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FL Bridge Collapse

 
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Old 03-27-2018, 03:02 AM   #141
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Re: FL Bridge Collapse


http://www.concreteconstruction.net/...629439130060c1


http://www.concreteconstruction.net/...629439130060c1
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Old 04-03-2018, 02:08 PM   #142
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Re: FL Bridge Collapse


More articles on the bridge:
http://www.miamiherald.com/news/loca...207358659.html

Engineering forum mentioned in the above article with loads of diagrams, photos, etc. :
www.eng-tips.com/viewthread.cfm?qid=437029
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Old 04-08-2018, 11:49 PM   #143
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Re: FL Bridge Collapse


I wanted to do this on the engineering forum, but they shut down the thread before I could sign up & post. Since I'm not an engineer, I originally felt funny about posting on an engineering forum, so I did not at the time. I hope they don't mind me copying a pict or 2. I feel the need to express what I believe is the root cause of the failure. It's been danced around in a lot of ways, but it's staring right at us the whole time.

The cross sectional area of the horizontal portion #11 ( about 8"? long) diagonal (about 8"? long) is less than 1/2 the cross section of the diagonal....<240sq in, vs 500 sq in. If #11 had been a continuous pore with the deck, the bottom of this whole section would have been on a sheer, and easily dissipated the estimated #11 1.7 million lbs of force into the deck. Since the deck was poured first, and it appears that little to no method was implemented to transfer #11 force to deck prior the short horizontal, the result is a potential 1.7 million lb of force applied to the short 8 long horizontal ( about 250sq inches). The #12 end column tied to deck is basically what held back #11.

When they de-tensioned the bottom tendon, it eliminated the some friction between #11 & the deck. That resulted in a large percentage of #11 force into the short horizontal, & hence a huge explosion of material out the end of deck, on a plane that was clearly above the deck. Momentarily, the bridge stayed up for less than 1 video frame, but once the deck & canopy failed, it fell at the rate of gravity. The #12 vertical column end stayed on the deck for about 1 or 2 frames, but due to abrasion from # 11 end pushing outward, about 5 ft of #12 was eliminated, & resulted in a vertical drop of the bridge end. In other words, there was a short delay to the end of bridge dropping. That is why I believe the short horizontal is what exploded, & not an end deck failure.

I simply don't see why the engineers would assume that small horizontal cross section would handle #11's force.

To make matters worse, when the bridge was transported, they had to traverse over approximately 10" of uneven roadway. In one drone video, you could see that they climbed up onto the divider curb, & afterwards adjusted the transporter height. By then the damage is done. Basically they tweaked the deck with about a 10" twist. In the middle of the transport, they stopped for a very long period of time ( possibly hrs?) while traversing said 10" curb. That could have been what caused the said crack, & to me is the most likely area for the crack to be. It's been documented that lead engineer Peete said that they were going to adjust the tendons to bring the crack back together. I feel that that short horizontal would have been the likely crack area, & it would have likely cracked at an angle due to the overhang load on it during transport.

The posted #11/#12 pict is BEFORE the bridge was moved.
The other pict was borrowed fro the engineering forum ( thanks Meerkat!)
The two distant bridge shots clearly show The top of #11 angle opening up. Just place a paper corner on screen to see the change. The bottom of #11 shot out the end of deck in later frames.
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Old 04-08-2018, 11:55 PM   #144
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Re: FL Bridge Collapse


edit....duplicate!
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Last edited by Railman; 04-09-2018 at 12:10 AM. Reason: duplicate
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Old 04-09-2018, 02:01 AM   #145
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Re: FL Bridge Collapse


Somebody doing a xesign on cad may just copy tbe detsil and use it for sll thise connections. Because of the way the triangulation was done baring from o e end to the other location should have the highest stress. If It's a detail copies from the right side, it would be a weak link.

Good work!
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Old 04-10-2018, 01:18 AM   #146
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Re: FL Bridge Collapse


The two decks appear a lot thicker then they are actually, the bottom deck is <24" thick at the centerline, and has a notch that is about 12" deep and ~9" wide directly on the bottom center-line for the drain pipe collector.

Additionally plans called for a 9' x 9.5" SQUARE box that reduced to a 6" pipe fitting for a Neehah bridge drain to connect the 8" cross 'down' spout ONLY 18 to 24" away from the # 10 truss member...

The two interior Pre stress cable conduits (3.25" in diameter) were only ~ 9" outside of the 24" wide trusses. On both the decks....

Finally, examining the photos of the bridge transporter rigging and cribbing, the cribbing instead of being installed DIRECTLY under the ~1000KIP load were installed about 5 feet outward creating a Moment arm and thus creating cracks in the "unconfined" concrete that later grew into the drain hole and voids created by the PS conduits. The dunnage should have been placed directly under the load/truss, and the outer dunnage installed just for balancing purposes.
better yet a I-beam parallel under the truss then dunnage over the wheeled jacks....

The square edge details where the trusses meet the decks and acute angles between the truss webs and deck create stress risers. IMO
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Old 04-11-2018, 10:22 AM   #147
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Re: FL Bridge Collapse


Quote:
Originally Posted by Fouthgeneration View Post
The two decks appear a lot thicker then they are actually, the bottom deck is <24" thick at the centerline, and has a notch that is about 12" deep and ~9" wide directly on the bottom center-line for the drain pipe collector.

Additionally plans called for a 9' x 9.5" SQUARE box that reduced to a 6" pipe fitting for a Neehah bridge drain to connect the 8" cross 'down' spout ONLY 18 to 24" away from the # 10 truss member...

The two interior Pre stress cable conduits (3.25" in diameter) were only ~ 9" outside of the 24" wide trusses. On both the decks....

Finally, examining the photos of the bridge transporter rigging and cribbing, the cribbing instead of being installed DIRECTLY under the ~1000KIP load were installed about 5 feet outward creating a Moment arm and thus creating cracks in the "unconfined" concrete that later grew into the drain hole and voids created by the PS conduits. The dunnage should have been placed directly under the load/truss, and the outer dunnage installed just for balancing purposes.
better yet a I-beam parallel under the truss then dunnage over the wheeled jacks....

The square edge details where the trusses meet the decks and acute angles between the truss webs and deck create stress risers. IMO
So I guess your implying that there was a deck punchout at #10 bottom?
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Old 04-11-2018, 06:48 PM   #148
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Re: FL Bridge Collapse


The "blisters" that the anchor bolts for the faux suspension pipes were mounted in reinforced the top nexuses of the truss webs to the "roof" deck, & the top chord didn't have 174' of a 9" wide and +12" deep notch for the deck drain plumbing...... nor any square cast pockets for Neehah drain fittings.....

10 pounds of P.S.cables and rebar in 5 pound sacks...


Imagine "Just" the truss webs with out the decks, they alone weren't self supporting, the loads had insufficient connections to the deck and roof IMO

Draw a detail at any of the lower nexuses, with all the penetrations ( 4-6- 8 pre stress cable conduits , all 3.25" in diameter) in the webs, two PS tendons parallel to the joint only ~ 9" away each side....+ the floor drain within inches of the Uber loaded concrete....

Then enlarge the voids to show the anchors of the tension cables....the ends of the post tension system should have been located outside of the overloaded volume....

Another "crap" detail is aligning the truss webs to 'Appear" to be functioning as a suspended bridge IMO forced the use of angles of webs that are much weaker than having a Symmetrical truss design....
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Last edited by Fouthgeneration; 04-11-2018 at 07:12 PM. Reason: missing )
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Old 04-12-2018, 04:36 PM   #149
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Re: FL Bridge Collapse


I'm not buying that, if for no other reason than the end dropped, and the other parts stayed together.
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Old 04-12-2018, 06:59 PM   #150
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Re: FL Bridge Collapse


Quote:
Originally Posted by hdavis View Post
I'm not buying that, if for no other reason than the end dropped, and the other parts stayed together.
That is one of many reasons!
The deck & canopy were resulting failures, & not the start. If you take out #11 bottom, the resulting deck canopy failures would happen right where they did.

Like I said, I believe the short level #11/#12 block cracked from the overhung transport load, that I estimated at least 250,000 lbs, pushing directly down from #12 to deck end. Do the math. It's cubic yds x 4k per yd for the deck, #12, deck end bottom side, & canopy weight. The 1.75" tendons should have been close to holding it, without considering the seismic type action of the transport. There is one more factor that made the 250k downpush on #12 a low value. While they where traversing the center road curb, they repeatedly shuffled the load side to side a bit on the north transporter. See time lapse video link. In order for that to happen, all of the lift was done by the inward transporter, making the overhung weight increase by a huge amount, without doing the math, easily 100k lbs more. This happened while one corner of deck was on center island, & diagonal deck corner was still up on south sidewalk curb. Their technique was to drive up on curb, & adjust twist after the fact.

As an aid to what went on, if the short level part was say 10ft long, you can easily see the effect of the overhung load, & vertical sheer potential of short block.
Initially the crack would have happened at an angle top to bottom, but if you look at the way #12 was abraded more to each side, the block most likely spit a second time vertically.
One more clue is that all the pipe, rebar etc at the bottom of #12 was virtually left in tact. What that tells us is that #11 rode upward on #12 as it pushed out, clearing the deck rebar & plastic pipe.
If the north end had just pushed off, & fallen over, it wouldn't have looked like it was blown up with dynamite.

Watch time lapse at 1/4 speed.The 1st one shows the weight shuffle to inward transporter in the range of 15 to 30 seconds in video.

Last edited by Railman; 04-12-2018 at 07:06 PM.
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Old 04-12-2018, 10:45 PM   #151
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Re: FL Bridge Collapse


Another great look at what went on.

http://files.engineering.com/downloa...tection_1a.mp4

Collapse starts at about 40ms , with expulsion out left end, & no it's not the deck corner. It is also when the #11 tendon starts to zipper out of bottom, with a resulting apparent bending of #11 bottom.

At about 50ms, the vertical drop starts at deck/canopy break points.

At about 105ms, the #12 vertical column north end of canopy starts to drop. The reason this is important is that the end stayed at full ht for a period of time after deck/canopy failed. If the deck end blew out first, the end would have dropped imediately. If you focus on the man lift cage, you can easily see when canopy end starts to drop. The man lift also has a black vertical mast that can be used a reference point for several movements, & perspective angles.

At 122ms, the bottom #11 tendon with hydraulic jack attached, shoots out of blister at a high speed, just past worker. There is no mistake the angle and location is correct in relation to the blister . The reason I say it's at high speed, is that it appears all at once. I believe it broke the tendon at this point due to the canopy smashing against #11 near top 1/3, resulting in a huge increase in it's tension, and subsequent breakage ejection, much like over stretching a rubber band. I still need to do scaling to determine resulting tendon length to be sure of this. The bottom of this #11 tendon is still attached to deck.

Video is over by 132

Last edited by Railman; 04-12-2018 at 10:56 PM.
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Old 04-12-2018, 10:56 PM   #152
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Re: FL Bridge Collapse


Quote:
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Another great look at what went on.

http://files.engineering.com/downloa...tection_1a.mp4
...
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Old 04-17-2018, 09:27 AM   #153
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Re: FL Bridge Collapse


I'll summarise by adding a few sentences in red ink to this picture which I have extracted from the FIGG - MCM design-build team's own document pdf proposing to FIU that they get the contract to build the bridge.



The bridge designers innovated (incompetently) a new I-beam design of bridge but where the I-beam's upright-supports (called an "open truss") join the deck of the bridge, the designers should have specified the necessary reinforcement to stop the severely stressed joints breaking apart - "should have" but negligently didn't and so the weakest link - the northern bottom end joint - failed first and it caused a catastrophic collapse of the whole bridge.

The failure which the evidence of the video and photographs suggests is likely to be with the design of the joint itself, an insufficiency of reinforcement in anchoring member #11 to the deck, leading to, I might suggest, shear fractures along the 2 planes either side of member #11 where they intersect with the deck which I have illustrated by annotating sheet B-8 as follows.



Using an online truss calculator, I modelled the bridge and calculated the forces on the members, deck and canopy.



Now I have calculated what the appropriate P.T. bar tensioning requirements really were and produced this table and bar chart.

I estimate the P.T. requirement as a factor of about 1.3 times the greatest tension force that the member experiences, being half-way between the minimum tension acceptable and that times the load safety factor of 1.6 for a live load. 1.3 x is half way between the two acceptable extremes.





As this photograph of the south end of the collapsed bridge shows, M2 was constructed 150% of the width of M3 and of all the other members, which explains why the south end of the bridge didn't collapse.



Here's my concept for a high shear strength steel joint for this sort of application.







Attached Thumbnails
FL bridge collapse-innovative_incompetence.jpg   FL bridge collapse-sheet-b8-cropped.jpg   FL bridge collapse-force-bar-chart.jpg   FL bridge collapse-trussmemberforcestable.jpg   FL bridge collapse-trussmemberforcesptsettings.jpg  

FL bridge collapse-comparing-member-2-3.jpg   FL bridge collapse-high-shear-joint.jpg   FL bridge collapse-i-beam-joint.jpg   FL bridge collapse-i-beam-joint-amd-stylite-fork-socket.jpg   FL bridge collapse-i-beam-joint-amd-stylite-fork-socket-join.jpg  

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Old 04-17-2018, 12:48 PM   #154
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Re: FL Bridge Collapse


That's a lot of good work
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Old 04-17-2018, 02:26 PM   #155
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Re: FL Bridge Collapse


Peter Dow,
Maybe you can enlighten me...

How much potential force was exerted on the short fillet block butting into #12?
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Old 04-18-2018, 08:20 AM   #156
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Re: FL Bridge Collapse


Tin.....Was it really necessary to ban Peter Dow??
What was the reason?
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Old 04-18-2018, 10:38 AM   #157
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Re: FL Bridge Collapse


Unfortunately rules is rules. Not a contractor.
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Old 04-18-2018, 07:56 PM   #158
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Re: FL Bridge Collapse


That was arguably the best first post of all time though. Clearly he had something to add to the discussion.
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Old 04-18-2018, 08:02 PM   #159
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Quote:
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That was arguably the best first post of all time though. Clearly he had something to add to the discussion.

It's on up tnere. Normally first timer non contractors get referred to the diy site....
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Old 04-18-2018, 08:10 PM   #160
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Re: FL Bridge Collapse


I mean, all it takes is a couple bucks to be a contractor.

Peter, if you're reading this, email me. I'll hire you to calculate the additional load a coat of paint puts on a house, then you'll be in our gang.

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