Kevin Normoyle wrote:
> Hi Steve.
> I had read Travanty's article. Travanty assumed the moment limit implied
> by manufacturer's wind load restrictions, is because of a max moment
> limit at the base of the tower. On page 5 he says
> I, along with lots of you folks, know that crankup towers don't fold at
> the base. There are anecdotal reports of top sections folding more than
> bottom sections right? Travanty didn't have more data than I have. You
> have to analyze each section separately. The manufacturers don't give us
> the data to do that.
I haven't looked at many of these things, but at least some mfrs do
provide dimensions (of tubes, braces, lattice dimensions) and materials,
so one could do the analysis. Although, you'd probably wind up having
to make some assumptions about things like welds and construction practices.
the analysis is complex, because of the telescoping aspect.
> So, like people say: It all depends on your situation. What we lack are
> the tools for fully analyzing towers. We have a mixture of good and bad
> data and models. Let's explore making that better, not making fun of
> each other like this is actually fully scientific. It ain't.
> (I notice there are good tower modelling programs available today. But
> they're not free. Too bad. )
I think it's an economic thing. Using a "single beam" or "single
column" approximation works for a lot of the cases, and lends itself to
a spread sheet. That's inexpensive.
Start getting more detailed, and the analysis gets more complex and
tricky. You don't want to wind up with a complex model, half of which
is numbers you estimated. The work involved in the analysis goes up
pretty fast, and it's also going to be very specific to a particular
design in a particular location, so you lose the general applicability
of a spreadsheet approximation.
Finally, there's a reason that good modeling programs cost a lot.
They're hard to develop, to cover all the cases and functions needed,
perform with adequate speed on a computer (e.g. they'd better be using a
good sparse matrix package, and have an awareness of numerical precision
effects). People who do complex designs for a living use these
programs, and they have needs well beyond analyzing a crankup tower.
They also are more than happy to pay tens of thousands of dollars for a
program that saves them 10% of their time, especially because it makes
running multiple cases easier. But, there's also a fairly steep
learning curve. They're not plugnplay.
There's a lot of similarity to programs like NEC, except that NEC solves
a simple set of equations by comparison. For NEC, each "segment" has
only one degree of freedom (the current in the segment) and the
"segmenet properties" are invariant with angle/etc.. For a structure
analysis program, each segment has many degrees of freedom (the
translation of the ends relative to each other, plus bending moments
plus torsional moments). And the segment itself has stiffnesses and
failure loads that are different in all of those directions.
Even if you had a free-ware program that takes all this into account
(and there are some around), it takes a fair amount of sophistication
and a lot of work to get all the parameters right.
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