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Old 04-15-14, 11:07 AM   #1
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Does your bike bridge have earrings?

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File Type: jpg Bridge damper.jpg (90.8 KB, 11 views)
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Old 04-15-14, 11:19 AM   #2
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Might that have been designed by the same guy who did the I-35 bridge?
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Old 04-15-14, 01:12 PM   #3
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I think they used the same engineering consultant out of San Francisco.

Here is a clip of the cable prior to the collapse.
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Old 04-15-14, 10:47 PM   #4
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The video surprised me, wow.
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Old 04-16-14, 10:46 AM   #5
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On June 8, 2012, a summary report[SUP][13][/SUP] of the investigation was released in which the failure was attributed to wind-induced "cable vibrations that induce damaging stress range cycles at fatigue sensitive details in the cable diaphragm plates." The effect of cable vibrations caused by wind was not included in the original design package for the bridge.[SUP][14]
[/SUP]
background : http://en.wikipedia.org/wiki/Martin_Olav_Sabo_Bridge

the bridges here is a truss, 1, and center lift, 2.. not cable suspension , and its for cars and trucks and not a bike bridge ,

though cyclists can ride across the river1, and the bay2, on the right side of the 2 lanes. on US 101.


but then again it does not fall below ZeroC more than a week or 2 here either

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Old 04-16-14, 11:31 AM   #6
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A Stockbridge damper. Neat little things.

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Old 04-16-14, 12:45 PM   #7
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Stockbridge damper - Wikipedia, the free encyclopedia.. not exactly, there is just a weight hanging from a shackle..
as I see the design tha 2 weights in mr Stockbridges design have a different damping effect ,

as the short cable connecting them can be tuned , by length if that crossbar, to have a frequency of it's own

& Inertia .. counteracting out of phase perhaps?

Napkin engineering level .. the inertial mass of the weight , i suspect is benefitting from the wooden rather than a metal clamping block ..

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Old 04-16-14, 02:02 PM   #8
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Originally Posted by More Cowbell View Post
I think they used the same engineering consultant out of San Francisco.

Here is a clip of the cable prior to the collapse.
Do you have a video of the cable afterwards? I wouldn't have guessed the weight would change it that much...
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Old 04-16-14, 02:35 PM   #9
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Do you have a video of the cable afterwards? I wouldn't have guessed the weight would change it that much...
It's probably the attachment point that raises the resonant frequency of the cable- a "false/virtual endpoint".
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Old 04-16-14, 03:18 PM   #10
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The light rail resumed service on February 24, 2012,
there a metro rail track on the bridge too?
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Old 04-16-14, 03:32 PM   #11
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It's probably the attachment point that raises the resonant frequency of the cable- a "false/virtual endpoint".
I would assume the effectiveness of that depends on the weight being added. My non-skilled eye thinks the size of the weight was relatively small compared to three mass of the cable..

All that being said, I'm guessing my eye is wrong as they bothered to add it to the cable.
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Old 04-16-14, 03:43 PM   #12
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i wonder how long they think that untreated piece of wood will last... i know i do.

or maybe it's just part of some school kid's science project. IDK.
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Old 04-17-14, 04:54 AM   #13
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i wonder how long they think that untreated piece of wood will last... i know i do.

or maybe it's just part of some school kid's science project. IDK.
Looked like ACQ treated to me, that stuff is a light lighter in color than the old stuff. I was more concerned with what would happen if the shackle pin comes loose. Needs to be wired (moused).

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Old 04-17-14, 07:34 AM   #14
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In Omaha there is a hiker biker bridge over the Missouri river that is a suspension bridge. Even in light winds it is amazing how much the suspension cables vibrate and buzz. As of yet anyway the cables on this bridge do not have dampeners like those shown. Considering the vibration tho, it might be a good idea.
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Old 04-17-14, 09:24 AM   #15
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Do you have a video of the cable afterwards? I wouldn't have guessed the weight would change it that much...
I didn't notice the vibration in the stay when I took the picture. There was a pretty good wind that day (15 - 20mph)
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Old 04-17-14, 09:26 AM   #16
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there a metro rail track on the bridge too?
This bridge is strickly bike and pedestrian. It bridges a divided 4 lane and a light rail line.
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Old 04-17-14, 09:34 AM   #17
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i wonder how long they think that untreated piece of wood will last... i know i do.

or maybe it's just part of some school kid's science project. IDK.
The wood was treated. This didn't have a secondary attachment point so is a risk for the weight being dropped onto the highway if the wood attachnment fails.
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Old 04-17-14, 09:37 AM   #18
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We have a fancy new $130,000,000 bridge down here in Hastings that was finished last December and the cosmetic finish on the concrete is flaking off. A mark of fine craftsmanship if I ever saw one.
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Old 04-17-14, 10:24 AM   #19
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We have a fancy new $130,000,000 bridge down here in Hastings that was finished last December and the cosmetic finish on the concrete is flaking off. A mark of fine craftsmanship if I ever saw one.
Minnesota sure knows how to build bridges.
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