Flexural analysis of 3-legged tower

[bjcure]

I've got a 3-legged trussed tower composed of three pipe columns in each corner. In plan view it is an equilateral triangle. Between the pipe columns are vertical and diagonal web members. If I apply a moment to the tower, about an axis that parallels two of the legs, how do I calculate the tension and compression? Does one column take all the compression and the other two columns take the tension (or vice versa if the moment is in the opposite direction)? In other words, would one column have double the axial load as the two other columns (except opposite direction)? Or is it more complicated than that?

 

[dhengr]

You almost shouted, in bold and all caps too??? Is that some sort of internet etiquette issue? I was going to use all double bold, all upper case letters, in 28 point print, and was thinking that would be just a grumpy whisper, given my feelings on the matter. You guys waste your time and effort to try to help someone with what appeared to be some kind of a real structural problem, and then after 13 posts or so he says; by the way here’s the important info. and details about the problem, and it changes almost everything you guys were guessing about my problem. Where is the internet and forum etiquette in these kinds of questions which waste your time, effort and willingness to pass on your knowledge and experience? Particularly, when the OP’er. won’t waste his time to fully lay out the problem, in the first place.

I think we should all take a few weeks off, let every OP go 20 or 30 posts, by which time we might have a better idea what the real problem was, from all the back and forth B.S.; even determine if the participants had a vague idea of the scope and nature of the problem was. Some of you more experienced fellows waste your time giving such good sound basic advice, and then have it turned on its head when all the cards are finally laid on the table. Someone on these forums has a sign-off which says something to the effect, ‘first define and be able to fully describe the problem, then solve it.’ Let’s all start doing that!

 

[BAretired]

dhengr,

I agree with you. Members of this forum are too eager to jump to the assistance of the OP without fully realizing the extent of the problem. In this instance, the very least we should expect is a sketch showing the tower, its supports and the loads acting on it.

BA

 

[hokie66]

Well, at least I think we gave some good answers to the original post, which could have been the real problem, but turned out not to be. And better yet, we might possibly have frightened the OP about trying to design a guyed mast with inadequate experience.

 

[rb1957]

the 3 struts 16" apart (on a tower 120' high) are going to be in equal compression and the moment is carried by the guys ... it's usual practise to pin the mast at the ground, i'd expect that you'd want the rotational freedom and i'd expect that if you did fix it, the moment it'd react would be a tiny portion compared to the guys.

 

[bjcure]

I think I gave everyone too much information. The tower has already been designed; I was just helping the contractor figure out the pick point locations when he lifted the tower. So I was only checking the maximum distance that the tower could cantilever when being lifted by the crane (moment in the tower and resulting column buckling as well as bolt capacity where the column pieces are spliced). My original question covers that issue, I believe. Of course the guys change everything for the in-place condition.

I apologize and I didn't mean to waste anyone's time.

 

[bjcure]

By the way, I essentially got the same result when calculating the overall "I" and using Mc/I as when I used M/d.

 

[rb1957]

IMHO, you didn't "gave everyone too much information" ...

it would have been helpfull if you'd said ... "i'm lifting a tower" ... that would've stopped several posts about loads (wind, etc). then we'd've known there is a distributed load (weight) and a single reaction (the crane) ... ok, probably two reactions (assuming a sling).

you could calc the max moment as a function of inclination, and the critical loading is probably with a single leg in compression and two legs in tension (possibly a single leg in tension, and one leg going along for the ride).

 

[dhengr]

The top set of guys should probably be more than a blade length below the top of the tower. On this tower, that wind turbine better not be to big. The thrust from the turbine will induce a fairly significant bending moment at the elevation of the top set of guy wires and this will tend to force the shape and orientation of the buckled shape, along with the guy wire system. The pinned base is a good idea too. All fairly obvious comments.

There has been a good deal of sound general advice given here, depending upon what the real problem is, and that keeps changing and getting more complex every 13 posts or so, but the best advice of all was hokie’s, which was in essence that the OP’er should clear about 140 or 200' radius around this tower, for when it falls over. And, low-and-behold, the question and the problem have changed again. Bjcure... That wasn’t too much info., it was just all the wrong info. for your real problem. And, you sent a bunch of very good, experienced, advisors on a goose chase, unrelated to your real problem, because of your bad problem definition or description.