Transmission tower analysis/design 7

[JTPE]

We are analyzing the existing condition of an existing transmission tower having long (30 ft plus) L2x2x1/4" braces. We have calculated the wind and ice loads per ASCE 7 and built a Finite Element model of the tower. we applied the wind loads and the ice loads to each individual member including the bracing members.

We are finding the Euler Buckling is exceeded for the long braces when they are modeled as two way members. Are these braces typically designed as "Tension Only"? When we make them tension only the model will not converge which is our next problem. Should the loads be applied to the joints of the structure only? If so, are both the wind and ice loads translated to the joints only?

We did change all of the bracing to fixed connections and only SMALL moments are generated at the joints. moments that are within the joints design capabilities. Is it typical to design these braces as pinned at the joints, we would have expected so.

We have modeled this tower in RISA 3D, not the RISA TOWER.

thanks in advance.

 

[BAretired]

transmissiontowers,

We have a local situation where the residents do not want power transmitted overhead because of a perceived health issue (which may be quite valid). There is a considerable push to have the portion near residences and schools built underground. Needless to say, the power company is indicating the cost would be astronomical, but the residents say "too bad, that is what we want".

In relatively open terrain, what do you suppose would be the comparative cost between overhead and underground? One to five has been cited.

BA

 

[transmissiontowers]

I'll preface my answer with the caveat, I am a structural guy and work for an evil power company.

EMF is a hot button issue with the local people in the residential neighborhoods. Many studies have been done on EMF exposure and as I recall there has been no linkage between EMF and cancer or leukemia. Our linemen are exposed to much higher E-Fields when they work on energized lines and not much ill effect has been seen.

We have E-Field meters and we go out and test the fields at the edge of our right-of-way to determine the strength. Your home will typically be farther from the source than the place where we measure. If you toast your bagel daily and stand in front of the electric element, you get a much higher dose of EMF. The same goes for a 1500 watt hair dryer and even your cell phone. EMF is all around you every day, so I wouldn't worry about a beautiful T-Line passing through a neighborhood. Knowing all that, all the statistics and evidence will probably not convince the people that they are not hurting anything.

Now to the relative cost. It depends on the voltage and amount of amps. We constructed a 6 mile underground 138Kv circuit about 10 years ago for $65 million. An overhead circuit would have cost $10 million. A lot of the cost depends on the price of copper and it has gone up lately. I would guess a factor of 10 might be right for 138Kv. The big issue is the heat rejection to the ground. Earth is a bad conductor of heat and you can get a thermal run away because the copper wire creates a lot of heat. The overhead wires are cooled by the wind blowing over the wire and are much smaller (around 1.25" diameter) than insulated copper cable which is around 10" in diameter.

As the underground circuit voltage goes up to 345kV, you need to space the wires farther apart to transfer much more heat to the earth. I have heard a figure of a 600 foot wide right-of-way to handle the heat generated. Considering a 345kV overhead line takes up 150 feet of right-of-way, the land cost will be 4 times more. The extra copper needed also runs the cost up. I would guess a 345kV underground to overhead cost ratio would be in the 25 times range, but again I am a structural guy and these numbers are heard over the cubicle walls.

Keep in mind electric utilities answer to the PUC and have to justify their projects. We also have the right of eminent domain where we can condemn private property for the public good, but do so as a last resort because it is even more of a hot button issue than EMF and very expensive.

You would probably have a better chance to get the power company to re-route the line than go underground. This is classic NIMBY (not in my back yard) where you force some other poor soul to put up with the T-Line. Everyone want their electric devices but nobody wants the T-Line.

Sorry to have bored 95% of the people that read this far, but I like to inform.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[BAretired]

thank you, transmissiontowers. I appreciate your response. I may be able to exert some influence on my neighbors in the coming months.

BA

 

[dhengr]

I’ll bet the really expensive part of the whole process is digging those 80' sq. holes 200' deep, every quarter mile, to bury those darn transmission towers in. Although one advantage is that if you compact the backfill really well, this should provide some lateral support for light angle compression members.

 

[transmissiontowers]

I'm trying out the quote so I hope it works. You might contact the local utility to come out and explain the EMF issue to a few open minded home owners willing to listen. Some will never understand. The utility has a public affairs dept that will be glad to talk to concerned people.

Typically, we hold open public meetings showing all the alternate routes and land owners within several hundred feet of each route are contacted. By the time it gets this far, the opposition will have whipped up the crowd into a frenzy and they can be very vocal, but come out for the free cookies and entertainment.

And, dhengr, pretty funny. They put the wires in special red concrete to tell the backhoe operator he is about to dig into a world of hurt.


BA; Keep me posted on the T-Line route. I don't know if it is OK to post my email here, but I would be glad to write back and forth. I doubt all these Engineers want to read more of my musings. The T-Line forum (

http://www.eng-tips.com/threadminder.cfm?pid=608

) might be a spot to post messages. Not many people post there.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[BAretired]

I will let you know how it turns out.

BA

 

[hokie66]

BA's query about overhead lines through residential neighborhoods brings up another question. Do you use higher safety factors for the structural design in suburbia as to opposed to remote or rural areas?

 

[JTPE]

All, Thank you for the clarifications. We are refining our model.

 

[transmissiontowers]

No, we use the same 50 year MRI and overload factors (we place overloads on the loads) in rural areas as we do in heavily populated areas. If you wait long enough, the rural area will become heavily populated eventually. We string wires across the prairie and when civilization reaches the line and roads are built under the line, we go back and raise the lines to meet NESC code electrical clearance. We protect cows and crops with the same diligence as we protect people. We don't discriminate against the vocally challenged.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[bill30206]

JTPE's original post sounds exactly like what was occurring in this office recently. Using RISA 3D and braces in tension only, the model did not converge. This was a stairs tower, not a transmission one. We eventually tried putting in ridiculously large members for diagnosis, but it would not work. Changing several joints to rigid got the model going and the moment levels were relatively small. One thing to try is to turn off the P-delta analysis. This may require you to go to ASD 89 for the code, and of course this is not an appetizing solution. But it may let you know a little better if the design is in the ballpark.

 

[hokie66]

transmissiontowers,
I was afraid that was the case. If they try to put a transmission line through my area, I will have another argument.

 

[shobroco]

Okay, we're all structural guys, and BA hijacked the post, but I'll take it further: why didn't anyone ask what the difference is in EMF transmission distance through the air & through the ground? Maybe the NIMBYs are actually agreeing to have their butts toasted because they can't see the lines & they feel better about it.

 

[transmissiontowers]

I don't know a whole lot about the EMF issue and air vs ground but I did a Google search and found a couple of articles:

http://www.gcpud.org/pudDocuments/energyResourcesDocs/2012-01-01_EPRI_-_EMF_and_Your_Health.pdf

http://www.sce.com/Safety/everyone/electric-magnetic-fields.htm

http://www.cvent.com/events/undergr...summary-b40ceeb8c4be4b238cfa23372067f517.aspx

bill30206;
If you have joints in truss members where they intersect, you may try to put a weak spring in the plane. We use GTStrudl for our frame and truss structures and there is a command to Determine Planar Joints that does the spring trick automatically. I don't use RISA, but there may be some similar command. I think I would try to get to the cause of your model instability.

Hokie66;
The T-Lines use a R-O-W width that is generally wide enough to prevent property damage to anything outside the edge of the R_O_W if the tower fails. If they do fail, they don't fall like a tree, they crumple on top of themselves. By the time the wires hit the ground, the T-Line circuit has probably tripped out.


_____________________________________
I have been called "A storehouse of worthless information" many times.