Flexural analysis of 3-legged tower
Flexural analysis of 3-legged tower
(OP)
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?






RE: Flexural analysis of 3-legged tower
RE: Flexural analysis of 3-legged tower
http://en.wikipedia.org/wiki/Burj_Khalifa
RE: Flexural analysis of 3-legged tower
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Flexural analysis of 3-legged tower
RE: Flexural analysis of 3-legged tower
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Flexural analysis of 3-legged tower
each leg will be in bending (with pinned reactions), the load in each leg will not be axial (which was my 1st guess)
there'll be a set of vertical couples (on the obvious two legs) and a set of lateral couples on all three legs.
not a trival problem !? made more complicated if there are fixed ends to the legs.
RE: Flexural analysis of 3-legged tower
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Flexural analysis of 3-legged tower
I'm mainly trying to confirm that the column axial tension/compression loads can be taken from the moment divided by "d". In this case, "d" would be (16")x(sq. root of 3)/2 = 13.85". A colleague felt that I should divide the moment by the distance to the neutral axis, or find the "I" of the entire configuration of columns and do an extreme fiber stress analysis. That doesn't seem appropriate to me in this case, but it did make me question it.
RE: Flexural analysis of 3-legged tower
btw, if "the legs are only 16" apart from centerline to centerline of each column", why wouldn't you use this as the moment arm for the couple, instead of "(16")x(sq. root of 3)/2" ?
not sure how the web members would affect the solution, obviously they provide an opportunity for load transfer between the legs. i think a quick FEM would provide insight and a better answer than a mass of assumptions.
RE: Flexural analysis of 3-legged tower
BA
RE: Flexural analysis of 3-legged tower
To check column buckling with wind perp. to a flat face, use pinned connections. To check interaction and torsion, use fixed connections or orthogonal springs to model some fixity (probably too much effort for such a small frame).
If you are hand-calculating this, the critical case will be with two columns in tension and one in compression, using your approach without considering extreme fiber stress.
I'm assuming this is a small antenna tower, likely less than 50 feet high.
RE: Flexural analysis of 3-legged tower
The center-to-center distance at the perimeter of the triangle is 16" between the columns. The distance from the midpoint between two columns to the third column is 13.85". I'll run this (or something like it) through an FEA at some point and see what happens. Just curious what you mean by a couple not being axial loads in the legs. Isn't the definition of a couple two axial loads acting in opposite directions separated by a certain distance?
Ron,
What you said made sense. I was just checking column buckling. It's a 120 foot tower supporting a small wind turbine, but is guyed near the top and at the 1/3 points.
RE: Flexural analysis of 3-legged tower
instead the moment is applied so that one leg reacts one of the couple loads and the other load is reacted by two legs. then 13.85" is the right couple arm. the reactions are vertical, not inclined along the leg.
i suggest you draw a FBD of a legs ... the one leg normal to the plane of bending (reacting P = M/d) is straight forward (i think !?) but the inclined legs are seeing some less obvious secondary effects (i think).
RE: Flexural analysis of 3-legged tower
RE: Flexural analysis of 3-legged tower
I agree with your colleague suggesting that you calculate I values to calculate reaction forces.
I would neglect the stiffness increased by the web members.
This is a spreadsheet I worked on for bolt groups but it can be used to calculate reactions.
RE: Flexural analysis of 3-legged tower
How can that be the critical case? The loading is less and the lever arm is greater.
Calculate I and use Mc/I, or just use M/d, you get the same result.
RE: Flexural analysis of 3-legged tower
As I look at it, the wind on the structure's leading leg is the worst case, all of the horizontal load is resisted by one set of guys and all of the vertical component from those guys goes into that lead column. The other guys are slack since the tower tends to move towards them.
All of this, without considering ice and temperature differences. Definitely black art territory.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Flexural analysis of 3-legged tower
RE: Flexural analysis of 3-legged tower
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Flexural analysis of 3-legged tower
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!
RE: Flexural analysis of 3-legged tower
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
RE: Flexural analysis of 3-legged tower
RE: Flexural analysis of 3-legged tower
RE: Flexural analysis of 3-legged tower
I apologize and I didn't mean to waste anyone's time.
RE: Flexural analysis of 3-legged tower
RE: Flexural analysis of 3-legged tower
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).
RE: Flexural analysis of 3-legged tower
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.