Miami Pedestrian Bridge, Part XII 34

[zeusfaber]

A continuation of our discussion of this failure. Best to read the other threads first to avoid rehashing things already discussed.

Part I
thread815-436595: Miami Pedestrian Bridge, Part I

Part II
thread815-436699: Miami Pedestrian Bridge, Part II

Part III
thread815-436802: Miami Pedestrian Bridge, Part III

Part IV
thread815-436924: Miami Pedestrian Bridge, Part IV

Part V
thread815-437029: Miami Pedestrian Bridge, Part V

Part VI
thread815-438451: Miami Pedestrian Bridge, Part VI

Part VII
thread815-438966: Miami Pedestrian Bridge, Part VII

Part VIII
thread815-440072: Miami Pedestrian Bridge, Part VIII

Part IX
thread815-451175: Miami Pedestrian Bridge, Part IX

Part X
thread815-454618: Miami Pedestrian Bridge, Part X

Part XI
thread815-454998: Miami Pedestrian Bridge, Part XI

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[Vance Wiley]

Thanks for that explanation. I have encountered similar results with digital measurements - tapes and levels - with a minimum increment of one "pixel" or "graduation", it either is or it ain't.
Perhaps the camera should be oriented at 45 degrees or the pixel rows staggered?

 

[Tomfh]

Regarding the aliasing explanation above, It’s worth remembering that very few lenses have the ability to resolve that finely. Typically each point of light is blurred across several pixels. You can see this in the images above. At light/dark edges, it take several pixels to go from black to white.

Except for the best lenses (or perhaps the crudest sensors) it is very rarely the case that each point in the image is triggering one pixel and one pixel only.


That’s why it’s such a joke to hear small sensor cameras with “20 mp resolution”. The lens isn’t capable of resolving anything like that.

 

[RAB678]

Guess I'm not quite done.
I can't seem to find inner truce with the crane vibrating question.
The little pixel blow up video was interesting and gave me an idea.
I went back and looked at the video dash cam video link that was provided here.

https://www.youtube.com/watch?v=Sb4npaRtPfQ

I believe that is the link to the dash cam video.
Sorry, I don't know how to embed it as well as MikeW7 did. Thanks Mike.

It made me bust out a BIG magnifying glass.
I also placed a movable wire in front of the screen to help track the cranes movement.
Amazing.... these new high tech gadgets they have today.

And then.......I saw something I missed before.

It would seem there is a large ball weight on the end of the crane cable. as expected.
and....It does swing back and forth about 15 feet? left to right.
Coincidentally like right after the bridge begins to fall.
There is a strap on it, traveling with it.
The ball also somewhat disappears and reappears. A couple of times.

I know...... that's because of camera angles, frame rates, pixelation and
the awkward gravity system in Florida.....

Using the wire as a reference.............
The crane boom top does move left, then waits, then seems to move a little faster. (Jumps a few pixels.)
It's not quite matching what I would expect from a stationary boom. Standard progression of pixels.
And repeats a few times. The streetlight does seem to progress as expected for reference.

That's not bouncing back and forth, is it......it is...is it.......it is...
Care to pixelate that one?

Almost forgot....while someone mentioned....you would need a larger crane to lift the bridge.
You are not exactly lifting the bridge. Just helping it, while cables are tensioned.

Then again I guess the size crane that was used is certainly needed to lift a
200 lb tensioner and support a 200 lb man in case something happens.......

Don't they use 2 cables actually? One for the main lift and maybe another for a lighter reason......
And what exactly would make a 200? lb steel ball go wildly swinging in the air like that?
Maybe.....it got a leak like a balloon...and went pffffffffffff..... all around.

Sorry to waste your time. I just think I see that.

 

[SFCharlie]

The PT rod was adjusted with a hollow hydraulic jack which was supported by a crane. It would have to be moved from upper to lower PT rod nut occasionally.

SF Charlie
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[MikeW7]

Sorry to waste your time.
:
It would seem there is a large ball weight on the end of the crane cable ... There is a strap on it, traveling with it.
:
Don't they use 2 cables actually?

I gave you links to a couple of YouTube Playlists a couple of days ago. They contain 3 videos of the collapse. If you'd also watched the Collapse - N view and Collapse - SW view videos, they would have answered most of your questions and clarified many of your guesses. Then you wouldn't have had to write such a long post, and I wouldn't have had to spend so much time answering it....

The crane block (pulley and lifting hook - looks like a ball from a distance) is located about halfway between the end of the crane mast and the work area. Attached to it are 2 slings:
[ul]
[li][EDIT to add more sling info] One sling supports the heavy equipment the workers are using. This equipment has to be repositioned multiple times as the workers adjust different tensioning rods. My guess is that they used a nylon lifting sling that is much wider than a thin wire sling of the same rating, making it much easier to grap and tug as the workers are positioning their equipment. When the bridge falls the bottom sling loop broke, and the crane cable tension caused the sling to shoot up in the air, as seen in the dashcam video. I'm sure there are news pictures somewhere that can shed more light on this.[/li]
[li]The other strap/cable is used as a connection point for a safety harness. You see it pulled to the side numerous times in the N View video so it's out of the way. One of the workers tried to put the harness on just before the bridge started to fail. When the bridge collapses you see the arms of the "Falling Man" shoot up over his head as he falls out of this harness, then the harness itself falls.[/li]
[/ul]

 

[Vance Wiley]

DAMMIT !! Someone figured it out! Someone call Mike Roman and tell him it's over - we can't cover it any longer. By the way - I'm selling my 3M stock tomorrow.

 

[MikeW7]

Someone figured it out! Someone call Mike Roman and tell him it's over - we can't cover it any longer.

I saw that too, but decided to let somebody else take the bait. Either Eng-Tips is testing the professionalism and overall quality of this thread, or we've been trolled. If the former, we've done a fine job. If the latter, I'd guess he's of the under-bridge variety, and I hope his home falls down....

 

[Spartan5]

I think the Russians are beta testing a new bot and using eng-tips to feed its neural network.

 

[SFCharlie]

Funny, does the affected foreign sentence structure disappear when he gets excited?

SF Charlie
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[Vance Wiley]

The discussion about lines and pixels and smallest unit of measurement digitally leaves me wondering what the smallest increment of measurement was for the instrumentation during the move? I may have skipped over it if it has been presented, so a reference is appreciated.
Thanks

 

[MikeW7]

...what the smallest increment of measurement was for the instrumentation during the move?

The Barnhart Movement Plan (Sheet LLT-01) says the following BDI instruments were used:
[ul]
[li]STRAIN TRANSDUCER BDI ST350[/li]
[li]ROTATION SENSOR BDI T500-030[/li]
[li]DISPLACEMENT SENSOR BDI STRING POT[/li]
[/ul]

Here's the link to the BDI sensor page. The page for each instrument includes PDFs for a Spec Sheet and Operations Manual.

 

[Vance Wiley]

Thank you,[highlight #EF2929] MikeW7[/highlight]
The ROTATION SENSOR BDI T500-030 has a sensitivity of 0.0003 arc degrees. If I did it right, that is 0.002 inches (2/1000) across a 30 foot wide deck.
That should be adequate. I suspect distortion from solar heating as the sun angle changes would be readable at that resolution.
I have no criticism of the capability of that instrumentation. sensor.

 

[Sym P. le]

11 Jul 19 20:46 - Refer to the cut-and-paste mashup ...

I've been thinking about this reality and roughly calculated that 3.75" needs to be gained on the long side, as per my mark-up below. This could be gained by various distortions or by movement at the bearing points. The south end has a 1.5" expansion joint to the landing and the north end has play around the hold down through the plastic duct. Needless to say, 900 tons does not need my permission to do what 900 tons does.

I also wondered if there was evidence of contact by the deck at the south end landing. Google Maps says, "Maybe ..."

 

[Vance Wiley]

roughly calculated that 3.75" needs to be gained on the long side
I am dense tonight, so I am having trouble following you. And the sketch/drawing is quite light on my monitor, but I see a green line (I think) from lower left to upper right. That seems reversed to me -
Question - were the PT anchor rods installed thru the deck and into the pylon at the time of the collapse? Did they shear off?

 

[Sym P. le]

Mike showed that the long south end of the span rotating about its base would push the top of 12 north. This has to be factored into the collapse sequence or another explaination provided. Given the 175' of span and 20' of height, I was curious how much distance the sweep would require. I picked three points that seem to be indicated by the videos, the bearing point of the south diaphragm, the upper canopy surface at the 10/11 blister, and the lower canopy surface at the 12/canopy node. I then scaled the two long sides off of the dwg. with the angle between them (easy with GIMP) and calculated the balance. The sum of the two short sides is 3.75" longer than long side.

The hold downs were loosely attached and seem to be sheared. They are visible in various OSHA report photos.

 

[3DDave]

Pushing the top of 12 North a few inches seems immaterial compared to the bottom of 12 being shoved about 10 feet, if the bridge was to stay together that long, before yanking the bottom of 12 back as the canopy heads for the ground. Obviously not all of 11 or 12 made the reverse trip intact.

 

[Sym P. le]

... a few inches seems immaterial compared to ...

We didn't have a number so I gave it one. Also until something breaks free, it all has to move together and that makes any number significant. It's neccessary to explain how the bottom of 12 ended up hanging four feet off of the pylon.

 

[3DDave]

I made it a 4-bar linkage, which explains how the bottom of 12 ends up off the pylon. Make the 12-pylon a slider, which is the motion allowed, a pin joint near 9-10, and a pin at 1-deck. The top of 12 cannot move at all until the bottom is already broken; the top is part of a rigid frame.

 

[SFCharlie]

Based on what I think I see, from frame to frame in the dashcam video, the first movements seem to show:
12 and the deck to 9 move as a unit;
11 and the canopy to 12 move as a unit; and
10 and the canopy and deck back to 1 move as a unit.
The angle between 11 and 10 appears to me to change.
If this is correct, 11 has to effectively change length and the canopy has to hinge or slide at 12.
I know that the forces on 11,canopy,10 are less than the forces at 12,11,deck by the weight of 11; but somethin's gotta' give at 11,canopy,10

SF Charlie
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[MikeW7]

New cut-and-paste mashup. Now in Color!
(Old mashup posted 11 Jul 19 20:46)

[highlight #FCE94F]EDIT ADD: Not a structural guy! I made this diagram to visualize the spatial relationship of the components based on how they seem to appear in videos. I will adjust the captions based on group suggestions.[/highlight]

Apologies for the tilty north pier...

[highlight #FCE94F]ADD: At this point both the canopy and deck have hinged and the 11-12-deck joint has been compromised - the order of occurance is not posited.[/highlight]
[highlight #FCE94F]EDIT: Next 2 pictures: reworded to change description of picture 2 to an "IF" scenario and moved a "pinch point" reference from pic 3 to pic 2.[/highlight]
IF the base of 11 wasn't immediately destroyed and the canopy-11-12 triangle managed to remain intact for a bit, the bottom end of 11 would have slid over the lip of the north deck and been crushed to rubble in the pinch point between 12 and the deck edge.

With the base of 11 gone, either by it's initial failure or after it was crushed in the pinch point, 12 drops off the deck and onto the pier. Member 11 is removed from this and following pictures.

12 drops behind the pier.

At this point the deck hinge has already impacted the highway and the 1-10 truss is starting to flatten out.
The deck hinge pulls the north deck section off the pier.
The canopy hinge drags 12 back onto the north pier.