certain percentage, how much and what is the justification for it.

Code seems to address the superstructure but not enough minimum design criteria for parts of structure or miscellaneous structures so engineers are left to second guess the force level that he/she is comfortable with, then building official goes, that is not conservative enough! And battle of "opinion" begins.

Just something to think about. Thanks all.

 

That's what we do!!

"Structural engineering is the art of molding materials
we don't wholly understand, into shapes we can't fully
analyze, so as to withstand forces we can't really assess,
in such a way that the community at large has no reason
to suspect the extent of our ignorance." ...Jim Amrhein

 

[Mrmike]

Wind load on flag (per ANSI/NAAMM FP1001-90):
W=0.0014*((1.3*V)^2)*(A^0.5)*Ch
where V - wind speed on mph
A - area of flag in sq.ft.
Ch - Coeffidient oh height for wind:
H Ch
>14 0.8
29 1.0
49 1.1
99 1.25
149 1.4
199 1.5
299 1.6

 

[vmirat]

I referenced a previous thread on this same subject. Here is the actual text of the thread from "oldrunner"...

I'm surprised that no one has mentioned the Guide Specifications for Design of Metal Flagpoles, ANSI/NAAMM FP 1001-97.

You didn't state the flag material, but the code provides values for nylon and cotton flags, and for polyester flags.

For nylon and cotton: Wf = .0010 x V^2 x Af^.5 x Ch
For polyester: Wf = .0014 x V^2 x Af^.5 x Ch

where V = design wind velocity in mph.
Af = flag area in ft^2
Ch = 2.01 x (z/900)^(2/9.5) where z = height of the
pole in feet.

It's probably a good idea to obtain this code, or if you have the AASHTO Standard Specifications for Structural Supports for Highway Signs, Luminares and Traffic Signals, 1994, there are mostly the same procedures except the AASHTO Standard doesn't have the flag load formulas.

VMIRAT here again...
It is interesting to note the difference between "MRMIKE" and "oldrunner" in what they have provided from the NAAMM manual. As a comparison, look at an example of a nylon flag at 20 feet with a wind of 85 mph:
"oldrunner" "MRMIKE"
44.6# 73.7#
The difference is in the 1.3 increase to the wind speed. So which one is correct?

 

[mapestru]

9/27/02--To whyun from mapestru--re: Flag Pole design:

1. I agree with most of the above, (including the confusions)( I would opt for use of the 1.3 factor since the cost would be miniscule, while the "uncertainty factor" in unit fabrications like flagpoles, is, in my opinion, somewhat substantial.

2. But an important aspect appears to have been left out of the discussions, above:---That is the fact that flagpoles are also subject to "Von Karmen Vortex" occillatory forces.
At certain critical heights, and flag "weights", this may cause the pole to oscillate laterally;--- ie in a plane perpendicular to the direction of the wind. ( I have seen amplitudes that appear to be several feet !)

3. The magnitude of the stress effects of that Von Karmen vortex phenomenon, may be of interest, --- but what is most important is that such phenomena are a serious cause of structural fatigue effects.

4. Because of those fatigue effects, it is important that the details of the attachment of the pole to the base plate be such that "Stress Raisers" are avoided,--- or, at least minimised. ( Certain of my previous forensic investigations have identified the cause of a flag pole failure to be associated with such stress raisers.)
A simple circumpherential weld, of adequate size, is usually best.

5. The use of vertical fins, occasionally welded between flagpole and base plate, so as to reinforce the base plate, can be very damaging to the flagpole, due to the stress raiser effects caused, where structural fatigue effects occur.

6. Also, another precaution to be noted is that if the flagpole is to be of Aluminum, there are special requirements for welding, which effectively significantly lower the allowable stresses for the Aluminum material.

7. Hope the above is helpful.......Good luck