In a message dated 98-06-29 23:13:30 EDT, n4kg@juno.com writes:
> One of the locals recently suffered the loss of a Force 12 6L20
> 12 ft. above a 2L40 / 2L80 interlaced Yagi.
>
> The tower was 130 ft of Rohn 45, guyed at 120 ft with a flat top
> section above the last guys. The rotor was 4 or 5 ft inside the
> top section with 13 or 14 ft of mast protruding out the top. The
> flat plate and approximately 18 inches of two legs broke loose.
> One of the welds for the top plate was alledgedly reported to be
> questionable. It is not known if the stresses were analyzed.
Let's take a look at the givens. First, the local windspeed rating is 70
MPH - the lowest they give.
Next, 130 feet of 45G is rated at 21 sq.ft. @ 70 MPH. The estimated load
of the aforementioned load was 23 sq.ft. including mast. Not dangerous but
aggressive.
MARC says that this is 32,010 in-lbs of bending moment at the tower
top/mast junction. If you take into account the TOTAL exposed structure above
the guy point (approximately 24 feet instead of 14), it increases the bending
moment dramatically. The bending moment TRIPLES to 93,828 in-lbs and the
recommended "mast" strength needs to be 275,000 PSI with 0.250" wall for a 2
inch mast.
No, this is not an apples-to-apples comparison - the 18" wide tower
section obviously has more yield strength than a 2" mast - but the 10 feet of
naked tower raised the ante to the failure point.
One of the often overlooked subtleties of the Rohn drawings is that they
are for commercial antenna installations. That's why there is 5-9 feet of
tower above the top guys - so that a commercial antenna mount (rated at 8.0
sq.ft. for 45 & 55G) will have enough clearance for the whip omni VHF+
antennas or dishes. Like Tom says below, attach the guys as close to the top
of the tower as possible for typical ham applications.
>
> IF you are planning to put a long mast in a tower, several steps
> should be taken to ensure the safety of your design. 1 - make
> sure the tower is rated for the proposed windload of the antennas,
> 2 - make sure the MAST is capable of handling the antenna load,
> 3 - place the top guys as close to the top of the tower as possible,
> 4 - consider placing the rotor at the top of the next LOWER section,
> (approximately 8 to 10 ft inside the tower) to minimize the bending
> moment on the top of the tower, and 5 - use an intermediate mast
> support just above the rotor to stabilize the mast when the rotor is
> removed.
Right, right, right, right and right.
Did the guy submit a wind damage insurance claim?
Cheers, Steve K7LXC
www.championradio.com
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