K7LXC wrote:
>
> In a message dated 98-04-01 12:48:24 EST, rudder1@ibm.net writes:
>
> > The Rohn 25G is rated up to 40 foot self supporting depending on wind
> > zone and wind loading per the Rohn catalog. The drawing for the self
> > supporting base specifies rebar on 12 inch centers in a 4 x 4 x 4 base
> > regardless of height or wind load. The base section must be either the
> > bottom tower section or an imbedded short base section. No other type
> > of base is permitted.
>
> As far as the self-supporting parameters, the latest Rohn info shows that
> it will take 1.5 sq.ft. @ 70 MPH and unrated @ 80 MPH. This is from the Rohn
> "Manufacturer's Technical Information for Bracketed or Self Supporting
> Towers". It's free from TOWER TECH for an SASE.
No different from what I posted above which is a response to a post that
said 25G is unrated for self supporting use. As I indicate below, I am
going to 35 foot where it is rated at 1.4 s.f. for 80 mph.
> >
> > The tower will be 35 foot high mounting only a lightning rod, omni TV
> > antenna, 2M base antenna, 40M inverted V, and one end of an 80 foot
> > inverted L. I calculate 1.35 sf wind load--all round section. This
> > will comply with Rohn's 80 mph rating. K7XLC will be quick to point out
> > that my county is a 100 mph zone--information he kindly provided me.
> > However the only way those kinds of winds occur here is during the very
> > (so far) infrequent hurricanes.
> >
> > I am not concerned about being under the rated wind speed for this
> > county since the property I bought came with a mobile home. Should
> > winds in excess of 80 mph occur, I fully expect the tower to fall on a
> > vacant lot. I will definitely not be anywhere near it.
> >
> It's your installation and you're violating the LXC Prime Directive (DO
> what the manufacturer says) and I would recommend installing it for safety and
> long-term reliability. 30 feet of freestanding 25G is rated at 1.7 sq.ft. @ 90
> MPH. That would make me feel better.
But I am doing what the manufacturer says--at least up to 80 mph. If we
ever get winds that high, any ham tower, no matter how high or stout, is
going to be knocked flat by trailers exceeding their design parameters
by a few hundred percent. Remember, Rohn's ratings are for wind
speed--not for debris speed. Not to mention the unguyed power lines
sitting atop 40' toothpicks out front.
A building inspector from one of the counties in this area gave me tips
on how to install my tower using his own 30 foot crank-up TV tower as an
example. I quote: "I mixed up a few bags of Sakrete and threw them in
the hole. When it set up I cranked her up and gave the tower a shake.
She wobbled some so I added some guy wires a guy gave me. That did the
trick."
I figure that makes me ahead of the game.
But seriously, I see no point in going for a tower rated well above what
most of the buildings around here can withstand. If this area
experiences anything but the most minimal of hurricanes, 90% of the
local structures will be knocked flat. Most of the houses are mobile
homes, virtually none of which meet the post Andrew standards. And I'm
only about 100 yards from a snowbird campground which is full of small
older camper trailers and mobile homes. The residents may go north for
the summer but the trailers remain. If we do have a hurricane, the air
will be a bit thick.
Lastly, it would not be a problem to rig temporary guys should a storm
approach. We will get plenty of warning and I have the materials and
available guying points. They wouldn't be at the normal 80% point but
with the massive self supporting base they would be more than adequate.
73 Malcolm KR4HP
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