Collapsed bridge did not have tower and suspension cables


Please consider donating to Behind the Black, by giving either a one-time contribution or a regular subscription, as outlined in the tip jar to the right. Your support will allow me to continue covering science and culture as I have for the past twenty years, independent and free from any outside influence.

The coming dark age: The Miami pedestrian bridge that collapsed yesterday only five days after installation did not have its required tower and suspension cables in place.

Engineering experts say investigators looking into the collapsed ‘instant’ bridge in Miami will want to know why a central tower which is usually built to support a suspension bridge was not in place when it collapsed onto Tamiami Trail on Thursday afternoon.

Last week, Florida International University’s official Twitter account posted a rendering of the bridge in its completed form as envisioned by the planners before its opening to foot traffic in early 2019. The rendering shows a tall central column with cables connecting it to the main span. Engineers say the design is known as a ‘cable-stayed bridge,’ which is a kind of suspension bridge, according to USA Today.

The bridge did not have the central tower in place, even though experts say it is usually placed at the early stages of construction. In the absence of a tower, there is usually a temporary support, though in this case it is unclear what the builders were using in the absence of a central structure.

I have never heard of any kind of suspension bridge ever being built in this sequence. Such bridges always install the towers and the cables, then the roadbed. Here, it increasingly looks like they put up the road bed before the tower and cables, an approach that practically guarantees the bridge will fail.

The images at the link show the presence of temporary support structures under the bridge, so it could be that the builders were using these to support the bridge until the central tower and cables could be installed. However, the bridge was made of concrete, which is far heavier and does not have the same structural strength as steel. It could be they overestimated the ability of this concrete structure to stand, without the cables.

Share

27 comments

  • MDN

    Seems like “Suspension Bridge 101” that you don’t have a viable bridge without the suspension part. What a monumental screw up that was 100% avoidable if a real engineer had been involved.

  • Dale Martin

    When constructing this type, temporary support is needed…Unless one is suffering from some kind of perception deficit.

  • Cotour

    Take note, all subjects covered yesterday, right here on BTB :

    1. “In the absence of a tower, there is usually a temporary support, though in this case it is unclear what the builders were using in the absence of a central structure.’

    2. “After the tower is built and the stay cables are installed, engineers then begin work on extending the central span.”

    3. “That tower had not yet been installed, and it was unclear what builders were using as temporary supports.”

    4. “US Senator Marco Rubio tweeted late Thursday that cables suspending the span had loosened, and the engineering firm ordered that they be tightened.” This is a confusion, Rubio is referencing tensioning cables and not suspension cables.

    5. “Authorities said Friday that the cables suspending a pedestrian bridge were being tightened after a stress test when the 950-ton concrete span collapsed over traffic, killing six people only days after its installation was celebrated as a technological innovation.” More confusion.

    6. “According to its web site, Structural Technologies offers ‘post-tensioning’ services to engineering firms.” This refers to tensioning cables that run through the slab and not support cables.

    7. “Engineering analysts are also vexed as to how a cable-stayed bridge built to carry pedestrians could buckle under no weight – even though a number of larger structures which can withstand cars and trucks remain in place.” This statement is misleading, there were no support cables in place so there should be no expectation of the bridge to carry any weight at all.

    8. “Andrew Hermann, a former president of the American Society of Civil Engineers, said it appears the engineers who built the FIU bridge didn’t follow the proper sequence.” As pointed out yesterday “Out of sequence assembly”.

  • mike shupp

    Design by eyeball rather than calculations.

    What struck me, in what I’ve read on the ‘net, is that the primary concern of the university was getting that 185 ft structure across those highway lanes with the absolute minimum of traffic disruption.

    Thus the absence of a central support tower — it was something to put in later, when it was convenient, when it could be done quickly, as just the one task without complications. No rush, in other words. After all, the official opening date for the bridge was a year off. And the main thing was getting those tension cables strung over the highways — that was the centerpoint of the design, the steel cables, which would carry the weight of the concrete easily as a clothesline holds up diapers.

    And that’s a university with an engineering school? Oh, those students will be simulating that design in MathCad and Maple in sophomore classes for the next 30 years!

  • Kyle

    They said the span was suppose to be able to support itself, if so I wounder if Structural Technologies over tighten the tension cables or something? Concrete has great compression strength but practically zero tension strength. That’s why we add rebar. Once that span was lifted in place by its ends, it would immediately experience compression forces on its top and tension forces on its bottom due to the structure sagging in the middle. If those cables were to snap, or be compromised in any way, the span would fail.

    Also, if they had to mess with the cables, the bridge’s weakest link, why wasn’t a detour or some other traffic control set up?

  • Mike Borgelt

    This sort of thing has happened before – in Australia. Search for Westgate bridge collapse. Assembly sequence error/omission.

  • Cotour

    The “central support tower” has no consequence to traffic, it is built out of poured concrete and rebar on site. The school may have been concerned about traffic while the bridge itself was moved into place and that is why it is done at night.

    You seem to be conflating things here.

  • Edward

    From the article:

    ‘When you’re doing staged construction like this, what you have to make sure is that at each stage that the structure is strong enough for the loads that are on the bridge,’ Hermann told USA Today.

    ‘The engineering, both design and the construction engineering, should have taken that into account with the bridge in that condition.’

    So, something went wrong even though the design included strength and support during that stage while the bridge was in that condition. Obviously, the support tower was not intended to be needed at that time.

    From the photographs, it looks to me as though one end fell off its support column. Although this may have happened due to the collapse, it may also be the cause of the collapse.

  • Edward: There is a video at the link that actually shows the bridge as it collapsed. There do not seem to be any support structures below it, and it seems to fall straight down, as if the center could not take the strain.

  • Cotour

    Oh boy:

    The following voicemail was left on a landline Tuesday by W. Denney Pate, FIGG’s lead engineer responsible for the FIU pedestrian bridge project:

    “Hey Tom, this is Denney Pate with FIGG bridge engineers. Calling to, uh, share with you some information about the FIU pedestrian bridge and some cracking that’s been observed on the north end of the span, the pylon end of that span we moved this weekend. Um, so, uh, we’ve taken a look at it and, uh, obviously some repairs or whatever will have to be done but from a safety perspective we don’t see that there’s any issue there so we’re not concerned about it from that perspective although obviously the cracking is not good and something’s going to have to be, ya know, done to repair that. At any rate, I wanted to chat with you about that because I suspect at some point that’s gonna get to your desk. So, uh, at any rate, call me back when you can. Thank you. Bye.”

  • Cotour: What is your source for this? And if you are making it up, you should make that clearer.

  • pzatchok

    Looks like fox news is running it.

  • Orion314

    Looks like Affirmative action engineering to me.

  • Andrew_W

    Bridge designer left state voice mail about cracks days before FIU bridge collapsed
    http://www.miamiherald.com/news/local/community/miami-dade/article205627294.html

  • Max

    Edward, I also noticed the bridge was not damaged on the very end. It looks like it slipped off the support column as you said. Or as the engineer called it,”some cracking that’s been observed on the north end of the span, the pylon end” (The support structures under the bridge on the day they moved it into place were gone in later pictures)
    I had assumed the cables they were talking about were for support… I would never have believed that there was no support cables at all. So it’s been established that these were tension cables that the crew were adjusting.
    Every day the bridge will expand several inches in the sunlight and contract at night making the tension cables seem slack. Is it possible that the adjustment of the cables pulled the bridge off of the support on one end? The movie seems to support it all came down at once. I would think a bridge that was cracking would break in two before it fell.

  • wayne

    pivoting to an actual “suspension bridge:”

    “The Five Mile Dream”
    1958 US Steel- Lowell Thomas
    https://youtu.be/eFsy_EHWM-Q
    9:36

    “The design of the Mackinac Bridge was directly influenced by the lessons from the first Tacoma Narrows Bridge, which failed in 1940 because of its instability in high winds. Three years after that disaster, Steinman had published a theoretical analysis of suspension-bridge stability problems, which recommended that future bridge designs include deep stiffening trusses to support the bridge deck and an open-grid roadway to reduce its wind resistance. Both of these features were incorporated into the Mackinac Bridge. The stiffening truss is open to reduce wind resistance. The road deck is shaped as an airfoil to provide lift in a cross wind, and the center two lanes are open grid to allow vertical (upward) air flow, which fairly precisely cancels the lift, making the roadway stable in design in winds of up to 150 miles per hour (240 km/h)…”

  • Cotour

    I have now seen some drawings and a particular picture and as I understand it there are tensioning rods that go down through the angled uprights down through to the slab and not just horizontally through the slab and the roof in parallel. Why did the break happen exactly where the men were working on the top at that exact spot? They appear to have been adjusting the rod tensions with a hydraulic tensioner and the rod possibly broke in adjustment and / or set things off in an associated way.

    There is a picture of that spot with the hydraulic adjuster and the rod sticking 4 or 5 feet out of it, I will see if I can get a screen shot of that particular evidence.

    So the upper end of that angled upright that can be seen in the video I posted does appear to be the point of failure, hydraulic adjuster brakes the supporting rod that tensions the structure and the upright collapses and destruction ensues.

    I think the overall lesson will be that the cable support pylon will always first be constructed and ready to take weight or their will be proper street support in place before anything else is even considered to be assembled. The design is very nice and aesthetically pleasing and the system is proven in many other installations, but that 174 feet and the weight must be held in place as a first priority.

  • Cotour

    Go to the 1 min point in this news video and you can see the hyrdraulic tensioner that was in use , apparently broke the rod and blew it out setting off the collapse of the angled upright.

    https://youtu.be/UKm8zGNZUMY

    Those guys working the tensioner, which was lifted to the top by the mobile crane I am certain are in very bad shape today if they survived the break and the fall. All around, just not good.

  • Andrew_W

    Cotour, I think you’re right, the tensioner is definitely located at the point of failure.

  • Edward

    Robert wrote: “Edward: There is a video at the link that actually shows the bridge as it collapsed.

    You’re right. How did I miss that? The one end fell off its support column due to the collapse.

    Max wrote: “I would think a bridge that was cracking would break in two before it fell.

    The video shows it crack into two parts near the support column that it slipped off of. This explains why it slipped off.

    Max wrote: “Every day the bridge will expand several inches in the sunlight and contract at night making the tension cables seem slack.

    The coefficient of thermal expansion for concrete is similar to that of steel. If the materials are chosen correctly, then this should not be a problem.

    I do not know why the cables seemed slack, that day, or whether this is an unusual occurrence during construction.

    For those who do not know, prestressing a concrete structure (e.g. bridges or building floors) allows for longer spans between support columns and better overall structural strength.
    https://en.wikipedia.org/wiki/Prestressed_concrete

    the resulting material has the characteristics of high-strength concrete when subject to any subsequent compression forces, and of ductile high-strength steel when subject to tension forces. This can result in improved structural capacity and/or serviceability compared to conventionally reinforced concrete in many situations.

  • Cotour

    I think what they were attempting to do was to adjust for the cracking that had occurred by adjusting the tension rods running through the angled upright and it just snapped setting it all off.

  • Cotour

    Another video angle approaching the bridge in traffic at the moment of collapse :

    https://youtu.be/p6L20i6_gzE

    The bridge crumbles right where the workmen are working on top with the hydraulic tensioner. No harnesses, no decelerators hooked up to the crane if they should have fallen. One big deadly mess.

  • Commodude

    OSHA is going to have a field day with that video.

  • Cotour

    Commodude:

    Isn’t it interesting how much information and how many correct assumptions can be made eventually coming to understand what more than likely happened in a construction collapse, in this case of this failed bridge section, by just sitting in front of a computer and reviewing video, specifications, drawings, pictures and applying common sense and a knowledge of how things work, mathematics and physics, and function in the real world?

    I am certain that WE right here on BTB were way ahead of the curve, just an hour or so in WE were well on the way to pretty much explaining what had gone down..

  • Scott M.

    It definitely seems as if one of the tensioning rods broke while they were trying to re-tension it. Warning, this video has an engineer speaking his native lingo (that is, lots of f-bombs):

    https://youtu.be/KtiTm2dKLgU

    The truss that broke was originally supposed to have a support under it. They moved that support towards the center, leaving the end of the bridge hanging.

  • Edward

    Scott M.,
    Interesting hypothesis, but it is a bit like a psychiatrist diagnosing someone by watching a TV interview.

    One thing that is missing from the admittedly grainy security video of the collapse is the hydraulic tensioner coming out of the strut just before the bridge falls. His cause/effect logic makes assumptions that miss the possibility that stresses during the collapse could have caused the rod to fail, resulting in the tensioner in the same position as it was found. He understands that everything is a spring, as he told us, but he did not consider the possibility of a collapsing bridge also behaving like a spring and causing the rod to fail. He only considers the case of the concrete breaking and crushing the rod.

    His speculation that the crane was helping to lift up the span is wrong. Not only is the bridge too heavy for the crane, making this use of the crane unlikely, but the crane would have fallen over during the collapse. Also, I do not understand why he thinks that tensioning the bridge while traffic is such a bad idea, because such tensioning obviously is not supposed to be a hazardous process; otherwise, they would not have had a plan to do post tensioning in place.

    The investigation will examine a lot of items in order to make sure that they do not miss the root cause by making the assumptions that the video made. They will not focus on any one thing unless they eliminate everything else, and they will probably note multiple areas for improvement in the Accelerated Bridge Construction process.

    I am going to wait for the NTSB report for the answer. There was plenty of speculation about the 2016 Falcon 9 explosion, too, and all of it was wrong.

  • Cotour

    Maybe NIST will weigh in and produce a 30 thousand page report and come to the conclusion that Global warming was the cause?

    A point of clear and unambiguous observation, that bridge fell at free fall speed, no doubt or argument about it. Thats what it looks like, you can clearly measure it and it makes perfect and reasonable sense, a structure is compromised and there is nothing between it and the street. Mathematics and physics.

Leave a Reply

Your email address will not be published. Required fields are marked *