guyed stack design 15

[harvey13]

Is there a 'standard' for the design of guyed stacks published by some organization?  I am familar with wind pressures (25-33 lbs/sqft - 100mph) and wind loads (pressure x projeted area).  I am less familar with accepted methods of calculating critical buckling loads, determining gauge thickness, effect of stiffeners, number of guys, etc.  Are there inexpensive programs that use the equations of this 'standard' and my design requirements (height, diameter, wind speed, etc.) to select the minimum steel gage, number of guys, etc.  I would appreciate any information.  thanks harvey13

 

[TowerEngineer]

I guess it's time for me to add my two cents to the discussion with regard to guy wire galloping and aeolian vibration, and guyed and self-supporting structural vibrtations in general.

Guy wire galloping is usually the result of the guy wire pretension force being set too low, whereas aeolian vibration is the result of guy wire pretension being set too high.

Guy wire pretensions are normally set between 8 and 15 percent of their ultimate strengths in order to prevent both of these phenomena.

There is a simple formula for calculating the approximate fundamental frequency of the guy wire. The formula is:

f = ( (F/m)^1/2 ) / 2*L

where:

f = fundamental frequency in Hertz
F = average tensile force
m = density of wire per unit length
and L = length of unsupported wire.

Excessive aeolian vibration is the result of vortex shedding that closely matches the fundamental frequency of the wire or one of its harmonics whose energy is not sufficiently dissipated by the wire or its connections. This is usually the result of wire prestension being set too high. Safe wire bending amplitudes as a result of vibration are generally very low because of the very high number of cycles (numbered in the millions of cycles) that a wire or conducter may experience over its lifetime of 50 years or more.

Galloping guy wires generally result as a response to wind gusting where the force from the gust acts as an impulse load. If the wire pretension is set too low, then the wire can be very responsive to gust loading and very large galloping motion can occur. This galloping motion can be especially destructive when cross-over occurs. Very high tension spikes frequently occur resulting in 'hammering' of the guy connections. These tension spikes and the resulting 'hammering' has resulted in the destruction of the guy connections as well as the vibration dampers.

Stockbridge type guy vibration dampers have been used successfully for more than 50 years. These dampers are an Alcoa patented product (Patent No. 3052747) that have very widespread use in the electrical transmission and guyed tower industries. They are very common on guyed towers greater than 700 feet tall. They are also very commonly used on guyed stacks that are greater than 200 feet tall.

A newer vibration damper called the 'AR Damper' has been on the market since the late 1980's. Research has shown that this newer damper can be up to 7 times more effective than the Stockbrdge damper under certain conditions.

You can read more on the subject in Alcoa's Electrical Conductor Accessory Products Handbook. It offers a good, concise discussion on wire vibrations and their elimination. There is a simple formula for calculating the period of vortex shedding and and the free-loop length of the wire (distance between anti-nodes). If the total length of the wire exactly matches a mutliple of the free-loop length, then you are likely to have a condition of resonance. This source also provides some damping efficiency curves for the Stockbridge damper in relation to wind velocity in MPH.

Furthermore, a good discussion of wire vibration theory is offered in the Resnick and Halliday textbook on Physics, Volume 1, pp. 404-445.

More can be found on the subject in the Telecommunications Industry Association document EIA 222-F 'Structural Standards for Steel Antenna Towers and Antenna Supporting Structures'.

With regard to eliminating vortex shedding on guyed or self-supporting stacks... One very simple method of vortex elimination is to weld short vertical segments of angle iron to the side of the stack in the upper portion of the stack where stack displacements are more pronounced. The angle iron can be attached with the two leg tips touching the stack to reduce wind load or with one leg bolted in full contact to permit removal.

I trust the foregoing will answer many of the questions that have appeared in this discussion forum in the last couple of weeks.

 

[IJR]

Respects TowerEngineer

Great discussion fellas and I am going to try and mark this one for future refs.

regs
IJR

 

[Dekar]

Husker,

If it is not much annoyance, I am thankful to send a copy to me of the article published by ASME.

Fax number : (507) 264-3545

Regards,

Dekar

 

[chrisp6]

Husker,

I also would be very gratefull for a copy of the article on 'Design Procedures For Guyed Stacks' i'ts one that I havent seen before and I work For an industrial chimney manufacturer.

Regards,

Chris Pegg

F.: (+44) (0)1874 623 725

 

[chrisp6]

harvey13,

I work for a steel cimney company in the U.K. 'Rodell Chimneys Limited' (

http://www.rodell-chimneys.co.uk),

we specialise in the design, manufacture & installation of chimneys and associated ducting/support structures.

I imagine after 12 months your chimney has been built and installed. If you wish to discuss any aspect of your design I would be happy to do so through our web site.

If your design was for proposal and has not yet been built I urge you to visit the C.I.C.I.N.D. web site '

http://www.cicind.org'

and look down the list of members for a chimney specialist in your area of the world who should be familiar with the relevant standards.

Regards,

Chris Pegg

 

[TowerPE]

STAAD does not have very good, if even adequate, cable elements, nor does RISA. They seem to use an equivalent tension only element which is not a true cable.
Check out

http://www.c-conceptsinc.com

for true cable behaviour for towers and guyed monopoles.

 

[zeemech]

Husker,could you please fax me a copy of the article on guyed wire supported structures that you mentioned.my fax number is 713-771-1642. Thanks.zeemech