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guyed stack design
15

guyed stack design

guyed stack design

(OP)
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

RE: guyed stack design

I have been designing guy wire supported stacks for many years and I am not aware of any published standards that are available.  There are some text books that address guy wire supported stacks a little, but they are usually not real helpful.  You mentioned wind loading, and you should be aware that guyed stacks are flexible structures and the wind loads can be significantly higher than that on non-flexible structures.

I have several programs related to stack design on my web site at www.mecaconsulting.com .  However, at this point I do not have a program to do the complete analysis of a guyed stack.

Stack design is my specialty, so if I can help you please send me an emal at chris@mecaconsulting.com .

Chris Rosencutter, PE
Meca Consulting
www.mecaconsulting.com

RE: guyed stack design

Harvey13:

First of all, you need to get a copy of ASCE 7-95, Minimum Deaign Loads for Buildings and Other Structures.
Just knowing the conversion from mph to psf pressure is only a small part of the problem.  

In addition to basic pressure, you will need to account for height, gust, shape and terrain factors.  The gust factor is affected by the shape and natural frequency of the structure.  If the frequency is less than 1 hertz, then the structure is treated as a flexible member and the gust effect goes up.

Yes there is a software package available that will do what you want, but it is not "inexpensive."  No software that can do this will be cheap. A guyed structure needs to be analyzed as nonlinear and will therefore require a series of trial and error iterations until convergence is reached.
The software I use is from Powerline Systems in Wisconsin and will even optimize the diameter and thickness of the steel needed to do the job.  It is used mainly by power companies to design large transmission poles, guyed or not.
Their website is http://www.powline.com.

Another source is from Eaglepoint Software in Iowa. Here you can get a DOS based finite element analysis program called C-Straad which will solve the nonlinear problem and then interface with a steel design module to size the member. They are at http://www.eaglepoint.com/Structural/.

I'm sorry there's no quick fix for you here. The nature of what you need simply demands some sophistication.

Good luck.


wyf (The Polecat)

SESI
I invite people to visit my website if you wish to discuss these issues further.

RE: guyed stack design

Followup note to my previous posting:

I just noticed the reply to you made by Meca Consulting.  I also have and use a very comprehensive and well layed out spreadsheet developed by them (called Wind95.xls) based on ASCE 7-95 that will give you the wind pressures I was describing.

wyf (The Polecat)

SESI
I invite people to visit my website if you wish to discuss these issues further.

RE: guyed stack design

Apart from just structural design, you should also study the possibility
of vortex shedding induced resonance in slender stacks. If the problem
 is likely you might need to provide spoilers at the top region, which
would increase the wind load significantly, affecting the basic design!

RE: guyed stack design

There is an ASME standard STS-1 Steel Stacks which addresses the design of steel stacks which are guyed or unguyed, single or multiple wall, and lined or unlined.  It also covers vibration issues including seismic and wind induced.  It is available from ASME for $72.

RE: guyed stack design

8
There is an article that addresses the non-linear nature of the guy wires.  It considers not only the elastic stretch of the cable, but also the sag/span ratio and the initial tension:

A Design Procedure for Guyed Stacks Applying Non-Linear Cable Mechanics, by N. Gilbert, Y. M. Chokshi, Foster Wheeler Energy Corporation, Houston, TX.

It requires a highly iterative manual solution.  I have gotten bad results using structural design software.  The cable sag and pre-tension is handled poorly by some.  There is a company that has invested in writing a program to perform the design, according to the article, but you would either have to buy the flare from them or possibly pay them to do the design.  

The article was distributed by ASME.  There is no date on the copy I have but the newest reference publication is 1968, so it is a vintage article.  It is 8 pages long, if you would like a copy please respond with your fax number.

RE: guyed stack design

Question:  Why are you designing your own stack?  There are half a dozen companies that specialize in this kind of work and can do a better job faster and cheaper.  The first time designer usually creates a stack much heavier than needed, and many times that can still be inadequate.

The first thing your design must consider is the type of stack required: bare wall, dual wall, insulated, lined?  This determined by your service.

The next thing is the physical location.  Is the stack mounted on a rigid foundation?  Flexible roof?  is it near another stack or structure that can affect the wind flow?

Someone mentioned the use of strakes to break up votices.  This is but one method to mitigate against votex induced vibrations, most designers opt for tuned mass dampers or even damping pads, but these are only required for a minority of the stacks built.  Here experience is the rule.

Then, the stack design needs to match up with fabrication and erection methods.  Is it to be erected as a single unit hoisted into place, or piece meal?  Of bolted or welded construction?

There are two available publications to guide the stack designer, the ASME publication listed above, and CICIND publication (they have a web site), but designing your own stack is a little like designing your own automobile - it's possible, but I won't ride with you!

RE: guyed stack design

3
Harvey13,

Sorry I didn't see your post earlier.   I have specialized in the design of guyed and self-supporting tower structures for almost 20 years.   This experience has also included structural consulting for the nuclear power generation and petroleum industries.   To my best knowledge, there is only one firm other than my own that has computer software specifically designed for non-linear computation that is capable of the precise and accurate analysis of guyed towers, stacks, and other structures that would be required in your situation.  This company is Weisman Consultants in Downsview, Ontario, Canada.

Recently, ANSYS released a new FEA package that seams to give good results for non-linear analysis, but this software seems a bit cumbersome to run.   Also, Formation Design is offering new software that has tension-only members with second-order analysis,  but I have not had a chance to check this software yet.

With respect to analyzing this type of structure yourself,  I strongly urge you to find a consultant who specializes in the design of guyed structures and has done so for a number of years.   This is especially true if your stack is very tall in relation to its diameter.   A great number of second-order effects will have a significant impact on the design of the structure.

I plan to write a book on the design of guyed and self-supporting tower structures in the near future (including guyed stacks), but I am unaware of anyone that has written comprehensively and at length on the precise methodology and rationale that goes into the design of these unique structures.   The analysis is very complicated.   For tall guyed structures the analysis can be performed by hand, but it would require about three or four man-months to perform the required calculations that can be performed in a matter of seconds by use of a computer.  Because the analysis is non-linear, it requires the use of an iterative solution that includes stiffness changes for both the supporting guy wire elements and the main stack or mast.  Any solution so derived will by necessity be approximate to some degree.  How approximate the answer is dependent on the number of iterations required to reach the specified convergence criteria.   It is very likely that large errors (blunders) will be made if this calculation is performed by hand based on the enormity of interdependent calculations involved.

At any rate, if you have not already done so, I strongly recommend that you find a consultant to assist you in the design and analysis of your guyed stack.   Most worthy consultants that specialize in this field have done so for more than 10 years and have designed structures up to 2000 feet tall (the current FAA limit).

You can visit my web-site at: tower-structures.com if you so desire.  My contact info is available on the site.    I would be happy to discuss your problem in more detail.

Best Regards,

D. Wayne Davidson, SE

RE: guyed stack design

Lincoln electric offers a very informative and inexpensive  (~$15) book, Tubular Steel Structures, Theory & Design.  It has an entire chapter devoted to analysis of guyed stacks.  The web address is where it can be ordered from is http://www.lincolnelectric.com/services/educate

RE: guyed stack design

Hellow Husker,

I just read your response, dated Oct./ 30/ 2000, on harvey13's guy wire design issue dated on Jul./ 28/ 2000, and I am very interested in the "A Design Procedure for Guyed Stacks Applying Non-Linear Cable Mechanics, by N. Gilbert, Y. M. Chokshi, Foster Wheeler Energy Corporation, Houston, TX" article that you own.  Could you send me a fax on this article?  My fax number is (787) 977-0173.

I appreciate your information.

Cordially,
Michael Troche Feliciano, P.E.

RE: guyed stack design

STAAD Pro 2001 has just released their latest version to the public.  Has anyone had experience using this version of the software with guyed stack designs?  Research Engineers indicates that the software now handle guy wires through non-linear analysis.  Additionally, the software also allows stack designs to remain stable even after the guy wires are made tension only members.  My findings have shown a substantial difference in the outcome of the design using the new STAAD Pro vs the Older versions.

What are your opinions?  Is Staad Pro 2001 output realistic for guyed stack design?

I will post this question in the STAAD Forum as well.

Thank you in advance for any additional posts.

RE: guyed stack design

I have not used the latest version of STAAD Pro, but I have used STAAD Pro 2000.  I am somewhat skeptical of Research Engineers claims, because I have seen them make similar claims in previous versions of their software.

My biggest issue is with the non-linear analysis, which they have told me on several occasions that their software would support.  In the prior versions they calculate the cable stiffness based upon the pretension, and then the remainder of the analysis is performed using that calculated stiffness.  This is not a non-linear analysis, and I would be curious to see if they have truly corrected this problem.

As a point of reference, I have just added a Guy Wire supported stack design program to my website (www.mecaconsulting.com).  You can download a demo for FREE, and compare to STAAD Pro 2001 results.  I would be interested to hear what you find out.  I would do this myself but I am no longer a STAAD User.

RE: guyed stack design

A guyed chimney stack is very similar to a yacht's mast - so before starting the final design, I suggest a few days out on the bay. Observe the rigging. On the windward side, it is all tight and straight. On the leeward side, the rigging hangs loose, and does nothing.

It is the same with a guyed mast or chimney. When the wind reaches the ultimate level, the guys to windward will all be tight, and there will be virtaully no load on the other two or three sides. So design each guy to react its share of the wind load. There is no need to worry about non-linearities - by the time the guy is ready to break, there will be virtaully no sag.

Design the mast/chimney for the bending loads between guy points, assuming a continuous beam fixed at each guy. Then check the deflection pattern of the mast, to make sure it is achievable, with an acceptable amount of plastic redistribution. If not, then it might be necessary to move a guy, or make some parts over-strong.

Vortex shedding from the guys or mast can be a problem. It helps to pre-tension the guys to raise their natural frequency, but take care not to buckle the mast as a column. (Note that when the loads reach the ultimate limit state, the down-wind guys have relaxed, and the preload in that direction plays no part, but the preload on the guys at right angles is still there.)

If the mast vibrates, the sailor's solution is to wrap a rope around it in a spiral. A 16mm rope will dampen vibrations in a 250mm diameter mast. (The light towers on the Stand at Townsville survived the last cyclone!) A similar solution can be used for chimneys. The spiraling vortex breaker does not need to be very large, and doesn't contribute greatly to the overall drag.

Cheers,


RHK


Russell Keays

RE: guyed stack design

Cool notes Russel.

Clear and simple. Dropped by only to appreciate the approach to engineering problems.

regs
IJR

RE: guyed stack design

The guyed mast analogy is good to a point, but there are a couple of considerable differences between stacks and masts (and towers for that matter).  

1)  Stacks are usually venting or flaring hot gases, and so there is significant thermal expansion involved.  A 300 ft tall stack venting 450 Deg. F gases will grow vertically 11.3 inches.  This thermal growth is resisted by the guy wires, and so big loads can be introduced to the guy wires, stack, and foundation.

2)  The non-linear behavior of the cable must be taken into account.  If you assume that the cable is linear, you could significantly underestimate the lateral deflection of the stack.  In addition, if you consider the cable as linear you will significantly over-estimate the axial load in the cable and stack due to thermal expansion.  

Guyed towers do not experience significant differential thermal expansion, and so their pretension is usually set very high (about 20% of Breaking Strength).  At a pretension this high, it is a good assumption to assume the cable is linear.  But stack cables must be more relaxed to account for thermal growth.  Typically their pretension is set at about 6% to 12% of the breaking strength.  Pretensions this low are very much in the non-linear range.  They will become linear as they reach the design load, but initially they are non-linear.

3)  Stacks are usually fixed at the base, not pinned.  This needs to be considered so that the stack stresses are correctly calculated.

4)  I have never seen a guyed stack with vortex shedding problems.  One simple precaution is to place the guy wire attachment points on the stack at uneven increments, so that they do not correspond with nodes for a mode of vibration.  If the stack has ladders and platforms, then that is an additional vortex breaker.  

Similarly to rope being wrapped around the mast, I have used helical strakes on the upper 1/3 of self supported stacks that are suseptable to vortex induced vibration.  However, the shape factor increases 50% when you add the strakes and the cost of the stack can be almost doubled.  

Regards,

Chris Rosencutter

RE: guyed stack design

meca

You and friends above have done a great job here. Do you guys want to go ahead and tell us the difference between guy dampers and the helical strakes? I was once involved in a guyed chimney tower and we were asked to provide dampers against guy vibration,but so far you pals only mention measures against vortex shedding. Which one is a must or are they the same thing?

Excuse my ignorance and thanks for the detailed explanation above

regs
IJR

RE: guyed stack design

IJR

The helical strakes are to prevent vortex induced vibration of the stack (mast).  I don't know that I have ever seen them on guyed stacks, but they are somewhat common on self supported stacks.

I have to plead ignorance on guyed dampers, since I have never used them.  It is my understanding that they are to prevent galloping of the guys on very tall towers.  Stacks are rarely taller than 500 to 600 ft, and the cables are usually very large to resist the wind loadings.  It is my understanding that galloping is an issue when the tower is tall and the cables are relatively small.  Someone please correct me if this is incorrect.

RE: guyed stack design

You are right on damping against galloping,meca. And I believe that galloping is a dynamic effect of wind on relatively thin guy rope-magnification of displacement at some wind frequency, or so I assume.

Unless my assumption on galloping is wrong(which I would like to refine), then yes, the length of the guy is what makes it critical. But how long is what I havent been able to resolve.

Thanks meca for the faith and for making me note the differences between the two.

RHK:    How do you calculate the natural frequency of a guy that has been pretensioned?

IJR

RE: guyed stack design

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.

RE: guyed stack design

Respects TowerEngineer

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

regs
IJR

RE: guyed stack design

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

RE: guyed stack design

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

RE: guyed stack design

harvey13,

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

RE: guyed stack design

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.

RE: guyed stack design

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

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