Michael Bruss wrote:
>
>I'd like to know if their are some guidelines for determining how much a
>chimney can support.
>
>I have a brick chimney that is in excellent condition. It's 2 x 3 ft and
>protrudes 3 ft above the roof. The center flu is 1 x 1 ft and has a
>fireclay liner. Currently there is a 10 ft mast (1.5" in diameter)
>attached to the chimney with two stainless steel bands. Since the chimney
>is not at the edge of the roof, no mast can extend to the ground, so the
>only supports for the mast are the steel bands. On top of the mast is a 6
>ft vertical antenna. This setup has worked fine for about 1.5 years.
>
>What I would like to do is put a TV-type rotator on top of the 10 ft mast,
>and then attach a 3 ft mast section to the rotator. To the 3 ft mast, I
>want to attach a Yagi (Cushcraft A505S, 12 ft long, 11 lbs, wind load 2.9
>sq ft) and put the 6 ft vertical on top of the 3 ft mast.
>
>Ice is not a problem here, but it can be windy.
>
>Can the chimney take it? Are there any other problems I need to consider?
Remember that chimneys are not really designed or built to support
anything other than themselves.
The sideways wind force on the mast translates into a turning moment
which tries to break the mortar bond line below the lower steel band,
and topple the chimney over. What resists this force is the weight of
the block of bricks and the adhesion of the cement mortar between the
courses of bricks.
Cement mortar is great in compression but has little strength against
being pulled apart. After years of rocking motion from the existing
mast, the bond may already be very weak. It may look good, but quite
possibly all that is holding the bricks in place is the weight of the
bricks above them... and all the time your present mast is rocking,
rocking, rocking away at that joint to continue to weaken it.
Chimneys can survive for years and centuries in this condition, but
your plan would vastly increase the overturning moment in a strong wind.
Also, a 10ft x 1.5in mast is way too thin to take a TV rotator and a
5-el 50MHz beam, with the existing vertical on top of that. That's a lot
of weight as well as windload, so it will sway alarmingly even if
there's almost no wind at all.
When Britain had VHF TV, chimney-mounted antennas were normal for that
frequency range, and the chimney construction was very similar to what
you describe. The experience was that 2-3 elements on a 6-8ft mast (with
no rotator) would survive on most houses; but anything larger probably
would not. Either the mast buckled, the steel bands broke, or the
chimney came down.
You need to think very seriously about those possible consequences. A
tangle of broken aluminum on the roof is no big deal - but you and your
family may not survive a brick chimney falling through the roof and into
a bedroom.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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