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]

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

 

[structSU10]

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.

 

[msquared48]

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

 

[JoshPlumSE]

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.

 

[MiketheEngineer]

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??

 

[dhengr]

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.

 

[honthan]

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.

 

[hokie66]

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

 

[racookpe1978]

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)?

 

[hokie66]

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.

 

[RFreund]

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

http://www.HowToEngineer.com

 

[transmissiontowers]

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.

 

[hokie66]

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

 

[transmissiontowers]

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.

 

[transmissiontowers]

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.

 

[SAIL3]

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...

 

[transmissiontowers]

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.

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

 

[TLHS]

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.

 

[transmissiontowers]

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.

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