[TowerTalk] guying

[Jim Jarvis]

I am reluctant to extend this thread...but there's a flaw
here, imho.  I'll make this my last on the topic:

Chris wrote:

I beg to differ. The function of sensible guying is to prevent the tower
from reaching its material failure point, or at least delay it.


>The function of the guys, is NOT to add strength to the
>system.  The function is to force a different failure mode,
>and the price of doing that will be reduced load rating, or
>earlier failure,

on the contrary, the function of guys IS to make the system stronger. After
all, you do guy Rohn 25/45/55 don't you? Sensible guying here, and
elsewhere, will increase the load rating and produce later failure.
-0-

No argument about "sensible guying".

The initial post concerned tower FAILURE, not the value of guys vs. no guys.
The function of a "safety guy" in the specified situation was to keep the
tower from hitting the power lines, if it fell.  i.e. to CHANGE the failure
mechanism.

THAT SAID, a few short comments:

* if you're going to guy it anyway, why bother with a self-supporter?
	save the concrete expense!  (WB9CRY's point)
* HBDX towers were designed for small loads, like TV antennas.  They are
	NOT specified to hold anything with a boom longer than 12'.
* HBDX towers, or look-alikes, with tapered, bolted, stamped-metal legs, are
considerably
	weaker than uniform cross-section, tubular legged towers with welded
cross-bracing.
* If you guy an HBDX or look-alike, you are imposing compressive loading on
	the smaller top-sections, which may not be designed to withstand it.
	So where you guy the tower, and how, may actually cause earlier failure.
* I intuit a failure mode where a gust imposes rotational load at the same
time
	as peak compressive load, resulting in a spiral, folding failure somewhere
	in the mid-section. (just above the mid-guys?) But without knowing the
design
	numbers and the dynammic reserve in same, all our words are just that.....

	Did anyone mention the value of adding torque bars?

* Back to the original post...this was a safety and liability question which
went astray.
	Only a structural engineer can satisfy building code requirements for
	a modified structure as proposed.  I have difficulty believing that a
permit
	would be granted for a tower without safe fall distance to the power line.
	I have difficulty, as well, believing that any insurance company would
cover
	the liability.   This has "bad idea" written all over it.

n2ea
jimjarvis at ieee.org