North Dakota tower

[Dave Doherty]

Hi Dan-

It does seem like if scaled everything proportionally it should work, but I 
suspect you would get to the point of diminishing returns at some point. 
You have the deadweight of the tower and antennas and such, and the very 
significant downward forces exerted as a vector result of the guy 
deadweights and tensions, and the downward vector result of the action of 
the wind working against the guys. The sum of those vector forces is not in 
proportion as you increase height, especially since the wind picks up 
dramatically with altitude.

Icing tends to occur in strata in the atmosphere, as any instrument rated 
pilot knows. (Descent into warmer air is the usual remedy for a light plane, 
because if a light plane ices up, it generally cannot climb well; but if you 
can climb, you can often climb out of it into colder air, as well.)  The 
taller the tower, the more opportunities it has to collect ice. Ice adds two 
problems:  A significant increase in deadweight, and an increase in 
cross-section of the members. That adds, sometimes dramatically, to the 
weight and windload.  And the likelihood is that some sections of the tower 
will be much icier that others, which breaks up any proportionality to the 
forces.

When ice starts to fall off a big tower, it comes in big chunks, sometimes 
several hundred pounds at a time.  As long as it hits the ground, no 
problem. But if it chances to nail a guy wire in a bad spot, or top slide 
down a guy wire to the attach point at the ground, that can spell the end of 
the whole structure. The higher the origination point, the greater the speed 
if it hits a guy wire near the ground.

So there is greater risk of taller towers failing due to weather-related 
effects, and that has to be factored into the design out of proportion to 
the height of the tower.

At some point, it seems to me that it all has to converge on a maximum 
practical height, regardless of budget, that varies quite a bit with the 
local climate.  And, of course, the maximum economical height would likely 
be much lower.

And as a practical matter, Garrett pointed out that every tower that has 
ever been built to a height significantly larger than 2063' has fallen down.

-d




--------------------------------------------------
From: "Dan.Strassberg" <dan.strassberg@att.net>
Sent: Monday, January 04, 2010 11:54 PM
To: "Dave Doherty" <dave@skywaves.net>; "Garrett Wollman" 
<wollman@bimajority.org>; "Scott Fybush" <scott@fybush.com>
Cc: <boston-radio-interest@bostonradio.org>
Subject: Re: North Dakota  tower

>
> Maybe I don't understand--easily possible; although I am an engineer,
> I am certainly not a structural engineer. But it seems to me that, as
> long as the ratio of height to cross-section of the entire tower as
> well as the ratio of the length to cross-sectional area of the
> individual structural members and guy cables were held constant, it
> appears to me that the tower would be just as strong as a shorter
> tower having comparable dimensional ratios. The taller tower would be
> much heavier than a shorter one with the same dimensional ratios
> (wouldn't the weight increase in proportion to the cube of the linear
> dimension?), so it would require a much sturdier foundation. Maybe
> the forces that would bring the taller tower down do not scale
> linerarly with the dimensional ratios, but it is not obvious to me why
> they wouldn't. So if you can explain, I would be appreciative.
>
> -----
> Dan Strassberg (dan.strassberg@att.net)
>