I'm also a ham and a one-time violist, as well as an amateur
instrument-maker.

Jon Teske wrote:
The only time both sides of a
stopped string vibrate is when we play what we
call a "harmonic" and there are several of them on violin strings. A
harmonic is when the violinist lightly touches the string so that the
unexcited side (e.g. not the bowed side) can also vibrate

In particular, you have to touch lightly enough that some energy
can couple into the other half of the string. Some of the energy
is transferred through up-down motion and some through the bending-
stiffness of the string itself. You could see the finger as a series
capacitor to ground, attached to each half of the string by a pair
of resisters bypassed by another capacitor. You can tweak the values
by the way you touch.

I don't think I have ever also considered my violins strings to have
impedance, or reactance, and I haven't quite considered the feedline
problem.

Mechanical impedance/reactance is a widely used idea in mechanical
and civil engineering. The dynamic motion of bridges and buildings
is sometimes even modelled using electronic circuit modelling tools
amongst other methods.

Clifford Heath.

The early radio pioneers were schooled in acoustics, mechanics en chemistry.
So they understood quite soon how to resonate the electric version of a wave.
That takes a bit of the mystery away.

Yes, 4 x ¼ = 1 lambda. So a folded dipole is 1 lambda and an open dipole is
a half lambda. But a folded dipole can thus be halved. A sort of trombone shape.

BUT the ends resonate with tension right ? So we have to feed one end with
high impedance also. Next detail and the big question for me; may the other end
be grounded ? In case of a situation where no counterpoise can be installed.

Because of the folded dipole which is earthted in the straight middle, i think the
half wave end fed can be earthed. Agreed ? The idea is to topfeed a mast which
stands in conductive grounds. So ¼ wire up and the tower is ¼ wave down,
which to my paper insight should work as an ½ wave antenna ? pls say yes. :-) ?

On Tue, 17 Mar 2009 15:18:35 +1100, Clifford Heath
wrote:

I'm also a ham and a one-time violist, as well as an amateur
instrument-maker.

Yeah! I am not alone. BTW I also play viola at a symphonic level.
Never tried making any life is too short to do both.

Jon Teske wrote:
The only time both sides of a
stopped string vibrate is when we play what we
call a "harmonic" and there are several of them on violin strings. A
harmonic is when the violinist lightly touches the string so that the
unexcited side (e.g. not the bowed side) can also vibrate

In particular, you have to touch lightly enough that some energy
can couple into the other half of the string. Some of the energy
is transferred through up-down motion and some through the bending-
stiffness of the string itself. You could see the finger as a series
capacitor to ground, attached to each half of the string by a pair
of resisters bypassed by another capacitor. You can tweak the values
by the way you touch.

Yeah I forgot to mention the energy transfer side of that. Since
harmonics are hard enough to produce in any event, a tunable capacitor
might be welcome. Unfortunately a fiddler has only two hand, both
heavily employed.

I don't think I have ever also considered my violins strings to have
impedance, or reactance, and I haven't quite considered the feedline
problem.

Mechanical impedance/reactance is a widely used idea in mechanical
and civil engineering. The dynamic motion of bridges and buildings
is sometimes even modelled using electronic circuit modelling tools
amongst other methods.

I guess that is the thing they forgot to calculate, or didn't know
how, on the infamous Takoma Narrows Bridge in Washington State which
collapsed when cross winds cause wild vibrations.

Jon Teske W3JT

Clifford Heath.

Mechanical impedance/reactance is a widely used idea in mechanical
and civil engineering. The dynamic motion of bridges and buildings
is sometimes even modelled using electronic circuit modelling tools
amongst other methods.

I guess that is the thing they forgot to calculate, or didn't know
how, on the infamous Takoma Narrows Bridge in Washington State which
collapsed when cross winds cause wild vibrations.


That was an unexpected coupling between the force from the wind and
torsional vibration of the roadbed. As the roadbed tilted, it "caught"
more of the wind and had more force applied, moving it further. The
torsional resonance was such that it oscillated with ever greater
amplitude (not much different than a flag flapping, or a blade of grass
in the wind.. not quite like a wind instrument reed, though)

As for whether it could have been anticipated? I don't know that
modeling was that advanced back then (1930s). The bridge was an
architectural feat, with a very delicate looking thin roadbed and much
longer than most other bridges (3rd longest when it was built, some 1500
feet longer than the Golden Gate, for instance). It was much longer and
thinner as compared to other suspension bridges of the time which were
double decked, (SF Oakland Bay Bridge) for instance.. making them
torsionally much stiffer). Interestingly, the designer of Tacoma
Narrows (Moisseiff) was also involved in the Golden Gate.

On Wed, 18 Mar 2009 11:11:42 -0700, Jim Lux
wrote:

I guess that is the thing they forgot to calculate, or didn't know
how, on the infamous Takoma Narrows Bridge in Washington State which
collapsed when cross winds cause wild vibrations.


That was an unexpected coupling between the force from the wind and
torsional vibration of the roadbed. As the roadbed tilted, it "caught"
more of the wind and had more force applied, moving it further. The
torsional resonance was such that it oscillated with ever greater
amplitude (not much different than a flag flapping, or a blade of grass
in the wind.. not quite like a wind instrument reed, though)

In fact, it was exactly like a reed. The Tacoma Narrows Bridge
exhibited the highest roadbed length to roadbed width ratio of the
designs of that era, and this was a contributing factor.

As for whether it could have been anticipated? I don't know that
modeling was that advanced back then (1930s).

Charles Ellis (an engineer for the GGB) is the inventor of the math
behind the modern suspension bridge. He developed 33 equations
embracing from 6 to 30 variable to account for shape, structure,
temperature, winds, and stress that were due to both dead and live
loads. The GGB was designed for a wind load of 30 pounds per square
foot at the roadbed and 50 pounds per square foot on the towers. The
thirty pound spec is equivalent to a hurricane, the GGB typically sees
only 10 pounds per square foot for 50MPH winds.

Under the wind load designed to, the towers would bend five inches
(they swayed free, unstressed, 12 feet during construction and an
earthquake).

Basically, Ellis designed the GGB to the sum of all probable stresses,
not their average, not their RMS. The only thing missing was harmonic
amplification. The GGB was closed due to wind in 1950 and later
retrofitted with 5,000 tons of cross bracing (as was the replacement
Narrows bridge).

The bridge was an
architectural feat, with a very delicate looking thin roadbed and much
longer than most other bridges (3rd longest when it was built, some 1500
feet longer than the Golden Gate, for instance).

You can't be third in the list to the GGB and longer both unless you
are speaking of the insignificance of approaches. The Verezzano span
(designed by GGB engineer Ammann) is only 60 feet longer but carrying
much more weight.

It was much longer and
thinner as compared to other suspension bridges of the time which were
double decked, (SF Oakland Bay Bridge) for instance.. making them
torsionally much stiffer). Interestingly, the designer of Tacoma

More the legacy of (GGB engineer) Russell Cone's assistants.

Narrows (Moisseiff) was also involved in the Golden Gate.

Moisseiff was also the weak link for both the Narrows bridge and the
GGB closure due to high winds in 1950. He underestimated the dynamic
wind load. Ellis was the inventor of the math, but not a chief
project engineer. In the field of bridge engineering, and especially
for the GGB, there were a lot of Prima Donnas - Strauss the first of
firsts. Moisseiff, by some accounts, appears to have been used as a
resource rather than a principle engineer in the Narrows bridge
construction. The bridge owners conspired to a lot of monkey shines
in cost-cutting choices which turned out to be fatal. They eliminated
the cross bracing from the bridge towers, above and below the roadbed;
and they dispensed with the roadbed stiffening truss.

Moisseiff, along with GGB designers Ammann and Cone, was appointed to
the review board to study why the bridge failed - that was doomed to
failure, too, by the bridge owners (who had their own insurance
problems because they declined to find an outside insurer and decided
to carry the risk themselves). The story of the back room feuding and
remarkable Reaganomic theories are case lessons in planned disaster.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
On Wed, 18 Mar 2009 11:11:42 -0700, Jim Lux
wrote:

I guess that is the thing they forgot to calculate, or didn't know
how, on the infamous Takoma Narrows Bridge in Washington State which
collapsed when cross winds cause wild vibrations.

That was an unexpected coupling between the force from the wind and
torsional vibration of the roadbed. As the roadbed tilted, it "caught"
more of the wind and had more force applied, moving it further. The
torsional resonance was such that it oscillated with ever greater
amplitude (not much different than a flag flapping, or a blade of grass
in the wind.. not quite like a wind instrument reed, though)

In fact, it was exactly like a reed. The Tacoma Narrows Bridge
exhibited the highest roadbed length to roadbed width ratio of the
designs of that era, and this was a contributing factor.

Reeds don't oscillate torsionally (at least not as the dominant mode)
They're more of a fixed/free beam that oscillates in longitudinal bending.



The bridge was an
architectural feat, with a very delicate looking thin roadbed and much
longer than most other bridges (3rd longest when it was built, some 1500
feet longer than the Golden Gate, for instance).

You can't be third in the list to the GGB and longer both unless you
are speaking of the insignificance of approaches.

The GGB is shorter than the TNB. It was third on the list on July 1,
1940 according to Wa DoT. (Verrazano narrows was built in 1964) George
Washington was built in 31 and was 3500 ft, and was longest until 37,
when GGB was built in 37.

TNB was 5939 ft long (per Washington state DOT). GGB is 4200 ft
(wikipedia gives 5000 ft for the length of the TNB)

T

Moisseiff was also the weak link for both the Narrows bridge and the
GGB closure due to high winds in 1950. He underestimated the dynamic
wind load. Ellis was the inventor of the math, but not a chief
project engineer. In the field of bridge engineering, and especially
for the GGB, there were a lot of Prima Donnas - Strauss the first of
firsts. Moisseiff, by some accounts, appears to have been used as a
resource rather than a principle engineer in the Narrows bridge
construction. The bridge owners conspired to a lot of monkey shines
in cost-cutting choices which turned out to be fatal. They eliminated
the cross bracing from the bridge towers, above and below the roadbed;
and they dispensed with the roadbed stiffening truss.

Moisseiff, along with GGB designers Ammann and Cone, was appointed to
the review board to study why the bridge failed - that was doomed to
failure, too, by the bridge owners (who had their own insurance
problems because they declined to find an outside insurer and decided
to carry the risk themselves). The story of the back room feuding and
remarkable Reaganomic theories are case lessons in planned disaster.

Interesting.

On Wed, 18 Mar 2009 17:17:14 -0700, Jim Lux
wrote:
Reeds don't oscillate torsionally (at least not as the dominant mode)
They're more of a fixed/free beam that oscillates in longitudinal bending.

And yet the failure mode I've most seen with them is splitting.
Perhaps this only reveals a limited experience of their failure.

The bridge was an
architectural feat, with a very delicate looking thin roadbed and much
longer than most other bridges (3rd longest when it was built, some 1500
feet longer than the Golden Gate, for instance).

You can't be third in the list to the GGB and longer both unless you
are speaking of the insignificance of approaches.

The GGB is shorter than the TNB. It was third on the list on July 1,
1940 according to Wa DoT. (Verrazano narrows was built in 1964) George
Washington was built in 31 and was 3500 ft, and was longest until 37,
when GGB was built in 37.

TNB was 5939 ft long (per Washington state DOT). GGB is 4200 ft
(wikipedia gives 5000 ft for the length of the TNB)

You have inadvertently summed in ordinary approaches. We have freeway
interchanges with more complexity. Bridge span is the significant
indicator of interest. The GGB comes in at 4200 feet suspension span,
yes, the Narrows is much shorter at 2800 suspension span.

If push came to shove about overall length over water, we, here in
Seattle, have vastly larger bridges that float. The I-90 bridge logs
in at 6620 feet, and the 520 bridge pushes that to 7578 feet. Nearby,
we have the Hood Canal bridge that is longer at 7869 feet.

I've been across all five many, many, many times, and I used to live
halfway across the bay on the way to Oakland, by way of the Bay Bridge
in Frisco. The one suspension bridge I refused to drive across is SW
of Colorado Springs, over the Royal Gorge - the highest suspension
bridge (1053 above the river below). Just standing on the approach as
a car goes over gives you the shakes (talk about harmonic coupling).

73's
Richard Clark, KB7QHC