[TowerTalk] sorry for multiple mails, but had another idea

[jimlux]

Donald Hofmann wrote:
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>  
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> -----Original Message-----
> From: jimlux [mailto:jimlux at earthlink.net]
> Sent: Sunday, December 14, 2008 10:12 AM
> To: Donald Hofmann
> Cc: w9rma at charter.net
> Subject: Re: [TowerTalk] sorry for multiple mails, but had another idea
> 
>  
> 
> Donald Hofmann wrote:
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>>
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>>
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>>  Also I don't much like the idea of just depending on guys to hold the
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>>  tower up. If they fail the tower comes down. If I mount the steel
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>>  base plate on three bolts at least it won't come down.
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>  
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> But that *is* the whole idea of a guyed tower.  If the guys fail, it
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> comes down.  It's true that typical tower sections are designed to take
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> some amount of bending loads (after all, the wind load on the tower
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> between the ground and the guy point is a bending load, as well as a
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> shear load)
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> The tradeoff between self supporting and guyed is that in the self
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> supporter, you need to have more bending strength in the structure,
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> which is why they are heavier and wider at the base (which reduces the
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> load on the components)
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> *The point is: who wants their tower to fall down?  Not me. If I can guy 
> it as well as make it self supporting then why not?*
> 
> * *

Towers are engineered structures: that is, they're designed to balance 
the various loads among the various components in a particular way, 
generally with the objective of doing it in a cost effective manner.  If 
you start changing the design, you change the distribution of loads. It 
might be better, and it might be worse.

A classic problem in introductory engineering classes is showing how 
adding a gusset to a 90 degree joint between two members lowers the 
ultimate failure load.  The gusset causes the stresses to be 
concentrated at the point where the gusset ends.

Amateur towers are interesting, because typically theres a lot of margin 
in the design, and there's also lots of "less than catastrophic" failure 
modes (e.g. if the antenna didn't fall down in the winter storms, it 
wasn't big enough).  Amateurs can also play the statistics game.  You 
might live in a X mi/hr wind zone, but that wind speed might only 
actually occur once every 10-20 years.

The design has to accommodate the worst case, while the user really only 
observes the average case.  If you make a modification, it will probably 
still work in the average case, but when it comes to the design case, it 
might fail earlier, or in an expected way.

There's also the "luck of the draw" on assembly and material properties. 
  A good design has margin in the design to account for the uncertainty 
in the welds and materials, so that it will stand up, even if the welder 
came in with a raging hangover on top of their cold and wasn't as 
careful as hoped, or that some other hiccup occurred.  These sorts of 
things are rare events, so the vast majority of towers survive beyond 
their "design loads".. but that doesn't mean that the original design 
was wrong.  It was just luck.

Jim