Paul_D_North_JrIt may surprise some here, but it's standard practice with most reinforced concrete structures to use less steel than would be necessary for a full utilization of the concrete's strength - say, 75% of the otherwise required steel as a typical figure - the technical term is "underreinforced".
It astonishes me. Here in the EU you would get your walking papers. I looked up "under reinforced" and found it recommended for single reinforced beams (rebars only in the tension zone, none in the compression zone). Is under reinforcing done on all concrete beams?
Single reinforced beams are not allowed in the EU. Beams need a minimum of stirrups and reinforcement in the compression zone to fix them.
I would say all European concrete beams are over reinforced according to American standards. Beside the stability proof that determines the amount of rebars we have to limit deflection and and the crack width. All might need additional reinforcement to comply with standards.
Paul_D_North_JrThe structure is still designed - even with the less steel - for full loading, so there's no safety issue.
The beam is designed for the full load, but as the safety factor is determined by deviding the yielding strength by the load stress you get a lowered safety factor.
There was a good reason not to use the hardening after yielding for dimensioning.
The beams to European standards are ductile and predictable. You realise overloading early enough through crack formation and growth.
Looks like there are quite different dimensioning philosophies in our countries.
Paul_D_North_JrOffhand, I can't think of any US freight railroad structures that are cable-stayed or use post-tensioned concrete - might be some, but not very many. In contrast, there are several in use for light-rail systems.
In the USA you find a number of pre-stressed freight railroad bridges. In Germany most of the railroad bridges on high-speed lines are post-tensioned continous concrete beams. You can find only few cable stayed railroad bridges around the world.
Interesting video ... Not in German ;) Warning Bad Language
EuclidHow is the tensioning performed? I am guessing that they use a very sophisticated hydraulic puller that shows the exact amount of pulling force applied to each cable.
Here is a press for a stranded prestressing tendon: http://www.paul.eu/fileadmin/_migrated/pics/buendelsp_3.jpg
Each strand is clamped with wedges: https://upload.wikimedia.org/wikipedia/commons/thumb/7/73/Stressing_anchorage.jpg/360px-Stressing_anchorage.jpg
The cable is stressed more than needed, the wedges inserted, and the cable released until the cable is clamped. The whole procedure is controlled by measuring the lengthening of the cable and the power of the press (manometer)
Pre-tensioning bars with thread are anchored with special nuts: http://civilengineeringbible.com/imgs/contents/201801/959_lee%20mcCall%20system.png
EuclidHow can they reduce tensioning on the cable if necessary?
When you know you have to reduce the tensining you leave the strands or bars long enough. Than you can place the press again to pull the cable/bar until the anchoring can be removed and than release the press to the predetermined length and anchor again or release completely. Regards, Volker
EuclidOne scenario that I can see that might fit uneplained cracking and unexplained cable looseness is that the truss was mishandled and damaged during the installation process using the big mobile jacking tables.
What I said earlier on page 1 is exaclty what the video posted above by rdamon is describing. There are questions about lifting the bridge from points other than the design support points; and how do compensate for that by changing the tensioning; the possibility of having damaged the truss during the lift and positioning by overstressing the rod in the north diagonal strut and stretching it to near its failure point; then finding it loose from the stretched deformation; then tightening the rod, not realising that it is deformed to be very close to parting under tension; not seeing the tension rising as expected because the rod is actually about to part; pulling on the rod and it finally parts and drops the bridge.
Vijayo? Gew-age?
Interesting claims. Looks like he's done his homework.
Larry Resident Microferroequinologist (at least at my house) Everyone goes home; Safety begins with you My Opinion. Standard Disclaimers Apply. No Expiration Date Come ride the rails with me! There's one thing about humility - the moment you think you've got it, you've lost it...
Volker,
Thanks for posting those pictures and your explanation of the tensioning process and equipment. That helps to understand how it is done.
rdamonInteresting video ... Not in German ;) Warning Bad Language
Thank you for posting. I hadn't seen the dash cam videos before.
It sounds like a reasonable explanation what happened. The why is not so plain for me.
When in the final position the strut, that might have caused the failure, is a compression member. When on the SPTMs during transport that same strut was a tension member as the bridge end cantilevered in this state.
So let's assume the strut was pre-tensioned for the transporting state so that there were no tensile stresses left to avoid cracks. When the bridge was placed on the columns there was compressive stress added to the already compressed strut it seems.
But not really. Additional compressive force means the strut gets shorter with the pretensioning bar inside getting shorter too and loosing part of is tensile force on the way.
So what was the reason to get to this tensioning bar again? And why let loosing this bar let the bridge collapse? According to the drawing there were three more tensioning bars in that strut. For me it looks like the last strut survived: https://cdn.cnn.com/cnnnext/dam/assets/180315142241-02-bridge-collapse-screengrab-0315-exlarge-169.jpgMight have been the second diagonal strut. During transport it was compressed but in the final position it was a tension member. From my point of view correcting the pre-stressing power in this diagonal makes more sense.
I feel it is still speculation. Perhaps the proposal reveals some more information.Regards, Volker
PS.: Here is the proposal for the bridge: https://facilities.fiu.edu/projects/BT_904/MCM_FIGG_Proposal_for_FIU_Pedestrian_Bridge_9-30-2015.pdf
Edit: Link replaced
For what it's worth I was able to build my table for the C40 with some help from my friends at the AAR. They provided a template and i populated it with dash 8 weights.
VOLKER LANDWEHR . . . The beams to European standards are ductile and predictable. You realise overloading early enough through crack formation and growth. Looks like there are quite different dimensioning philosophies in our countries. . . .
Looks like there are quite different dimensioning philosophies in our countries. . . .
That's basically what I was trying to say, simplified for the readership here.
Simple beams will often use the normal temperature & shrinkage reinforcing steel up in the compression zone to hang the stirrups from.
I suspect there are some differences, but not in the factor of safety. We'd have to compare notes in detail to see where we're miscommunicating.
- PDN.
Found this on youtube. Bridge drops in one second between 0.08 and 0.09 on the slow motion video.
https://www.youtube.com/watch?v=p6L20i6_gzE
Poor drivers never had a chance. This is going to be a big liability case.
Another youtube video explanation.
https://www.youtube.com/watch?v=sBDDQLcp6iI
Electroliner 1935Another youtube video explanation. https://www.youtube.com/watch?v=sBDDQLcp6iI
Good video but his follow-up one day later has an even better explanation.https://www.youtube.com/watch?v=GxQJj8D_FE0Less show than the first one and it shows that you can do it without foul language.
I stand corrected. After viewing this second video and looking at a number of photos he is correct that they worked on the tensioning bar in first diagonal not the second as I assumed. Doing that I realized a mistake in the video at 5:09. What he indicates as the blown through member 22 is in reality it is a concrete pedestal on top the canopy as shown at 3:13.
I still don't understand the reason for working this tension bar again. The member 22 went from tension during transport to compression in the final position thereby reducing the tension in the bar. But it isn't needed anyway I think.
I hope the NTSB report will enlighten me.Regards, Volker
Volker, good info .. Noticed in the dash cam video at about 1:07 that the worker performing the tensioning falls with the bridge. If the crane was holding the harness one would think that he would have been suspended in air or his fall protection would have slowed his decent.
VOLKER LANDWEHRGood video but his follow-up one day later has an even better explanation.https://www.youtube.com/watch?v=GxQJj8D_FE0
That is an excellent presentation. As I understand it, the bridge was being built by Munila Construction, and the engineering was being done by Figg. There was also additional engineering confirmation work being done by one or more other companies.
The customer in this case was not Florida DOT as would be the case with a typical highway bridge, but rather Florida International University. Also, I understand that FIU itself is deeply involved in the development of the Accelerated Bridge Construction (ABC) process and methods routine. I believe that news reporting has referred to the FIU Design Build Team, although this reference may mean the Figg Design Build Team which has also been referenced.
In any case, what role did FIU play in the design and construction of this bridge? It was their bridge and they have a large role in the development of ABC practice of bridge construction. Did FIU design the bridge to appear as it did with the tower and false suspension stays being provided mainly as cosmetic and theatrical elements?
Nobody likes to pay more for a bridge than they have to. However, this bridge seems to be about a lot more than just getting across the road.
EuclidIn any case, what role did FIU play in the design and construction of this bridge? It was their bridge and they have a large role in the development of ABC practice of bridge construction. Did FIU design the bridge to appear as it did with the tower and false suspension stays being provided mainly as cosmetic and theatrical elements?
I can't say for sure as many links in following Request for Proposal are dead ends:https://facilities.fiu.edu/projects/BT_904/Request_for_Qualifications-Request_for_Proposals/a._FIU-DB-AD_2014-06-24_FINAL.pdf
It doesn't contain any drawing so I thing the bidders had to interpret the test.
Quote: OWNER’S INTENT: The OWNER seeks America’s best designers and builders as members of a design-build team to design and build an innovative signature bridge that will become a respected and valued design landmark in Miami. It will serve as the critical element of a pedestrian-oriented shared-use corridor between FIU and Sweetwater, igniting the development of UniversityCity. We envision a wide pedestrian bridge (20’ minimum width to perhaps even more than 30’) that would serve not only as a means to cross from one side to the other, but would become a destination in its own right where community members might linger, gather, and create an urban social space -- a linear park.
I think the design came from Figg.Regards, Volker
rdamonIf the crane was holding the harness one would think that he would have been suspended in air or his fall protection would have slowed his decent.
It is possible that the crane had only hauled the press. The crane would need a special permit to secure workers in Germany. Perhaps it is different in the USA. Or may be the worker's harness failed.Regards, Volker
Paul_D_North_JrThat's basically what I was trying to say, simplified for the readership here.
Perhaps it was so simplified that I misunderstood. For me it sounded like making a balanced dimensioning (concrete and steel fail simultaniously) and than taking out an amount of steel.
After looking in internet I think I understand. It get if technical but I'll try to keep it simple.
Concrete dimensioning is not straightforward. There are more unknowns than equations so you have to make assumptions leading to more than one balanced condition between external loads and internal force from steel and concrete.
Making the right assumptions leads to a dimensioning that guarantees that the steel fails before the concrete ---> under-reinforced. It is a balanced condition without compromising the safety.
I said we don't know it in Europe. That is true for Eurocode. The UK had it in BS 8110. In Eurocode and DIN stards the under-reinforced design is implemented as standard into the dimensioning system without naming it.
How crack wigth control influences this I can't say. To stay within crack width limits you have to use smaler bars or lower steel stresses ( more steel)Here is a comparison of ACI, BS, and EN standards: https://www.iasj.net/iasj?func=fulltext&aId=10136I hope I got it correctly now.Regards, Volker
VOLKER LANDWEHR rdamon If the crane was holding the harness one would think that he would have been suspended in air or his fall protection would have slowed his decent. It is possible that the crane had only hauled the press. The crane would need a special permit to secure workers in Germany. Perhaps it is different in the USA. Or may be the worker's harness failed.Regards, Volker
rdamon If the crane was holding the harness one would think that he would have been suspended in air or his fall protection would have slowed his decent.
You are correct, it is the same in the US. I confirmed with an expert on fall potection and he said that the worker was probably secured on a horizontal line that fell with the bridge and cannot hook to the crane.
VOLKER LANDWEHRQuote: OWNER’S INTENT: The OWNER seeks America’s best designers and builders as members of a design-build team to design and build an innovative signature bridge that will become a respected and valued design landmark in Miami. It will serve as the critical element of a pedestrian-oriented shared-use corridor between FIU and Sweetwater, igniting the development of UniversityCity. We envision a wide pedestrian bridge (20’ minimum width to perhaps even more than 30’) that would serve not only as a means to cross from one side to the other, but would become a destination in its own right where community members might linger, gather, and create an urban social space -- a linear park.
Earlier, I said there is something about this bridge project that makes it seem like so much more than just a bridge. My conclusion is certainly confirmed by the FIU statement of their intent for the bridge to be a linear urban park. So while Figg did the engineering and design, I assume that the concept and architecture was provided by FIU.
After the bridge design was complete, there was a new requirement introduced in 2016 that necessitated moving the central support footing north by eleven feet. This required changes throughout the entire bridge design which contributed to putting the bridge over budget and behind schedule. These revisions affected a very large portion of the engineering work and double checking.
Although what fell a few days ago has been constantly referred to as “the bridge,” it was only one truss. The completed bridge was to have two trusses separated by a central support and tower rising 109 feet and supporting five symbolic suspension rods radiating down to each truss on each side of the central tower.
Moving the central tower eleven feet required the following design changes:
Move the central pylon with its 109 ft. high tower 11 ft. northward.
Revise the central tower foundation to be near the canal, at a considerably lower elevation than the previous location.
Make the south truss (the one that fell) 11 ft. longer.
Make the north truss 11 ft. shorter.
Change the length and end-cut angles of the 10 symbolic suspension rods (which were actually 16 in. diameter pipe).
Change the interval spacing and angles all (20) of the diagonal struts in the web areas of both trusses.
The nightmare of every design engineer, starting all over. Properly done it cost time and money but wouldn't compromise safety.
Here AP report: http://www.miamiherald.com/news/article206070889.html
The article states that a number of engineers found the move of the pylon an invitation for errors. And they quote a Duke University civil engineering professor saying quote:even seemingly minor changes in a bridge's design can lead to failures.
and: quote: "Once a design is completed, subsequent modifications tend to be suggested and approved without the full care that went into the original design. This has happened time and again in bridges and other engineering structures," he said.
I know from my experience that minor changes might be analysed only locally overlooking consequences on the whole system. Here our two-person integrity (four-eye principle) helps a lot.
But these were not minor changes. This should have been a completely new structural design including analysis and drawings. Time will tell if it was handled this way. At least the company got more than $200,000 for the redesign.
This change explains I think, why one transport support had to be moved inward.
In a different video I heard that the workers had realized that one post-tensioning bar in the last diagonal strut (at the support) had almost come loose.
I wasn't able to verify through other sources yet. I don't link this video as it contains faults explaining post-tensioning.
I'm not surprised by the loose post-tensioning bar. It depends on the ratio of tensile force during transport, compressive force in final position and the primary post-tensioning force.
I'm still unsure if the post-tensioning of the end diagonal was really needed for the final position. I hope the NTSB report will show.Regards,Volker
Edit: I just found this design drawing in the NTSB Twitter Newsroom:https://pbs.twimg.com/media/DYhaJGMVwAElEnv.jpg:large
NTSP isued a press release on March 21, 2018: https://ntsb.gov/news/press-releases/Pages/NR20180321.aspx
It is widely a description of the ongoing and future work on site.
Interesting is one paragraph about the post tensioning of the last diagonal strut.Quote: The investigative team has confirmed that workers were adjusting tension on the two tensioning rods located in the diagonal member at the north end of the span when the bridge collapsed. They had done this same work earlier at the south end, moved to the north side, and had adjusted one rod. They were working on the second rod when the span failed and collapsed. The roadway was not closed while this work was being performed.
This just confirms that post-tensioning done during the collapse. The why is still unanswered.Regards, Volker
Besides just being a means to cross the road, this bridge was intended to showcase several other purposes and causes. But the main cause was to show the world how to build bridges much faster. This race to completion was essentially marketing for FIU being the customer for the bridge and having a strong interest in promoting their Accelerated Bridge Construction engineering program.
Then along comes this surprise design change that changes almost every detail of the initial design. Not only does this cost extra money, but more importantly in this case, it requires more time. It slows down the accelerated bridge demonstration.
This raises the question of how much institutional resistance there is to speeding up the bridge construction business. How could the project have gotten so far without realizing that the location of the central tower was unacceptable?
EuclidBut the main cause was to show the world how to build bridges much faster. This race to completion was essentially marketing for FIU being the customer for the bridge and having a strong interest in promoting their Accelerated Bridge Construction engineering program.
To avoid misunderstandings, Accelerated Bridge Construction (ABC) doesn't make building the bridge on a site adjacent to the final location faster. It minimizes the road traffic disruption. A bridge build in-situ takes month to build closing the road completely. Using ABC reduces the disruption to hours needed to the the bridge to the final location and build it.
In my point of view ABC makes sense.
BTW ABC is not new. A remember a number of steel bridges erected this way, mostly across waterways. Here is a link to the erection of a bascule bridge using a 900 tonnes shearleg in the Port of Hamburg: http://www.hanse-dampf.de/bruecken/retheklappbruecke_1/17retheklappbruecke_3610.jpg
And a continous bridge with two shearlegs.
https://kreuzfahrt-3d.de/inhalte/2013/06/letztes-br%C3%BCckenteil-der-baakenhafenbr%C3%BCcke-in-hamburg-hafencity-wird-montiert.jpg
From steel to concrete is just a question of weight and available mode of transportation.
EuclidHow could the project have gotten so far without realizing that the location of the central tower was unacceptable?
Why would the new position of the pylon not acceptable? You can't build bridges as you like but as the boundary conditions permit. And is the art of the bridge engineers to adjust to these borders.
Nothing with bridge made this failure inevitable. Correctly designed, analyzed, as well as build and erected in the correct working order every thing would be OK by now.Regards, Volker
This guy sounds like the profane John Madden calling the 'Stress Failure Bowl' btwn Stanford vs. MIT. "And ya see over here, this rod woulda been mangled by the concrete and not been able to come outta this hole here..."
VOLKER LANDWEHR Euclid How could the project have gotten so far without realizing that the location of the central tower was unacceptable? Why would the new position of the pylon not acceptable? You can't build bridges as you like but as the boundary conditions permit. And is the art of the bridge engineers to adjust to these borders.
Euclid How could the project have gotten so far without realizing that the location of the central tower was unacceptable?
I am referring to the original location of the central pylon not being acceptable. I would expect that the organization that declared the original position to be unacceptable would have done so much earlier when the project was being laid out in the concept stage. Instead, they are either not informed about the bridge layout; or they are informed and drag their feet before stepping in and asserting that the central pylon must be moved.
In any case, the design and engineering had been completely finished before the project was informed that the central pylon had to move, which required much of the engineering and design to be done over, and introduced the the risk of overlooking some details that needed to be done over and re-checked. This seems like a major communication blunder that cost a lot of time and money besides imposing new risk.
The reason why the central pylon could not occupy its intended position after the bridge was designed was completely evident before the bridge was designed.
Aaaah, who cares about the original post when we gots more innerestin' stuff to look at now!! JK, Randy, JK!!
Let the law suits begin.
Here is an interesting summary of the fact known about the bridge failure of 03/30/2018: http://www.miamiherald.com/news/local/community/miami-dade/west-miami-dade/article207358659.html
They state that external structural engineers have offered explanations in engineering forums but acknowledge that the lack of knowledge final structural drawings and analysis still makes them speculations.Regards, Volker
Edit: While browsing through the enginering forum linked in the above article I found the following super slow motion version of a dash-cam video: https://www.youtube.com/watch?v=D6pGzgm4zZ4&feature=youtu.be
For me it looks like the support diagonal failed at the joint with the top chord leaving all load carrying to the bottom chord which failed in bending fracture. Still only a description and no reasoning.
Does anyone know of an online downloadable calculator for moment-shear-floor?
I need to play with some different weights, the last table I made took three days, I want to try something a little faster.
Suspect most of the modern 'free' calculators (are SkyCiv and BeamGuru the sort of tools you want, or do you need more complexity) will do their calculations in a 'cloud' rather than be downloaded to run on a local machine.
There is surely something 'out there' that can give you a go/no-go assessment for a specified factor of safety instead of just giving you numbers out for numbers in. Such a tool with sufficient expert-system logic might be relatively usable as a 'second pair of eyes' backstopping emergent design even as redline changes are being entered...
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