"Richard Harrison" bravely wrote to "All" (14 Jul 05 12:04:53)
--- on the heady topic of " AD5TH Tower - Storm Damage Photos"

RH From: (Richard Harrison)
RH Xref: aeinews rec.radio.amateur.antenna:34153

RH Some metals such as steel and titanium have lower stress limits below
RH which they are mostlly im,mune to failure from repeated flexing. Other
RH metals such as aluminum and its alloys have no such immunity and
RH eventually fail under repeated flexing. In these, designs are kept
RH below the fatigue threshold for the number of stress cycles expected
RH within their lives. Or, a lifetime is specified and replacement is
RH required. Another technique is non-destructive testing to find cracks
RH and the piece is retired for cause.


How can one estimate stress cycles from a wind load?

A*s*i*m*o*v

.... All the tea in china: 356,000 metric tons.

Asimov wrote:
"Richard Harrison" bravely wrote to "All" (14 Jul 05 12:04:53)
--- on the heady topic of " AD5TH Tower - Storm Damage Photos"

RH From: (Richard Harrison)
RH Xref: aeinews rec.radio.amateur.antenna:34153

RH Some metals such as steel and titanium have lower stress limits below
RH which they are mostlly im,mune to failure from repeated flexing. Other
RH metals such as aluminum and its alloys have no such immunity and
RH eventually fail under repeated flexing. In these, designs are kept
RH below the fatigue threshold for the number of stress cycles expected
RH within their lives. Or, a lifetime is specified and replacement is
RH required. Another technique is non-destructive testing to find cracks
RH and the piece is retired for cause.


How can one estimate stress cycles from a wind load?

A*s*i*m*o*v
Believe it or not, but from historical weather data. Historical wind
profiles do exist. Statistical analysis will yield an 80 percentile, 90
percentile or even 99 percentile probability of all wind being below the
profile limit.

Asimov wrote:
"How can one estimate stress cycles from a wind load?"

Tower failure usually results from a single event or cycle of
overstress.

Radio towers are rated according to the mph or wind load ( pounds per
square foot) they will withstand while carrying all their other loads,
dead or live.

Towers do vibrate in a breeze as any tower climber can affirm. Resonant
frequency depends on construction.

Wind vibration has brought down structures. A famous example is
"Galloping Gertie", a suspension bridge in the western U.S.A.

Deflection is limited in radio towers and elastic limits are not
exceeded. The tower section returns to its original form after each
flexing.

Stress cycles enlarge certain microscopic cracks. Examples are the Comet
airliner. Three aircraft flew apart in mid-air. It was determined that
stress cracks in the angular corners of its windows were enlarged by
pressurization / depressurization cycles. This was fixed by rounded
corners but it was too late. The first jet airliner was scrubbed.

Another example is found in high pressurre gas pipelines. Their internal
pressure cycles and this tends to enlarge microscopic cracks, if any, in
the pipe. Pipes are hydrostatically tested before placed in service (no
energy storage in water as it is incompressible) so a pipe blow out of
water won`t likely hurt a bystander. After the pipe is put in service it
is retested each year.

Radio towers are not usually flexed beyond limits by vibration but come
down due to an accident and / or extreme wind.

Best regards, Richard Harrison, KB5WZI

Yes, the Tacoma Narrows Bridge !

They forgot to design it for torsional forces.
Expensive lesson to learn.

It was rebuilt without the solid sides,
this allows the winds to pass between the
steel members instead of against them.

jake

Richard wrote:

Asimov wrote:
"How can one estimate stress cycles from a wind load?"

Tower failure usually results from a single event or cycle of
overstress.

Radio towers are rated according to the mph or wind load ( pounds per
square foot) they will withstand while carrying all their other loads,
dead or live.

Towers do vibrate in a breeze as any tower climber can affirm. Resonant
frequency depends on construction.

Wind vibration has brought down structures. A famous example is
"Galloping Gertie", a suspension bridge in the western U.S.A.

Deflection is limited in radio towers and elastic limits are not
exceeded. The tower section returns to its original form after each
flexing.

Stress cycles enlarge certain microscopic cracks. Examples are the Comet
airliner. Three aircraft flew apart in mid-air. It was determined that
stress cracks in the angular corners of its windows were enlarged by
pressurization / depressurization cycles. This was fixed by rounded
corners but it was too late. The first jet airliner was scrubbed.

Another example is found in high pressurre gas pipelines. Their internal
pressure cycles and this tends to enlarge microscopic cracks, if any, in
the pipe. Pipes are hydrostatically tested before placed in service (no
energy storage in water as it is incompressible) so a pipe blow out of
water won`t likely hurt a bystander. After the pipe is put in service it
is retested each year.

Radio towers are not usually flexed beyond limits by vibration but come
down due to an accident and / or extreme wind.

Best regards, Richard Harrison, KB5WZI

On Fri, 15 Jul 2005 10:16:02 -0500, (Richard
Harrison) wrote:

Another example is found in high pressurre gas pipelines. Their internal
pressure cycles and this tends to enlarge microscopic cracks, if any, in
the pipe. Pipes are hydrostatically tested before placed in service (no
energy storage in water as it is incompressible) so a pipe blow out of
water won`t likely hurt a bystander. After the pipe is put in service it
is retested each year.

A major problem for pipelines are where both corrision and cracks are
within limits but close to each other. The pipeline folks have been
analyzing and fixing those for over a decade now.

Retesting is not done by pressure, as far as I know, but by sending
"pigs" down the line to analyze the pipeline.

Tony

Tony wrote:
"(Pipeline) Retesting is not done by pressure, as far as I know, but ny
dending "pigs" down the line to analyze the pipeline."

No surprise. Hydrostatic testing pressurized pipes to maximum pressure
or slightly more to prove their safety. This itself tends to aggravate
cracks.

A pig is a carriage (usually wheeless and propelled by differential
pressure) through the pipe. They serve many purposes. They can be used
for video inspections, ultrasonic tests, or X-ray review of a pipe.

When I was a pipeliner, they were often blown through a pipe to clear it
of trash and liquids.

Best regards, Richard Harrison, KB5WZI