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Sag and Tension Limits and their use

Sag and Tension Limits and their use

Sag and Tension Limits and their use

I browsed through the threads refering to sags and tensions and realized the wealth of knowledge available and was impressed.  I decided to start a new threat so as not confuse the others looking information.

I have been viewing some printouts from the sag10 program that illustarted calculated sag and tesion values for different conditions.  I believe that these calculations were based on some given design (loaded or unloaded) criteria or limits.  It is my understanding that at least one of these design limits is needed to achieve sag and tension values at different temps, condistions, etc.

The printouts that I have use (for simplicity) design limits A, B, and C to list other values at different tempertures.  Now lets say that my initial design criteria was A, C, and E, whould this sag and tension table be different?

I have been reading what I can about this and now I'm here.  It seems that you design a T/L you determine various unloaded and loaded limits and the program converges (or does something) on one of your design limits.

I know the formulas for Sag and tension (they use each other) so I'm curious how a computer program starts to do sag and tension not know one or the other.  I assume it some how uses the beginning limits to starts and then converge to the others you need.

I have a list of sag and tensions for a 400 ft ruling span and I do NOT agree with all the limits the last engineer used to do this.  I am just curious the final sags and tensions would have been acheived anyway regarless of the initial limts.

The program used to do this is DOS and from the late 80's.  It runs but it is VERY user abuser and NO documentation.  So reproducing with my criteria can only be done by purchacing SAG10 and that is $5k.  Well before I do that I must know what I'm talking about before I talk my boss into spending that $.

Thanks for any input...sorry so long..but you get exceited when you may be getting close an answer.


RE: Sag and Tension Limits and their use

I'm not sure I understand your question.  Any transmission line design must be based on a set of design requirements for required clearances, allowable conductor tension, maximum loading, maximum ambient temperatures, wind loading, ice loading, etc.  Changing any of these could conceivably result in different results for sag/tension.

The computer software will attempt to compute appropriate tensions and resulting sag based on the criteria given to it.  In some cases, the software may use built-in criteria based on standards such as the NESC or other codes.

Consider the fact that it was not long ago that all of the calculations were done with a slide rule.  Not that I want to go back to those days (although my slide rule is in my desk drawer), but just to point out that the computer is just speeding up the computations.  

I'm not familiar with Sag10, but maybe a review of the program documentation would reveal how the design criteria are defined.  

RE: Sag and Tension Limits and their use

I agree with dpc, but maybe I can add something to start you thinking.  You have hopefully gotten as far as choosing the conductor size.  You have come up with some design criteria as dpc states - ice loading, wind loading, conductor temperature under emergency loading, maintained clearances above roads, waterways, grade, etc.  You have an idea what kind of structure and what ruling span will be most economical for the installation.

Now, the higher the tension, the greater the demands on your angle/end structures and guying.  The lower the tension, the greater the sag.  Fix one parameter and play with it.

For example: you may choose 20% of the conductor ultimate tensile strength (UTS) as your design tension at -18 C.    From this you can determine sag and tension at all temperatures for your ruling span.  You can also determine a stringing chart for other spans, because probably not all your spans are going to be equal in length to the ruling span.  The maximum tension will occur at either coldest temperature or maximum ice and wind loading.  The maximum sag will occur at maximum temperature under emergency loading.

If the maximum tension exceeds what your structures or guying can take, you have to either lower the tension or upgrade your structures and guying.  Maybe the tension is ridiculously low for the structures and you can use a cheaper design.

If the maximum sag lowers your clearance unacceptably, you have to increase tension, raise structure height, or decrease span.

So, it is a bit iterative but you should be able to make a fairly educated guess to start with based on past experience.  Most utilities have standardized line designs so you can't go too far wrong if you stay within their limits.  My guess is you have given the computer program span length and conductor size.  As dpc states, the program must use default criteria if you don't give it any.  It is probably assuming tension as a percentage of UTS at one temperature and calculating sag for that temperature, then calculating sag and tension at all other temperatures.

RE: Sag and Tension Limits and their use

One other thought:  if this is a one-off, it is cheaper to pay a cable manufacturer to prepare a set of charts for you based on your criteria than to buy the program yourself.  In fact, I got Alcatel to do it for me for free.

RE: Sag and Tension Limits and their use

First of all thanks to both of you.  I appologize for not being to good at explaining my problem, however, you both did answer my question.  I am sorry I could not reply earlier but I had a surprise meeting and it took up the whole day.

I want to continue my conversation if you do not mind.

In RT's example of a design criteria (A) of 20% of the conductor UTS as your design tension at -18 C.  I set this up as a single design criteria and let the program calculate (or get them done by a manufacturer) sags and tensions at different temps.

bare with me as I appear stupid, to make sure I understand.
a. By using design criteria (A) am I saying that at -18C the tension will 20% of UTS?

Now lets say I add another design criteria (B) of 60% UTS, wind 4psf, ice 1/2in, at 60F.
b. does the program or (manufaturer) use both of these criteria as a starting point?
c. will this now lead to another set of sag and tensions at different temps? like at 60F, ice 1/2, wind 4psf, it may not be 60% UTS and at -18C it would not be 20% UTS because it used both criteria as startin points?

because I missed today, I want to also begin to understand something else.  Assume that I'm in the future and have a sag and tension chart for initial and final condition at given ruling span (300ft for example). And lets assume I know the equation for sag to be a function of span length (L) and tension (T) sag(L,T).

If in my T/L design (RS=300ft) I want to know the sag of a 362ft span.
d. is safe to say that the sag in the 362ft section is = sag(362,Trs)?  where Trs (tension at ruling span) is the tension from my Sag and Tension chart at desired temp.
e. if this is true, is it still okay to make this calculation (assumption), if I use a structure with rigid pole insulators instead of suspension insulators?

Okay I put enough nonsense forward for one night.  Thanks very much



RE: Sag and Tension Limits and their use

Work on the clarity and conciseness of your postings and we may be able to help you better.

There is a good description of line design factors in Chapter 5, "Installation Practices" of the Aluminum Electrical Conductors Handbook published by the Aluminum Association.  It is still in print and available - search the web for "Aluminum Association".  I advise you to purchase this book.

There you will learn about sag-tension charts, stringing charts, ruling spans, sag-span parabola, stress-strain graphs, etc. in better detail than I can describe and 99% of your confusion should clear up.

RE: Sag and Tension Limits and their use

re: my last post.  I didn't mean to sound so snotty.  It's been several years since I did an overhead line design.  I'm at the point where I have to reference my technical material to remember what I did.  You are further ahead to get hold of the Aluminum Association Handbook than rely on my explanations.

That said, try to think a little about what you are saying rather than writing down rambling thoughts.  I will have a go at answering your questions as I understood them.

a) Yes.  I chose 20% as an arbitrary number.  The -18 C came about because this is the default temperature used for application of the ice/wind loading in the absence of better information.  I can't see applying your Heavy loading at 60 F; ice won't even form at that temperature.  In reality the tension will be chosen so that the kneepoint of the conductor strength on the stress-strain graph won't be exceeded for the governing tension - either the Heavy loading at -18 C, or the tension at some colder temperature without loading.  If a prolonged temperature lower than -18 C could exist, then you may be wise to apply the Heavy loading at the colder temperature.

b) the criteria inherently conflict - you must pick one only.

c) I can't see how this is physically possible.  If you fix a criterion, then that number shouldn't change.  If you input 20% UTS at -18 C and find the tension changed at -18 C in the printout, then I wouldn't trust the program.

d) You have lost me.

e) I don't think it matters.

RE: Sag and Tension Limits and their use

Suggestion: I agree with the statement of dpc (Electrical) Apr 30, 2002 marked ///\\\
///I'm not familiar with Sag10, but maybe a review of the program documentation would reveal how the design criteria are defined.\\\

I used several electrical power system analysis programs and made comparisons of their outputs for the same input. The comparisions revealed differences in the outputs. It may be difficult to obtain equations that are incorporated in the programs because of proprietary rights. However, some spot check calculations by pencil and paper will reveal how the software is designed, its accuracy and what may be needed to obtain more acceptable or reasonable results.

RE: Sag and Tension Limits and their use

Thanks to all.

I researched it today. and now have a good understanding of what I was trying to acheive.  I do appreciate all your input as it all helped.


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