Loads on foundations - TOWER 1

Heverton · Aug 29, 2014

Hello everyone, first I would like to say that is a pleasure to join such a great forum with such valuable and helpful information. I’m a trainee in a company that bought TOWER license a time ago, but no one actually knows how to use it, so I’m trying to figure it out by myself.

My main goal is to calculate the loads on foundations. I’ve read some articles saying that (one from Luiz Seiti Hatashita describes almost everything I’m doing) is possible to use PLS-CADD and TOWER, in a simple model, without any bolts, information about KL/r, etc., and in the Summary Report for All load Cases obtain the correct values.
Whenever I run a low tower (<35 m), the test runs fine and I get results that are pretty accurate to the numbers I have. However, when I run the full tower, with extensions and legs, the structure fail to converge (the exact message is: “Problem performing analysis: the model contains error, failed to converge or was canceled. Edit the structure for more details”). I’ll try to be as specific as I can:

1) I’ve created the tower from a full project we have here, with some bolts, and almost the same members (Model 2 Structure);

2) At first I edited the members KL/r and other info, but I found out later that this step isn’t necessary if I just want to know the foundations strengths (I’m not 100% sure about that);

3) Then I open the model on PLS-CADD, using the same cables, tension and others conditions that I have on the tower project. (Using IEC-60826 wind model).

4) In the Menu SECTIONS of the Tower program, I edited the Transverse/ Longitudinal Area Factor (CD from code) with the correct values.

*The problem seems to happen when I apply wind to the structure. It’s projected with more than 95% of its members and still can’t handle a simple wind.

Then I have those problems descripted above. I tried to find some information about this in the web but I couldn’t find enough, so I hope you can enlighten my path in this situation.
I just need to know how to find an error, I tried to edit the structure, adding redundant members, and others to keep it stable, but I just want to know how to properly test it.
If I missed any details, please let me know.

PS: I’m Brazilian so forgive for any mistakes.

Regards,

Heverton

leo5cn · Aug 29, 2014

can't you get the footing reactions by hand calculation ?

transmissiontowers · Aug 30, 2014

leo5cn is correct that you can get the foundation reactions close enough with hand calculations. Tower analysis is a very specialized field and not many Engineers do it. If you want to know why your model failed to converge, you can run Tower in the non-linear debug mode after it crashes to see where the instability might be. If you have a valid license, you can send the model to PLS-CADD tech support and they can look at it and tell you where the problem lies.

If you want to try it yourself, I would run the model in TOWER and apply some small point loads on the attachments and no wind on the tower to see if the geometry is the problem. You should probably have the legs as Beam elements to help with the stabilization. If you really want to learn TOWER, you should take the class PLS teaches in Wisconsin. I don't deal with metric units at all but I suspect you have too much wind on the tower if it works without wind and fails with the wind.

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

Heverton · Sep 1, 2014

Thanks for the answers.
I'll take another look at the model and see if I can find any errors.

msquared48 · Sep 1, 2014

If this is a lattice tower, considering the various load combinations of dead, wind, seismic, ice, and shape factors, that will be one h@/\ of a lot of calculations.

Easier for a monopole...

Mike McCann, PE, SE (WA)

transmissiontowers · Sep 2, 2014

Mike;
In many parts of the world outside the USA, lattice towers are used for T-Line and even substation structures. It is easier and cheaper to order a rolled steel angle, cut it to length and punch some holes as compared to the capital investment to buy a huge press brake to bend 60 foot long plates into tubes and a long seam welder plus all the hand welding to attach plates.

As far as calculations, the PLS software handles all of the mundane stuff. (we don't use seismic loading for lattice towers although we do consider them for substation structures that have lots of mass mounted up high).

Heverton;
Please post back here what you find out was your problem. You can send a BAK file of your Tower model to PLS Tech support and they can tell you what went wrong with your model. In some cases, the model will fail to converge because you have too much load and the members are failing so bad that the stiffness matrix blows up. If you just need foundation reactions, you can run the tower in linear mode. Go to General-->General Data and select linear analysis mode and it does not go through the converge routine because the deformed joint positions are not used to reform the stiffness matrix each time for each load case. If you use this linear analysis mode, be careful because it will not find the instabilities in the tower and it will give you a false sense of security that your tower is fine when it is unstable.

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

msquared48 · Sep 2, 2014

transmissiontowers:

Well, many jurisdictions are now requiring seismic loading of these structures, but there are outs in the TIA222 and similar codes too.

Mike McCann, PE, SE (WA)

transmissiontowers · Sep 2, 2014

I'm a corresponding member of ASCE 74 and I don't recall any talk of seismic for T-Line towers. They are such a distributed system and the wires don't transmit much motion or force. I'm the Chair of ASCE 113 and we do cover seismic loading for Substation structures and various load combinations. I could see the TIA 222 using seismic because it is a single structure with some mass mounted up high.

And you can call me George. I picked this handle a while back.

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

Heverton · Sep 2, 2014

transmissiontowers:

I found out my problem. It seems like the project I followed had some "problems".

The problem seemed to be the planar joints. I added a few more members in the original model, and it runs fine now. If I must say so, I used beam elements, however the problem persisted.

Thanks for the help. Now I'll focus on using the right conditions to test the tower.

transmissiontowers · Sep 2, 2014

Glad to see you discovered the error. When trying to debug an error, the first option is to make the legs as Beam elements where you have a planar joint (like non-square tower bodies)to give some stiffness out of plane. If you still have problems, run the analysis in Linear mode and look at the joint deformations which will indicate where the problem is. PLS-Tower has used Fictitious truss elements for many years before they allowed beams and you may need to stabilize the planar joints with a few fictitious (or dummy) truss elements. When the joints are constrained by either the legs or arms as beams or with dummy truss members, then you can put the analysis type back to non-linear and see if it will converge.

I like to turn on the graph of the non-linear convergence in the Output window and look at how many iterations it takes to converge. If it takes too many (like over 50 to 100) you can look for other small problems with the joints.

Good luck with your tower models and come back here if you need help. I don't read this forum too often and it may be a few days. Your fastest way to get help is to contact PLS tech support. Your company paid a lot for the software and it comes with support.

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

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