On Tue, 17 Feb 2004 17:47:39 -0000, "Richard"
wrote:

I'm wondering about the spacings, A, B, C, and D.

Hi Richard,

You should be thinking more of the tensile failure of the brick mortar
leveraged by the moment of the load above. Chimneys are very strong
to compressive loads, and as brittle as candy to torsion.

The upshot of this is that on a windy night you may find the chimney
in your bed.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Tue, 17 Feb 2004 17:47:39 -0000, "Richard"
wrote:

I'm wondering about the spacings, A, B, C, and D.

Hi Richard,

You should be thinking more of the tensile failure of the brick mortar
leveraged by the moment of the load above. Chimneys are very strong
to compressive loads, and as brittle as candy to torsion.

The upshot of this is that on a windy night you may find the chimney
in your bed.

73's
Richard Clark, KB7QHC

========================

Better get in a stock of viagra.

Brickwork is weakest when under a tensile stress. So the chimney brickwork
is most likely to fail under tension due to sideways thrust of the wind.

Placing the chimney under torsion causes only a horizontal shear force on
the brickwork to which it is more able to resist.
----
Reg.

Reg Edwards wrote:

"Richard Clark" wrote in message
...
On Tue, 17 Feb 2004 17:47:39 -0000, "Richard"
wrote:

I'm wondering about the spacings, A, B, C, and D.

Hi Richard,

You should be thinking more of the tensile failure of the brick mortar
leveraged by the moment of the load above. Chimneys are very strong
to compressive loads, and as brittle as candy to torsion.

The upshot of this is that on a windy night you may find the chimney
in your bed.

73's
Richard Clark, KB7QHC

========================

Better get in a stock of viagra.

Brickwork is weakest when under a tensile stress. So the chimney brickwork
is most likely to fail under tension due to sideways thrust of the wind.

Placing the chimney under torsion causes only a horizontal shear force on
the brickwork to which it is more able to resist.
----
Reg.

All this presupposes the chimney is brick.

Mine is cast concrete and has had a big TV antenna on it for years and
this is a high wind area.

There is a smoke and soot problem and I only run the fireplace a few times
a year. I wouldn't put an expensive ham antenna (or a rotor) up there.


--
Jim Pennino

Remove -spam-sux to reply.

On Tue, 17 Feb 2004 20:01:40 +0000 (UTC), "Reg Edwards"
wrote:

sideways thrust of the wind.
=
Placing the chimney under torsion

When a body in the form of a rod or a tube, albeit a short rod, is placed
under torsion, ie., it is subjected to a TWISTING MOMENT, then SHEAR
stresses are set up in it.


The shear stresses are in the plane of the cross-section - adjacent
cross-sections tending to slide over each other.


When the body experiences a sideways thrust, as from wind, then a BENDING
MOMENT is set up in it. One side of the body is under vertical TENSION and
the other side is under vertical COMPRESSION.


Brickwork and concrete are much the weakest when under tension. Not so weak
when under shear. Much the strongest when under compression.


The weight of brickwork exerts a uniform compressive stress over the
cross-section. Initially there is no tensile stress. But the bending moment
due to wind on ONE side of the structure eventually overcomes the
compression. On THAT side the stress becomes tensile and the brickwork parts
company with itself, ie., the structure initially fails under tension.


It is only AFTER failure has occurred due to tension that shearing takes
place and bricks begin to slide sideways, one over the other.


The art in the design of brick and concrete structures, eg., as for
chimneys, bridges and gravity damns, is NOT to allow any TENSILE stresses to
be set up. Hence the frequent use of steel reinforcement.


Memo: In view of the variability in such materials apply large factors of
safety.
----
Reg, G4FGQ

"Richard Clark" wrote in message
...
On Tue, 17 Feb 2004 17:47:39 -0000, "Richard"
wrote:

I'm wondering about the spacings, A, B, C, and D.

Hi Richard,

You should be thinking more of the tensile failure of the brick mortar
leveraged by the moment of the load above. Chimneys are very strong
to compressive loads, and as brittle as candy to torsion.

Hi,

I confirmed too that you will experiment problems with the maconnery.
I attached the end of a G5RV to my chimney, fine, no problem for years, but
the vertical damaged the bricks and the cement udner high wind.
A small roof pylon (2-3m high) is by far preferable and more secure.

Thierry
ON4SKY
http://www.astrosurf.com/lombry/menu-qsl.htm



The upshot of this is that on a windy night you may find the chimney
in your bed.

73's
Richard Clark, KB7QHC