New Miami Foot Bridge Collapse Bad Engineering?

[Quarlo Klobrigney]

And this.....

[floobydust]

It's an engineering failure- CivE napkin calcs and discussions about the design are brow raising. Mass incompetence in many areas surrounding the build as well.

Why does engineering fail?

We want it cheap as possible. Don't use steel, use concrete as a "truss".
We want it artsy as possible. The bridge should be all unicorns and roses. Fake cable-stays and tower please.
We want it designed as fast as possible. Hurry, get the drawings out asap so everyone else has plenty of time for marketing, costing, and to procure and build and install.
We want it as convenient as possible. Don't block traffic for any reason.
We want it "inclusive". Add students and an under-qualified team to do kindergarten math.

I could go on, but my point is it's very difficult to do decent engineering in the first place.

Corporate pressure on engineers is winning, over safety and ethics.

[Cerebus]

We want it cheap as possible. Don't use steel, use concrete as a "truss".

Don't knock concrete, the Pantheon in Rome was built of concrete in 125AD, is still standing and the dome is still the largest unreinforced, unsupported concrete dome in the world at 43 metres diameter. Nothing wrong with concrete if you know what you're going.

[Bud]

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[floobydust]

Concrete is roughly 1/2 the cost of steel and 1/10 the strength. It's fine in some applications, but here in this truss, concrete can't take tension or shear after a crack.

If they spent $100,000 more on steel I don't think it would've collapsed so I'm saying this is too cheap CivE's.
950-ton span to support pedestrians, seem a little heavy, on the concrete?

“Why they had to build this monstrosity in the first place to get children across the street?” said an anguished Joe Smitha, whose niece, Alexa Duran, was crushed in Thursday’s collapse at Florida International University. “Then they decided to stress test this bridge while traffic was running underneath it?”

[coppice]

We want it cheap as possible. Don't use steel, use concrete as a "truss".

Don't knock concrete, the Pantheon in Rome was built of concrete in 125AD, is still standing and the dome is still the largest unreinforced, unsupported concrete dome in the world at 43 metres diameter. Nothing wrong with concrete if you know what you're going.

He didn't knock concrete. The Pantheon has no trusses, because you can't make one from any known form of concrete. Their tensile strength is very poor. The Pantheon was designed to keep all the concrete in compression at all times, including during its assembly.

[Cerebus]

We want it cheap as possible. Don't use steel, use concrete as a "truss".

Don't knock concrete, the Pantheon in Rome was built of concrete in 125AD, is still standing and the dome is still the largest unreinforced, unsupported concrete dome in the world at 43 metres diameter. Nothing wrong with concrete if you know what you're going.

He didn't knock concrete. The Pantheon has no trusses, because you can't make one from any known form of concrete. Their tensile strength is very poor. The Pantheon was designed to keep all the concrete in compression at all times, including during its assembly.

Did you really think I didn't know all of those "facts"? The Pantheon has no trusses, not because they are purportedly impossible to make "from any known form of concrete" (one takes it that you don't believe pre-stressed concrete counts as "a known form of concrete"), but because it was designed as a dome.

BTW, the Romans actually did make use of reinforced concrete, copying from the Greeks before them who made tension members like lintels out of stone by pouring lead reinforcement into channels carved in the stone. Yup, pouring molten lead into stone to make reinforcements.

Perchance my comment was related to the "steel good, concrete bad" implication. There's nowt wrong with concrete, used properly, including in tension as long as you apply appropriate reinforcement or compressive pre-stressing. The issue here isn't the basic material choice or a choice of 'cheap' materials, on the best evidence available so far it's a either a design flaw or poor construction practices related to the way the concrete was post-stressed as, or after, it was moved onto site.

[Gyro]

Iirc, I think they found than the Pantheon has lots of volcanic pumice in the mix too which takes the weight right down.

P.S. I have a vague memory from the documentary that there was some evidence that the original (higher density) attempt had actually (partially?) collapsed, the pumice filled one being the second attempt.

[donotdespisethesnake]

Iirc, I think they found than the Pantheon has lots of volcanic pumice in the mix too which takes the weight right down.

P.S. I have a vague memory from the documentary that there was some evidence that the original (higher density) attempt had actually (partially?) collapsed, the pumice filled one being the second attempt.

I wouldn't be surprised if they built the same way later cathedrals were built. Keep building until it collapsed. Rebuild with more strengthening until it didn't fall down.

In many ways, we still do the same thing. Until we reach a time were *every* factor can be perfectly simulated in a computer, then some trial and error will occur. That applies to space shuttles and bridges. There is always a desire to make things cheaper, faster, better so there will always be failures.

Prof Henry Petroski wrote the book on engineering failures To Engineer Is Human: The Role of Failure in Successful Design

With the self-confidence gained from their initial triumphs, engineers designed bridges that were structurally lighter and more economical. Of course, this meant that they were also more likely to fail — and they did.

When a bridge failed, engineers received a wake-up call and answered it. The bridge type that failed was often abandoned outright and safer alternative designs were adopted. This naturally led to a period of successful bridges, which once again lulled engineers proud of their achievements into complacency — until a new kind of failure occurred.

This cyclic behavior has continued for a century and a half, with a significant bridge failure occurring about every 30 years.

At least this bridge collapsed now, not when it was filled with pedestrians crossing the bridge. No consolation for those killed already.

[coppice]

Iirc, I think they found than the Pantheon has lots of volcanic pumice in the mix too which takes the weight right down.

The volcanic ash doesn't just reduce weight. It greatly reduces cracking, and makes the structure of the concrete much more stable over the long term. Whenever you see concrete from the ancient world that is still serviceable it seems to be of this volcanic ash loaded type.

[floobydust]

Dashcam video with partial photogrametric reconstruction of the bridge collapse.
Still a lot of speculation on what bit failed; a member, anchor blister, PT tendon, canopy etc.

Over budget and behing schedule:
"... the project was already running about $2.6 million over its $9.4 million initial budget, cost-tracking documents from February show. Originally scheduled to be completed in July, the finish date had been pushed back to January 2019."

Late design changes:
the bridge has to be 11ft longer, to accommodate a future extra lane of traffic. So the pylon was moved, at the end that failed.

[Tomorokoshi]

It reminds me a bit of this disaster:
https://en.wikipedia.org/wiki/Hyatt_Regency_walkway_collapse

[TerraHertz]

Two interesting commentaries.

Seems they were tightening the tensioning rod in the end diagonal, when it failed.
But, there were two tensioning rods in that strut, with only one being accessible from the top. The other one's adjuster was underneath the span. Did they try to put all the tension on one of the rods, because no one wanted to do the one underneath?

Also finally I understand why the struts are at all those funny angles. To line up with the future 'suspension bridge' structure. (Shown in 2nd video.). But that is virtually a fake visual frill, with no actual suspension cables.
Hmmm... how karmic. The bridge refused to be a fake. Or one could observe that it was designed by people who think fakery is a good idea.


They seemed to think the tensioning operation was risky - hence the crane for the worker's safety harness. (Which he for some reason took off, and so fell with the bridge.) But they didn't stop the traffic... I just... why?

[floobydust]

Even if the roadway was closed for this operation, the work crew would have been killed or injured. Nobody planned on a bridge collapse.

The crew had tightened the PT tendons at the south-end and was then doing the same on the north-end.

It looks like the PT tendon broke free while tension was being adjusted. One of two PT tendons in member #11.
In post-collapse pictures, the blue hydraulic jack A shot out but the second tendon B did not.

Member #11 was highly stressed already and changing tension caused a failure, still exactly unknown, but it looks like the member's lower end or (11-12) attachment point failed.
Still a lot of speculation, the structure was basically weak and adjusting the PT tendons was just was the proverbial straw that broke the camel's back.

CivE's are saying it's a terrible design. Trying to take a huge leap forward in bridge technology.

[raptor1956]

If it turns out the cable stay portion was, as has been reported, just for show I would have serious concerns for the design.  It looks like the collapse was proceeded by re-tensioning the rods, however, the whole design approach gives me concern about what the priority was and it looks like safety was not the design goal at the top of the priorities.  If you're going to go to the trouble of designing and building a bridge with a cable stay structure why not make the cable stay portion handle some of the loads.  This just looks like the University was looking to make an aesthetic statement and to hell with safety...


Brian

[glarsson]

If you're going to go to the trouble of designing and building a bridge with a cable stay structure why not make the cable stay portion handle some of the loads.  This just looks like the University was looking to make an aesthetic statement and to hell with safety...

They can't build a cable stay bride using the ABC (Accelerated Bridge Construction) method. The bridge span must be built offsite and moved in place in one piece. Therefore it can't rely on cables and pylons. The clearly had "low impact on traffic" as a higher priority than safety.

Big, heavy and ugly bridge. Why?

[ez24]

There is a lot of talk about failure during tension.  But in one of the videos, he says because of work changes the member had to be tensioned more to move it then released after because of the cantilever.  But all talk is that it was tensioned again.  So my question is - to release tension, is a hydraulic ram used?    I wonder now if the mistake was that it was supposed to be released but the worker got it wrong and increased it by error?  I haven't heard - did the worker get killed?

thanks

[floobydust]

When they moved the span, supported underneath mid-span by the transporter; compared to in-place, supported at each end- the forces on the deck and end trusses changed from tension->compression;

So the tendons had to be adjusted. I think to back off the nut you use the jack to stretch it first. That might have added stress which caused the failure.

There were strain-gages monitoring while the bridge was moved and installed. Hopefully the two wires were not flipped. It's too bad they didn't leave the sensors in afterwards. They'd notice the forces, if they were different than the design engineer expected.

One guy got killed in the crew, two others hospitalized. Their safety lines seem to have gotten snagged by the falling canopy.

[ez24]

So the tendons had to be adjusted. I think to back off the nut you use the jack to stretch it first. That might have added stress which caused the failure.

This makes sense - thanks

[donotdespisethesnake]

The safety harness was to stop the worker falling off the bridge, not because they expected the bridge to fail.

What is not explained so far is the relation of the cracking to the tensioning being performed when it failed. Those beams were already under compression, so maybe they though adding a little tension would not hurt. But if the cracking was an early sign of shear stress, then adding any tension may have made it fail completely.

It sounds like an off-the-cuff quick fix response to the cracking, without understanding the underlying causes.

To me the angle of the truss members was compromised by the aesthetic consideration and were the wrong angle. Instead of transmitting weight of the bridge down to the pier, it tends to rotate the strut around the joint, creating a lot of shear force.

The lessons for concrete trusses might be learned, but new mistakes will made all over again when the next "new design" becomes vogue. I've come to the conclusion that disasters like this are not preventable, unless people stop doing new things, which isn't going to happen. Human life is the price we pay for progress.

[amyk]

Over a year later comes the preliminary investigation report: https://www.ntsb.gov/news/events/Documents/2019-HWY18MH009-BMG-abstract.pdf

[Quarlo Klobrigney]

Over the next 19 days, the cracks grew until the bridge collapsed. The construction and
inspection firms working on the bridge were aware of the cracks and reported the cracks
to the design firm, asking for guidance. The engineer of record at the design firm
repeatedly indicated that the cracks were of no safety concern. Yeah....

[coppice]

Over a year later comes the preliminary investigation report: https://www.ntsb.gov/news/events/Documents/2019-HWY18MH009-BMG-abstract.pdf

They are pretty damning about both the incompetence and indifference of the engineers, which fits with what we heard before. However, the statement "The  emergency  response  by  local  fire  departments  and  law  enforcement  personnel  was  timely and adequate" seems like rather faint praise. I'd hate to be called adequate.

[magic]

I presume these days there are only two grades:

adequate - according to procedure, can't sue
inadequate - violating procedures, can sue

I would rather be adequate out of these two

[rstofer]

In the reconstruction of the San Francisco Bay Bridge there were also problems with the tensioning rods.  Initial inspection found fully 1/3 of the rods had failed from hydrogen embrittlement.  Later on, another 400 rods were found to have failed from environmental corrosion.

http://www.materialsperformance.com/articles/material-selection-design/2015/11/failed-anchor-rods-on-the-san-francisco-oakland-bay-bridge-a-corrosion-discussion

The previous span fell during the 1989 earthquake and it took nearly 20 years to build the replacement and I sure wouldn't want to drive the new one in the next earthquake.