Design and construction of 126m tall flagpole Design parameters 9

[Raja6]

Hello Gentlemen,
Recently I have been entrusted with the assignment of reviewing design and construction documents for a 400 ft tall flagpole.
I am looking for answers to the following questions:
1. (ANSI/NAAMM FP 1001 07 Guide specifications for design of flagpoles) Which is the latest version of this specification? 2007 or 2013?
2. The maximum height specified in this spec is 300 feet only. Which spec is applicable for heights beyond 300 feet?
3. ASCE7 is referred in this specification. Do we need to use the latest version of ASCE7 2016 or 2022? Which ASCE7 is compatible with this spec?
4. What is the reference for coating for the steel?
5. What should be the minimum design life?
6. What is the allowable deflection? H/50 or H/60 or less?
7. Is any other reference code or Guide specification available in the industry?
8. What is the Risk category applicable? I or II or III or IV? Is this structure considered as a monument or special structure?
Thanks for your input in advance.

 

[Raja6]

Little Inch,
Yes you are right. I agree.

 

[BridgeSmith]

You don't find strakes on flag poles as they tend to be a tapered design especially at that size or incorporate other anto vortex mitigations.

And they look terrible.

The problem we discovered with our tapered high mast light towers we termed as a galloping effect when there was ice buildup (although it may be a vortex shedding phenomenon). Not sure if strakes would have worked, but we couldn't use them anyway, since the light ring has to be lowered occasionally for maintenance. We settled on hanging chain dampers inside the top of the pole, which appeared to work in the wind tunnel testing and computer modeling.

 

[BAretired]

You don't find strakes on flag poles as they tend to be a tapered design especially at that size or incorporate other anto vortex mitigations.

And they look terrible.

OK for a chimney, not OK for a flag pole.

Gee, I thought the helical strakes would like kind of groovy!

 

[Raja6]

JStructsteel,
Please acquire a copy of the attached standard for wind turbine supporting structures.

 

[Raja6]

BridgeSmith
Happy new year to all.
Thanks so much for all your valid inputs.
After so many meetings and discussions with the client and the designer of the flagpole, the design is not finalized yet.
As you had suggested the only way forward for this scale and complexity of flag pole, with many aeroelastic phenomena etc the design parameters, it seems can only be quantified by scale model testing and analysis for wind, vortex shedding, fatigue, connection details etc.

Hence we intend to employ a consultant who can do the advanced computer modeling and wind tunnel testing, to capture the effects of wind etc.Hope we will end up with the provision of appropriate dampers for vortex shedding and fatigue in the proposed design.
One of the many critical aspect in the flagpole design is the selection of the flag size.
The history of tall flagpoles built since 2000 suggests that the length of the flag varies from 33% to 47% of the height of the flagpole.
Every one felt that the current codes and standards proposed for the design of tall poles ( say more than 400 feet tall) are either having limitations or they are expired and hence not valid anymore.

 

[oldrunner]

Raj6:

I am the EOR on a number of tall flagpoles.

https://en.wikipedia.org/wiki/Aqaba_Flagpole

Your latest summary of the criteria required for the design is very good and is what I used on my “monster” poles as these flagpoles were initially called. Today the highest flag pole is the Cairo Flagpole at 679 feet.

https://en.wikipedia.org/wiki/Cairo_Flagpole

The original analysis for the Aqaba pole was done with a spreadsheet as there are so many weight items that have to be considered. Eventually my spreadsheet checked the pole section at every foot as I could include what ever cg of a weight item at that level. This included every flange and wall stiffeners, the internal ladder system, the weights at the top of pole – i.e. hoisting cables and equipment. This allowed me to perform a fairly accurate P-delta analysis. For the Aqaba pole, I had to consider both wind and earthquake, so the lower sections are controlled by the wind criteria and the upper sections by the seismic. This pole is basically divided into 39 foot sections, each a different thickness. All of the flanges are a minimum of 1.5” based on previous monopoles details that were established by a different pole company. I think the spread sheet was about 25 spread sheets linked together with the 1st 3 pages summary's of the various load conditions. (Lotus 123 and Dos). On these sheets were the D/C ratios with a ok or fail indicator.

When I was doing monopoles, they were all galvanized and where done to TIA/EIA code where a deflection criteria is required. I didn’t pay a lot of attention to the deflection criteria on the flag poles but a lot of attention to the P-delta, the section buckling, vortex shedding, fatigue, and the connections. Especially connections: On a previous 410 ft pole, I used A325 bolts where I was limited to the maximum diameter availability which became a problem tighthened them without special inspection. I used squitter washers there. On the 426’ pole I used A354 bolts and the erector rented two tensionmeters where I could get the tension required with any diameter. (3” max in that pole). Both poles I got complaints from the erector because of the cramp ed area. Sorry.

This is what you don’t want:

https://www.youtube.com/watch?v=tB82mqEyBBg&t=6s

I don’t like bolted flanges but didn’t have time to develop a welded connection or a something like a step tapered pole where the upper section is inserted into the lower sections. I don’t like double rows of flange bolts because of possible flange flatness issues. I don’t like the pole to be connected to the base plate with welds either side of the shell. I like chairs. I also like to do my own foundation design with a good geotechnical report.

Then I checked it with SAP90.

And when you are all done, send it to someone else to review, which I did.

 

[Raja6]

Hi Old runner,
Thanks for your post.
ANSI/NAAMM FP-1001-07 – Guide Specification for Design Loads of Metal Flagpoles- allows 33% increase in allowable stress for bending and shear. When wind is the predominant force in the case of Flagpole( when compared with Dead load and live load)how can we allow 33% increase in allowable stress for bending and shear? When we calculate the deflection, it is around 5m for the extreme wind case. No one accounts self weight of the flag pole in the P delta analysis. They consider only the tip load in the flag pole.The flag size is another grey area in the whole design. Should we consider 25% or 40% of the flagpole height as the length of the flag? Moreover when we called the technical department of ANSI/NAAMM, they wrote to us saying that as per ANSI norms this Guide is already expired in 2017.
ASCE7 16 advises to go for method 3( wind tunnel test and special approved literatures) for dynamically sensitive flagpoles ( more than 400 feet tall) with natural frequency of the order of 0.25 Hertz. see below.
The height of the building or other structure is more than 400ft ( say 122m)
o The height of the building or other structure is greater than 4 times its minimum effective width.
o The lowest natural frequency of the building or other structure is less than 0.25 Hz.
o Bridges, cranes, electrical transmission lines, guyed masts, highway signs and lighting
structures, telecommunication towers, and flagpoles.
How can we consider the AASHTO standard parameters for the design of flagpoles which are dynamically sensitive and more than 400 feet tall?
It is high time the authorities recognize the need of a comprehensive code for the design of tall flagpoles and come out with one as soon as possible. Because vortex shedding, fatigue design are the grey areas where lot of unknown factors exist which needs to be confirmed with tests and experiments.
There is also no proper guidance available for calculating the threshold frequency for fatigue analysis for flagpoles.
For vortex shedding, which is the correct damping ratio to be considered for flagpoles is not clear in AASHTO standards for flagpoles.
So a comprehensive design guidance is needed for damper design, base connection design, pretension design,fatigue design, vortex design etc.