Juggy done a bridgedozy

[Shoulders?-Stomach!]

http://www.bbc.com/news/world-us-canada-43418898

Costing $14.2m (£12.5m), the cable-supported footbridge was funded by the US Department of Transportation.

According to a brochure on the university website, it was designed to withstand a Category Five hurricane and last 100 years

The bridge was erected on Saturday in just six hours.

It was built using a method called "accelerated bridge construction" to avoid traffic disruption. A major section of the bridge was assembled on the side of the road and then raised into place

The 862-tonne, 174ft (53m) bridge fell over an eight-lane motorway on Thursday afternoon, crushing at least eight vehicles, and killing six people, police said.

Fucking hell, the kind of thing you expect in a third world country, even more horrific because this was intended to prevent further deaths due to students getting hurt and killed trying to cross the motorway.

[Fambo Number Mive]

Very sad. There needs to be a full enquiry.

[madhair60]

More like a bridge over troubled slaughter.

[remedial_gash]

Guns don't kill people, bridges do.

Seems that thoughts and prayers  don't reverse bullets or architectural mistakes. There's been a minor stink about H+S and deregulation that I don't understand,so I hope that Buzby weighs in.

[biggytitbo]

A Bridge Too Fast :(

[Dr Syntax Head]

Sakes. Should have installed a pedestrian crossing. People love pushing the little button and waiting for the green man.

[bgmnts]

One of those horrible situations where i cant help but laugh because it is that stupid.

[biggytitbo]

It was built using a method called "accelerated bridge construction"

And it fell down using a method called 'accelerated bridge deconstruction'.

[gilbertharding]

I only dabble in structural and civil engineering, but I'm fairly sure the short time it took to put the span in place is a red herring.

[Alberon]

Also more likely to be a bad mistake in the construction rather than in the plans.

[biggytitbo]

Was a giant mothman seen resting on the bridge before it fell down?

[bgmnts]

Or the muffled cries of a gerbil?

[buzby]

I only dabble in structural and civil engineering, but I'm fairly sure the short time it took to put the span in place is a red herring.

Yes, ABC is relatively common construction method (there was a replacement pedestrian bridge over the end of the M62 in Liverpool built using it a couple of years ago).

The bridge was split into to 2 unequal deck sections (a longer one over the road, and a shorter one over a river or canal) with a suspension tower in between them.

Only the longer of the two spans, the one over the road, had been put in position and it's that one that has collapsed.

The span was lifted into place onto it's piers (which, as a suspension bridge it's not meant to be supported by), but without the central suspension tower or cables in place. This is unusual, the normal method of construction for a central tower suspension bridge is to build the tower first and then hang the deck sections off either side of it from the cables.

The longer span was supposedly designed to be strong enough to support itself until the tower and cables were installed. The webbed construction of the deck was presumably considered similar enough to a truss bridge (a self-supporting trinagulated beam bearing on buttresses at either end) to allow this to happen.

This would reduce the amount of time the road was closed for, but it appears to have failed under load (or even under it's own weight) while they were working on it (some reports say they were performing a stress test, but I can't see how that would be possible without the tower and cables in situ). It's also unusual for a truss structure to made of reinforced concrete instead of steel, which would have made it much lighter.

Both the design agency and construction contractor reportedly have previous history of structural faults including at least one collapse (after a concrete deck beam was modified during construction).

[Shoulders?-Stomach!]

If this bridge was so quick to construct I don't get why it was only due to open in 2019? I mean, it's a good job and all but...

[biggytitbo]

Let's not be too harsh on them, we've all suffered from erectile dysfunction before.

[buzby]

A bit more information has come out overnight, including some CCTV and dashcam footage of the collapse.

Rather than being a conventional suspension bridge, the two deck sections were designed to act as at least partial trusses, fixed to the centre pier where the tower was supposed to be and floating at the other ends.

Normally if you are making bridge spans out of concrete they pre-stressed - concrete is weak in tension but strong in compression, so you can increase it's strength by tensioning the steel reinforcing rods from each end with hydraulic jacks  before pouring the concrete into the mould. Once its cured, you can then release the jacks, and rods will try to pull back to their resting position, putting the concrete into compression and  increasing it's strength (the big concrete beams on road and rail bridges and building frames are normally made this way).

It's becoming more common these days to use post-tensioning on concrete structures. In this, threaded anchor plates are embedded into the structure at the end of hollow channels. Long threaded steel rods are then inserted into the channels and threaded into the anchor plates. At the other end o the rod a spreader plate is placed over it and a large nut it threaded on. you can then apply tension to the rod by pulling on it with a hydraulic jack and tightening the nut, compressing the concrete member. This has the advantage of being easier to construct (you don't need a lot of heavy hydraulic tensioning gear during the concrete pouring process, so it can be made at the job site instead of at a structural concrete factory), and you can adjust the tension if required after the concrete has cured and the member is in position. However, it does mean the part is potentially weaker until it's in it's final position and the correct amount of post-tensioning has been dialled in.

The deck section was built along the side of the road and then swung 90 degrees and lifted onto the piers last weekend using 4 hydraulic crawler/jacking units working in two pairs. The original plans for this operation showed the pairs of crawlers positioned at each end supporting the span on spreader beams between each unit of the pair (effectively in the same places the truss was designed to be supported from once in position). The video of the jacking operation shows no spreader beams were used, and at the north end the pair of crawlers was repositioned further inboard so it could run along the roadway instead of having to pave over the soft verge and remove a concrete barrier. This would have turned that end of the span into a cantilever rather than a truss, imparting forces on it which it was not designed to handle.

Before the deck was moved, some post-tension was applied to a number of the tensioning rods (which ran through the diagonal truss members with the tensioning nuts accessible in the cable mounting 'blisters' on top of the canopy) to add strength to the deck. Once it was in position, the initial tensioning of each rod would have taken place.

One of the design engineers noticed cracking on the north end of the span (where the crawler positions had been changed) and notified the Florida DoT. He presumably was accompanied by engineers from the construction contractor, as at the time of the collapse they were working on the bridge, and in the post-collapse pictures a crane is present at the north end of the deck, and one of the post tensioning rods at that end has a hydraulic jack attached to it and has been half-ejected from it's channel in the concrete.

It looks like the north end of the deck may have been weakened during the transportation and jacking procedure, the contractors noticed the cracks and then tried to add more post-tension on the rods at that end and one of the rods has either snapped or the anchor plate at the other end has failed or torn free. The sudden release of tension (on an already potentially weakened structure) has then taken he compression force off the concrete and caused a catastrophic failure of the deck.

Even if there hadn't already been cracking observed on the structure, they should never have been adjusting the post-tensioning with traffic passing underneath. The road should have been closed and the surrounding area sealed off to pedestrians. Someone should be going to jail for this.

[Johnny Yesno]

I'm glad I get my news from buzby because the actual news outlets are offering very little insight. I've not seen mention that the bridge was unfinished, for example. Just that it was due to be 'officially' opened in 2019.

[BlodwynPig]

I just hope his knowledge is used in mechanical engineering/construction/gun manufacturing/aerospace engineering and he doesn't work in a Starbucks.

[ollyboro]

Bridge of dies

[Alberon]

If there's any justice there it should be more than one person going to jail. A good bunch from the construction firm and whichever bright spark kept the road open while work like that was going on on the bridge. I understand wanting to reduce closure times, but some times it is just essential.

Health and Safety often goes overboard here, but it does seems like that in America it doesn't go far enough.

[petril]

If they'd just armed all the students, they could've shot the traffic out of the way and been able to cross in peace.

[kittens]

The bridge was erected on Saturday in just six hours.

It was built using a method called "accelerated bridge construction"

the bridge was built quickly.

it was built using a method called "building a bridge quickly"

[Ferris]

One of those horrible situations where i cant help but laugh because it is that stupid.

I said the same thing about a ski lift engine that suddenly (and for no apparent reason) switched into reverse gear at full speed. The image of people suddenly whizzing back down the mountain was hilarious, until someone mentioned injuries.

Edit: here it is, the Graun has edited out the more hilarious/terrifying footage https://www.theguardian.com/world/2018/mar/16/ski-lift-hurtles-out-of-control-georgia

[hamfist]

Just want to thank you Buzby, always enlightening to read your posts.

[madhair60]

Thread title keeps making me laugh

[buzby]

I've found a couple of photos illustrating what i was talking about above.

This shows the bridge being transported last weekend. The two crawler/jack units an the right side of the picture (the north end of the bridge, next to the canal) are the ones that had been moved. The original plans showed the pair straddling the concrete barrier. The sandy verge would have had to be paved over, so they moved the crawlers inboard. The final diagonal truss member on the section overhanging the jacks is the one they were working on at the time of the collapse.


This is the north end of the bridge after the collapse. The large block on the centre is the 'blister' the suspension cable would get attached to (you can see the threaded mounting rods sticking up out of it). This is at the top of the last diagonal truss member in the first photo. You can also see a steel rod hanging out of it with a blue cylinder on the end of it. That is the post-tensioning rod with the hydraulic jack attached to it.

The rod should be inside the diagonal truss member. The fact it's straight and sticking out that far indicates it came out prior to the collapse (it would have been bent and trapped by the rubble otherwise), and is likely what precipitated the failure of the structure. There would have been many tonnes of tension being applied to that rod by the hydraulic jack, so when the rod or it's mounting in the base of the deck failed it would have been shot out of it's channel.

The truck crushed under the rubble is from Structural Technologies VSL, who were presumably perfoming the post-tensioning operation. Navaro Brown, one of their employees is one of the victims.

[gilbertharding]

Splitting hairs - but it is (was) a cable-stay, rather than a suspension bridge.

At first it reminded me of the time a few years ago when a footbridge collapsed onto the M20, quite near where I live, luckily killing no-one. That was caused by a lorry hitting it, knocking it clear off its bearings.

This one... damaged when it was maneuvered onto the piers, are you saying? Then failing when the reinforcement was over tightened?

[checkoutgirl]

I'm glad I get my news from buzby because the actual news outlets are offering very little insight. I've not seen mention that the bridge was unfinished, for example. Just that it was due to be 'officially' opened in 2019.

The news providers in America are a complete joke though. But thinking about it, I barely understood what he meant. Imagine what the average American viewer would make of what he said.

[buzby]

Splitting hairs - but it is (was) a cable-stay, rather than a suspension bridge.

At first it reminded me of the time a few years ago when a footbridge collapsed onto the M20, quite near where I live, luckily killing no-one. That was caused by a lorry hitting it, knocking it clear off its bearings.

This one... damaged when it was maneuvered onto the piers, are you saying? Then failing when the reinforcement was over tightened?

Yes, cable stayed. After I looked at the design proposal it said the trusses were supposed to be capable of supporting themselves (at least when unloaded, otherwise there's no point going to the expense of adding the cable stayed element)  and the cable stays were mainly to add additional defection support when the bridge was loaded, or for purely fro aesthetics. The tower was supposed to be fixed onto the top of the two deck sections once they were mounted on the central pylon rather than directly onto the pylon, which would point towards it not having much load-bearing capability.

It could be one or any combination of the following factors:
Structural engineering or construction fault
Damage from unplanned stress while being positioned due to the compromise placement of the jacks
A faulty post-tensioning rod

On the day after the lift was completed (Tuesday), the engineer noticed cracking at the north end of the span, left a voicemail at the Florida DoT but didn't consider it serious enough to compromise the structure. On Thursday morning an FIU worker heard a loud 'whipcrack' noise while using the pedestrian crossing next to the bridge.

At the same time a review meeting was taking place to discuss the cracking, where it was decided there were no safety concerns and the span was safe. Around noon, Structural Technologies were on site to re-tension the tensioning rods at the north end of the bridge (presumably as a result of the discussions at the review or privately between the contractors, as the other parties seem to have been told this was due to a 'stress test') and at 1:30pm the span collapsed when the tensioning rod being tightened became disconnected and the final diagonal truss on the north side fractured, causing the canopy and deck of the span to shear under a load they were not designed to carry.

Here's a still from the CCTV of the moment of failure:

The diagonal truss has fractured from the canopy, and both decks are in the process of shear failure. Here's another picture of the north end post-failure from the side:

The bulk of the diagonal truss is still intact, but it has fractured at both ends from the deck and canopy. From the photos showing the blister it looks like there were two tensioning rods running down though that truss and the end of the other is still in situ. It may have failed first (possibly the noise heard by the university worker), or it may have failed instantaneously due to overload when the rod being tightened failed.

[Shoulders?-Stomach!]

I say telescopic dampers I mean rigid staaays