Maximum Span Length of a Structure

I suppose each country would have its own standard. On a global perspective the IEEE standards are normally referred to when undertaking transmission line projects. however, according to one source, "There is currently no industry consensus design standard that specifically addresses transmission line structure loads."

have a look at the following:

transmission lines standard

transmission line design process

POWERLINE

HTH!

The span depends on so many factors.The terrain mainly is a matter to look into.The plain ground and hilly terrain span will vary for the same height of tower.Economy is another factor.With high towers large spans and few er towers requd for entire line length as against towers of lower height and more in no with reduced span.Tower height is again determined by a no of factors like the wind load, soil etc which can make weight of tower more for same height.It is not a simple jobs.there are formulae for calculating the eqivalent span which can be taken from any hand book on electrical engineering

I think we have studied this portion in Engineering mathematics (statics) - I remember the name - some Catenary uniform strength (no that was the suspension bridge) - or was it parabola ?

where one of the portion was to design the span - based on strength of wire, ice formation, wind speed, ground height, economy etc.

But too long back. Can not even search out the book. But may look into this aspect if it is for pure design purpose.

Else there are always guidelines available.

It is catenary.Now when a wire is suspended between two supports on aplain ground the span can be longer on supports that are higher.But other rfactors like wind speed etc determine tower wt. andconsidering a line length most optimum tower ht that gives minimum tonnege for the line has to be adopted.Normally a route survey followed by aprofiling the route is done and then span and tower hts are worked out.

The Conductor laid at the Power Transmission Towers

will take or follow the profile of CATENERY and NOT PARABOLA.

The equation for the catenery and the Parabola is known to us.

DHAYANANDHAN,

INDIA.

There are several key factors and in no particular order:

- wind on cables and on towers including possible dynamic effects on the cables.

- ice build-up with wind blowing: when the cables get very cold and then a misty breeze comes along and freezes on the cable leading to a very substantial increase in diameter of the cable.

- cable installation and repair: when you are initially installing the cables, you have an unbalanced load on the tower. Same applies when the cable breaks for whatever reason.

- bend in transmission line; when you need to go around a corner, the horizontal forces will be out of balance.

- type of cable: the cable has to be able to support its self weight along with the other forces described above. This is one of the main determining factors in span lengths.

The country in which you are located will have codes covering this. Otherwise, the basic calculations are relatively simple with the cables as catenaries.

An additional significant factor is required or allowable mid-span sag of the conductor dictated by the tension on (and mass of) the conductor(s).

As well as the max sag at highest temp likely to prevail.This is getting interesting.

The temperature of the lines is also a factor of the power transmission of the line. The overall sag is not too much affected by the change in temperature (to be discussed . . .). Compare it with the ice build-up weight. Also the lines allowable working stress changes with temperature.

well the conductor that will be used is the 477KCMIL HAWK conductor, 26/7 stand, 0.656 lbs/ft (weight), 19500 rated tensile strenght, and 0.858 diameter. The line will be on a hilly terrain with a wind design basis of 145mph, rocky soil. also the structure types use wood poles from southern yellow pine and we would like to use a maximum height of 70' poles. its a 3 phase 115kv transmission line with a phase spacing of 8 feet. if anyone can help point me in the right direction that would be of great help.

On hilly terrains the spans would differ considerably and also u may encounter negative wt.span( already discussed in the forum) which is an up ward pull on the supports.You have to carry out a survey of the route and profile be taken of the line.Once this is done u can decide the span.

temperature determines the max. sag which occurs at max temp and conductor must be strung taking this into account.Sag temp curves will be available./in addition with regard to ice loading the wind on ice coated conductors makes it severe with more area for wind force to act and thereby on the support.Under these conditions what is termed as Galloping of conductors can happen which is to be taken in to account while deciding the phase to phase and phase to earth clearence.

Normal Span in metre is =(CSq Root P-L/D)x0.3048

where P is height of conductor support

L conductor clerence above the ground in metre

C ruling span in metre

D conductor sag at rulin span C in mtre

Ruling span =Sq Root Of sig L cube /sig ma L

It require more detailing and not so easy to present here.Hope u cam make something out of the above.

for the structures that have negative wt. span we will use a structure that is used to accomodate for that which uses taller poles and uses suspension insulators. so far the longest span needed so far is 300 meters (across a river in a deep valley) however i am not too sure which wood pole structure can support that long a span. i have been looking at a structure on page 71 of this RUS bulletin.

http://www.usda.gov/rus/electric/pubs/1728f-811.pdf

I live in a tropical country and we don't have any ice loading on our lines so that isn't a factor and the stringing temperature we use is 90 F.

The hill terrains more of tension off is done .Each span is different.Suspension inulators and towers are used if u have a same level along the route.Also easy to restring whenever conductor snaps.You will be surprised when we did a 220KV line in Hilly himalayan terrain we had all towers as tension towers except those that were on planis or n level terrains.For negative weight span your foundation has to be heavier.Negative wt span means there is an up ward force on the tower which is on lower .To resist upward pull a heavier foundation is required ,The back filled soil alos helps in resisting up thrust and it may be nessecery to design aheavy foundation for such locations.

For river crossing use steel lattice type towers which can be lower in wt and give better wind resistance.300 M is not a big span(In our 400KV Lines we use normal span of 400 M)The rive crossing towers can be a special design with suspension insulators and on eithr side of the river should have a Anchor tower where you can tension off.Anchor towers are those whcih can still remain vertical when conductors on one side snap.You normally see them at the end of the line where one side you have the line and the other side a slack span (without tension) to gantry of the substation.For river crossing consider the following to arrive at tower height considering the sag point is well above

Highest fllood level that occurs

Max temp in that zone and sag point shoul be above highest flood level

Hope this clarifies.