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Transmission tower analysis/design
6

Transmission tower analysis/design

Transmission tower analysis/design

(OP)
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.

RE: Transmission tower analysis/design

Braces may be designed as acting in tension only for very slender members or tension and compression if they meet a slenderness ratio of 200 or less. The members you describe can only be acting in tension.

Ice loads on a thirty foot long brace would have to be considered as a uniform load and carried as a catenary, producing tension throughout in addition to tension from lateral loads. It would be normal to disregard the end moments of such bracing members.

BA

RE: Transmission tower analysis/design

I would think that the members were designed as tension only if they are in an X configuration.

If the braces are in an X configration, there is a way to determine if the center of the brace configuration can be considered braced if you are designing them as tension-compression elements. See "Design of Diagonal Cross Bracings" by A. Picard & D. Beaulieu from AISC for some information.

Is the non-convergance case also including P-delta effects on the system? Otherwise i don't see why you would get an error if it were modeled properly. I do not use RISA, only have minor experience with it, so I might be missing something.

RE: Transmission tower analysis/design

I do not think ASCE7 is the governing code here.

You really need to check TIA-222-G and apply that codes criteria, especially considering the icing and wind load combinations for lattice towers.

There are other special programs out there to apply this criteria directly without very many special gyrations.

Are you sure that there are no interior braces normal to the 30 foot long angles that would decrease the L/r ratio? I notice them all the time, and 30 feet seems a bit long.

Mike McCann
MMC Engineering
http://mmcengineering.tripod.com

RE: Transmission tower analysis/design

Using Tension only X bracing with the braces connected at the center of the X can frequently lead to iteration issues. You might just try modifying the model so that the braces don't connect. That might solve your iteration issues.

RE: Transmission tower analysis/design

And RISA has a habit of "failing" if you try to use a 2-d mode without a "Z" bearing - usually fixed. Might that be your problem??

RE: Transmission tower analysis/design

JTPE:
You might get better, more knowledgeable/experienced, subject and codes specific, help from the engineers who frequent the “Electric Power & Transmission & Distribution” forum, here on E-Tips. It’s right around the corner from the “Structural” forum, look it up. I’m not intending to suggest you don’t get good advice here too.

RE: Transmission tower analysis/design

I have also been working on the model with JTPE and like the idea of removing the node at the crossing of the x-braces that seem to be exceeding their Euler buckling limit. I will try this and see what occurs.

RE: Transmission tower analysis/design

Why not just remove the members which are in compression from your model?

RE: Transmission tower analysis/design

It appears to me/concerns me that the direction of the questions seems to vear off from

"How do I accurately model an existing tower (as-built complete with flaws or old/invalid/incorrect design assumptions for long braces and (possibly) incorrect and inconvenient connections!)?"

into

"how can a I change the model to make the model look like it works and can calculate faster by deleting braces, connections, failure modes and parts - that actually exist?"

Aren't we putting the cart (the theoretical model) in front of the horse (the real tower, as-built with possibly incompatible/incalculable connections)?

RE: Transmission tower analysis/design

racookpe,
I don't think so. Everything we do in structural engineering involves modeling. Structures are always smarter than we are, but we just do our best.

RE: Transmission tower analysis/design

I would make sure you are using the correct code as mentioned earlier however the Rev G is very similar to ASCE7-05. Also make sure that you are using the wind speeds that correspond to the ice loading.

EIT
www.HowToEngineer.com

RE: Transmission tower analysis/design

3
ASCE 10 is the standard you need for transmission towers. Everything is assumed as pinned connections in lattice towers and we adjust the KL/r based on connection fixity. We do not apply ice on members of the tower (not required by ASCE 74 or NESC), we just load up the conductors with ice. The de facto standard program for analyzing towers is from PLS-CADD from Power Line Systems. They have about 95% of the tower market.

There is another forum for these questions and I just came across this thread by accident. I've been analyzing towers since 1973 so I can answer most questions about this very narrow subject. You typically leave out the redundant bracing members and adjust the leg and lacing KL/r to assume that the redundant will brace the leg. Of course you have to account for the missing area for the wind drag by adjusting the section parameters. All this is taken care of in PLS-Tower. The latest version will allow you to have the redundants in the structural model but analyze the tower as if they are not there.

In my part of the world, the hurricane winds are much more severe than the ice loads. The NESC-2012 has wind maps from older ASCE-7 so you can get the right loading. ASCE 10 gives stress allowable because nobody else is crazy enough to put single angles in compression bolted on one leg. We have towers on our system that are about 100 years old, so somehow it works.

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

RE: Transmission tower analysis/design

Thanks for chiming in here, transmissiontowers. I think this is an area of structural engineering best left to specialists like yourself.

RE: Transmission tower analysis/design

2
We walk a thin line between success and failure. Unlike a bridge or building with human life considerations, we use very low safety factors (on the order of 1.2) and design for a 50 year event. We are willing to accept a few failures in the quest for a light structure. We don't have to worry about people climbing our towers during an ice storm or a hurricane. The occasional drunken teenager climbs them and electrocutes themselves. For any of you Jackass, SteveO, wannabees considering this endeavour, a 345 KV conductor will flash-over to ground through your head if you come within 8 feet of the wires (no they are not insulated) under the right conditions of humidity. The wires also are very hot under a heavy electrical load and can reach 250°C which is why you rarely see birds on the conductor.

We are also one of the few areas of Engineering where our designs get a full scale load test to prove that we knew what we were doing when we put the joints and members into a program that told us the design would be safe. We test to destruction so we see how much fat was left in the design. We can afford to do this because a design may be built 500 to 1000 times and if we save 1000 pounds, the savings continue to pay over the next 50 years.

So as you drive across the country and see a lattice tower line, maybe you can appreciate that it stands there for 95% of it's life just loafing around waiting for that once in a 50 year wind event to come along so it can prove it's stamina.

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

RE: Transmission tower analysis/design

OK, to get back on topic, the OP asked about Tension-Only members. When you just model the main legs and lacing members without the redundants, you leave out the crossing joint and designate the lacing as a T-O member. The L/r can get up to 500 and when the program correctly analyzes the system, the non-linear analysis will allow the T-O compression member to go to 100% of its compression capacity and not simply remove it from the stiffness matrix and give it zero load. It will take compression up to its capacity and will remain there as it bows out. The tension member gets load and you get a partial brace point where the 2 cross as the tension member will stop out of plane buckling.

Take a look at ASCE 10 in example 7, 8, and 9 to see how crossing diagonals are analyzed. Most general FEA programs do not handle transmission towers easily. GTStrudl has a tower module where you can check member capacities but it requires lots of input for each member. I have not used Risa Tower but you can see some T-Line towers on the Power Line Systems web site. It is a very specialized FE program just for T-Line towers (and communication lattice towers)

http://www.powline.com/products/tower.html

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

RE: Transmission tower analysis/design

transmissiontower..I have alot of respect for what you guys do...I consider myself fairly flexible when it comes to relying on engineering judgement over codes, but some of the design you guys do would make my hands sweat....the key, ofcourse, as you mentioned, that it is backed up by real-world full-scale testing to destruction...

RE: Transmission tower analysis/design

The bridge and building engineers have to contend with buses of children driving over their bridges and doing so safely 1000's of times a year so they have to have big safety factors.

A few years ago I was at a conference in Orlando and we got on an elevator and we packed in fairly tight when a lady wanted out from the back. She was scared that the elevator was overloaded. There were 7 or 8 structural engineers on it and we told her you could not pack enough human flesh on an elevator to overstress the cables, but she insisted that we stop on the next floor so she could get off. The elevator designers must have huge safety factors.

I just designed a river crossing tower that is 330 feet tall and weighs 220,000 pounds. This would be about 30 stories tall if it were a building but I'll bet a 30 story building weighs quite a lot more.

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

RE: Transmission tower analysis/design

I've done some stuff in the lower end of distribution style works. The factor of safety is definitely nerve wracking. When you're talking about pole structures that aren't critical, you can sometimes basically design them for failure at the design load case (i.e. factor of safety of one) and still meet code. They're also allowed to degrade significantly in the field before replacement is required. Normally you don't really want to do that, but the whole factor of safety can sometimes just be based on engineering judgement.

I'm pretty sure towers would scare me if I couldn't find an experienced person to help me. Once you start going that far down to the wire you really want to know what industry standard practice is or you'll do things that are reckless and open you up to liability, or over conservative and uneconomical.

RE: Transmission tower analysis/design

Since nobody is around when the hurricane blows them down, we either blame it on "Tornadic Winds" or a rogue gust. ;) We don't actually have many failures that are not related to material problems. I looked at one where the base shoes called for a full penetration weld from the leg angle to the base plate and we actually just got a fillet weld and the weld failed which brought the tower down. The tower had been in service for 15 years and the fillet was strong enough for all winds until we got a hurricane severe enough to expose the fault.

The ASCE wind maps are approximations as are the drag factors on angles. Since we don't have wind tunnels big enough to test a tower and the Computational Fluid Dynamics programs are not advanced enough to handle a structure as complex as we have, the wind on the tower and the wind on the wires are probably overstated. There is also the fact that there are multiple load paths as members buckle so the actual tower is stronger than the computer predicts.

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

RE: Transmission tower analysis/design

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

RE: Transmission tower analysis/design

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.

RE: Transmission tower analysis/design

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

BA

RE: Transmission tower analysis/design

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.

RE: Transmission tower analysis/design

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


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.

RE: Transmission tower analysis/design

I will let you know how it turns out.

BA

RE: Transmission tower analysis/design

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?

RE: Transmission tower analysis/design

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

RE: Transmission tower analysis/design

Quote (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? )


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.

RE: Transmission tower analysis/design

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.

RE: Transmission tower analysis/design

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

RE: Transmission tower analysis/design

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.

RE: Transmission tower analysis/design

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/energyResourcesD...

http://www.sce.com/Safety/everyone/electric-magnet...

http://www.cvent.com/events/underground-transmissi...

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.

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