Steel tower wind loading 11

[phuduhudu]

I am trying to get good guidance on the dynamic effects of wind on a 30m high steel tower. The first 20m are 0.8m diameter tube and the next 10m are 0.3m diameter tube. This works under static wind loads but all the texts I have found on dynamic wind loading, especially vortex shedding, are incredibly complex and really aimed at researchers rather than designers. Is there no way of simplifying the problem? Is there some practical design guidance on steel towers? Carl Bauer CEng MICE
Bauer Consult
PO BOX 2224
Gaborone
Botswana

 

[Ron]

To a great degree, codes address the transient dynamic effects of wind with the factors that are applied. Many codes are now using short duration wind events at higher speeds (ASCE 7), in addition to the gust factors and other pseudo dynamic considerations.

For a tower, considering that they are not usually in confined spaces, vortex shedding would be a microcosmic action occurring only at certain points on the structure and then only to very small areas. I would be more concerned with longer duration winds on very thin members and the aolean vibrations that might ensue. These effects can be introduced on the tower, though more often they are on the guy wires (galloping is the extreme result). These in turn, introduce true dynamic effects on the tower, though difficult to quantify, so in design we usually keep limits on deflections and make sure pre-tensions are adequate to prevent these actions.

 

[butelja]

Get your hands on a copy of the ASME Standard STS-1. The description of the standard is given below. You can order it from

http://www.asme.org

. I think it is exactly what you are looking for. I hope this helps.


Steel Stacks
STS-1
Published: 1992

INTRODUCTION The following Standard applies to steel stacks; that is, those stacks where the primary supporting shell is made of steel. It applies to both single and multiple-walled steel stacks, either of which can be lined or unlined. It also applies to steel stacks that are guyed, or to certain aspects of stacks. The stack may be supported on a foundation or from another structure. This Standard covers many of the design of steel stacks. It outlines the consideration which must be made for both the mechanical and structural design. It emphasizes what consideration must be taken for wind-and seismic-induced vibrations. It gives guidelines for the selection of material, linings, and coatings. It states the requirements for lighting and lighting protection based upon existing building and federal codes. It gives the requirements for climbing and access based upon current Occupational Safety and Health Administration (OSHA) standards. It emphasizes the important areas regarding fabrication and construction. It outlines areas requiring maintenance and inspection following initial operation.

Price: $ 72 .00

Number of pages: 104

ISBN#: 0791822109

ASME order #: L06992

 

[IJR]

Respects Ron Boss

The subject interests me too. And I find your transition between short duration and long duration wind design very fascinating. You should let us know more on the subject. I have carried out steel tower designs in collaboration with some university professors who always force me to do conservative wind design with large pressure coefficients. To discourage this behavior I ended up using TIA-EIA-222-F, which is used in transmission tower designs. The towers even with this code would undergo large wind drift. And I would be told to take only 70% of this as design drift since the winds I have used would be very short lived.

By the way, Carlbauer, next to Ron's comment let me add. Unless you have too much time to spare, make a discovery that 30m height is relatively a reasonable height and unless your design specification suggest a rigorous analysis, I suggest you follow codes like TIA-EIA-222-F(US) and ASCE 7 (US) or CP3(UK) or DIN 1055(German).

What is the purpose of your tower? Is there any cladding. For a 30m tower to acquire a 0.8m seems to me that you have a serious vertical loading. If that is true then are you in seismic zones?. If yes, then unless you have more than 120kg/m2 of wind over there, then you probably have to do dynamic analysis, not for wind though but for quake.

Ron, please comment on your great duration-basis approach.

Respects

IJR

 

[phuduhudu]

To straighten a couple of things out:

1.There are no guy ropes. It is a cantilever structure.
2.The reason for the 0.8m diameter base is that access for the first 20m is internal up to a platform at this height.
3. There will be a couple of small dishes on the last 10m section with a maintenance ladder.

IJR, I have used the South African wind loading code (very similar to British CP3)and as you commented the tube has plenty of stiffness to deal with this.

So it seems none of you would be concerned by dynamic effects on this type of tower. Carl Bauer CEng MICE
Bauer Consult
PO BOX 2224
Gaborone
Botswana

 

[Hariharan]

Vortex shedding is a problem in cantilever stacks. There could
be serious damage if it is not addressed to. I have seen 4'
diameter tubular members (1" thick!) collapse due to vortex
shedding resonance.

ASME Stack design specification (Refer Butelja's Post)
is usually followed by many
industrial chimney designers around the world. I don't think
much of research work is required.

For the chimney dimensions you have indicated, the top
portion is certainly prone to vortex shedding. You will need
to provide helical strakes or similar spoilers at the top
portion. The details are available in ASME or in some
text books (3 strakes at 120 degrees, thickness 0.1*dia of
pipe, pitch 5*dia, I think. Please check.). Note that these
strakes would increase the wind load by 50-100% (increase
in drag coefficient due to roughness , projected area).
Normally, such margins would be available in stacks.
Dynamic magnification due to wind is to be evaluated
as well.

 

[meca]

I ran some quick calculations on the tower that you described using the criteria in ASME-STS-1-1992 (See butelja post). Since all details of the structure were not provided, I did make some assumptions.

Using the vortex shedding criteria of ASME-STS-1, I calculated a critical wind speed of 5 mph (8 kph). Based upon this critical wind speed and another calculated parameter, ASME-STS-1 makes recommendations. Unfortunately, your tower falls in the range where the code does not provide any clear guidance; However, based upon my experience with stacks when you have a critical wind speed less than about 30 mph there is insufficient energy to create uncontrolled vibration. Some may dispute this rule of thumb, but I have seen in used quite a bit in the Stack industry.

(If you will all excuse this cheap plug, I did these calculations on a stack design program which I sell at

http://www.mecaconsulting.com

. It is based upon ASME-STS-1-1992 and has an ASCE 7 wind load generator built in to the program)

 

[IJR]

Friends

So carlbauer, your structure is a pole?.And the only loading is wind?. If so, comments from friends above need your attention. I personally am not so pessimistic on vortex shedding given your height of structure, but I advise a quick check. Because POLES DO VIBRATE. You might also want to provide a very good connection at the base of the 0.8m diameter pipe because, man, you are going to have cyclic loading there with a lot of range limits.

If you confirm this, then I guess you should get one of the codes mentioned above and just like "meca" did, run a check. In South Africa you probably have access to British standards, and you may want to check BS**** on Design of Steel Chimneys(I dont have the number on my mind right now, but I will post it here for you tomorrow). I once used this code and there was a straightforward simple check for wind-induced vibration of steel chimneys.

Good luck

IJR

 

[Ron]

I agree with MECCA. (I also should have read the problem statement better the first time around!)

Vortex shedding is related to the Reynolds number for the aerodynamic considerations. Using a different criteria, I also calculated a critical wind speed of about 5 mph for your monopole. The critical vibration frequency is given by the Streuhal (sp?) number which is a square wave shape based on the Reynolds number. If the Re value is between about 5x10^5 and 1.5 x10^6, the Streuhal frequency is critical. For the small diameter of your monopole, high wind pressure does not correspond to the critical wind speed, so there should not be a problem in this respect.

We most often use the TIA standards for tower design. As IJR pointed out, the lateral movement at the top of the tower is sometimes very large, but if you look at a beam analogy to the tower, these too are rational.

Ron

 

[IJR]

Respect Ron, and I am back here only to support your flawless points of views

Carlbauer, while at the subject you might as well visit a library, get an old book by M.S.Troisky "Tubular Steel Structures-Theory and Design". In the fast few pages there is a chapter on the basics of wind induced oscilations.

There is Rons formula that goes V=fD/S where V = critical wind speed for vortex shedding. S = Strouhal number and D is a stack diameter, watch the US units.

This for your interest and I believe it is a great book despite some reportedly active errate, but as Ron mentioned, you are not likely to have problems

Respect all and to The Late Mr Egor P. Popov

IJR

 

[oldrunner]

Carl:

I'm a new (just today) member of this group, but I do have some experience in communication poles as well as tall flagpoles. Here in the U.S., the design code is the EIA/TIA code as mentioned by some of the other engineers. I didn't see any wind velocity nor what windage your dishes are to resist. The design of the pole is usually dictated by the allowable rotation of the dish - i.e. 1 degree, 2 degrees etc., and this is either provided by your client or can be determined from a nomogram in the code knowing the diameter of the dish and it's frequency. Usually we design for a survival (maximum wind load) and a service wind load. The service wind load means that the pole won't rotate so much that the signal is lost.
Most of my poles are set on a concrete pier. For the design of the concrete pier, I need to know the allowable passive pressure and the maximum allowable passive pressure and where the passive pressure starts. To determine the depth, I use the Ranking formula. Many times we use a spread footing; for this I need to know the allowable bearing pressure. I will check the pressures and overturning factor of safety. We usally use square footings because of the cost saving in time and labor. I will check two directions; ie. across the square section for the factor of safety and across the diagonal for the maximum pressure.
You can write a spreadsheet for this if you are going to do a lot of poles (like I did) or you can determine the stresses and tip rotation using a finite element program like SAP90 or SAP2000.

 

Carl, I am a consultant who specialises in wind loading of structures. The problem of estimating dynamic reponse due to vortex shedding is a fairly difficult one which has not been well handled by codes and standards. It has been extensively researched for the new draft of the Eurocode on wind loading, but I think their methods are still too complex for the average structural engineer to handle. In our draft new Australia/New Zealand Standard on wind actions we have tried to come up with a simple diagnostic tool to determine whether this is likely to be a problem. The key parameter is the Scruton Number equal 4 PI times the mass per unit height times the critical damping ratio divided by the air density times the diameter squared. If this parameter is greater than about ten you should not get any significant cross wind vibrations.

I have a new book on wind loading of structures published by Spon Press of London (

http://www.sponpress.com)

which discusses this phenomenon as well as other aspects of wind loading (including extreme wind information for 55 countries). I have tried to write this book for practising engineers not researchers. If you (or any other forum member) wishes to contact me my email address is : jholmes@bigpond.com. My web site is:

http://www.users.bigpond.com/jholmes.

 

[austim]

Hi, all and sundry,

Just one minor supplement to John Holmes's posting (g'day John, by the way).

Anyone wishing to read the new draft Aus/NZ standard to which he refers may download it free of cost at

http://www.standards.org.au

Follow the prompts to amendments/downloads, and download DR 99419.pdf (my copy is downloading now, and has just gone past 1.8MB )

 

The above is helpful. Thanks to those who have contributed. Particularly John Holmes.

Your "Dynamic Analysis for Wind" paper that was distributed at a SESOC seminar in Auckland in Septemeber 1996 has been helpful for determining the natural frequency of a cantilever mast or pole. n=(0.56/h^2)*sqrt(EI/m)

I have a 12m x 0.45m diameter chimney with 3 guy rope stays at 9m.

What effect do the stays play in damping the natural response. Is it just a matter of using the free cantilever length in the formula? I suspect not, and a second mode of response is involved.

Can anyone advise a source, on-line or otherwise, where this data could be obtained? Thanks.

Regards

BF

 

[meca]

Bruce,

If the chimney is steel, I have developed a program that will calculate the natural frequency for a guyed steel chimney. If you email me more information I can run it for you. It is not as simple as a formula, since an eigenvalue solution must be performed. The stiffness of the cables must be considered as well as the stiffness of the stack.

In addition to the information provided, I also would need:

Thickness of Chimney
Material of Chimney
Dimaeter of Guys
Type of Guy Wire
Location of Deadman
Any other significant attachments to chimney (for weight considerations).

My email is chris@mecaconsulting.com .

Thanks,

Chris Rosencutter

http://www.mecaconsulting.com

 

[ishvaaag]

This is a new question.

"damping the natural response" (without them, we must think)

Every structure whatever is made of has the set of properties called modes of response.

Your problem I see can be modelled in RISA-3D and alike programs, since it has tension only elements like cables are. The pipe you can model either as made of plates or as a line element. There you can find the frequencies corresponding to the natural modes of vibration.

Of course other more complex or focused programs such meca's can maybe do a better job.

On Wind Vortex Shedding I have posted a worksheet in the Mathcad Collaboratory for free download, based in one Bolton's text, which gives forces from natural principles. Of course no substitution to the kind of knowledge holmes or oldrunner may have access to.

Other interesting thing is that at someone commented (except the application other thing dictates) almost any wind code should be sufficielent to design a pole of this size, our codes cover life safety. On the other hand the observation of fixing it very well to the foundation is no joke, buffetting is everyday ripping baseplates and loosening bolts.

 

[yinkuse]

Hi

Has anyone heard of master series? It's an application for analysis, designand detailing. It's supposed to be integrated.


Yinka

 

[tvanniekerk]

I see you are from Gabs, Botswana. I am from Jhb, SA. Have you looked at SABS 0225, the SA lightmast design codes? We use these to design the cell phone monopoles, and from your sizes, this falls right into this category. Vortex shedding is mentioned, but only briefly, as local buckling in the shaft seems to be the dominating factor, especially with a thin wall thickness.

Contact me if you want more details.

Theo van Niekerk
GM Selby Africa
+27-11-8838982

 

[chichuck]

austim,

I tried that link, and I got a site by National Fertility Management. Where can I get that standard to which you refer?


Regards,

chichuck