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A10 Autostrada Bridge collapses

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A10 Autostrada Bridge collapses

Old 26th Aug 2018, 08:16
  #81 (permalink)  
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Originally Posted by tescoapp View Post
Material properties mostly.

Then cost. In theory it should be sealed against O2 so shouldn't oxidise.
I asked the question in the 50s as to why the steel was rusty when used I concrete. As you said, the perceived wisdom then was that the concrete would seal the steel and prevent corrosion. This was clearly less expensive than applying anti -corrosion material first. As said above, poor concrete mix of inadequate sealing lead to corrosion. It was not inevitable however as WW2 bunkers in France will testify.

Now you see all constructional steel properly sealed.
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Old 26th Aug 2018, 13:45
  #82 (permalink)  
 
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I will admit in a previous life I wasn't a concrete cube crusher...

But I did have to sort out their screwups.

Two types of truss bridge, truss and inverted truss.

On a normal truss with the triangles above the deck the top span is under compression and as concrete is 5 times stronger in compression than tension the top needs normal rebar for load. It will have a tension rod through it for tuning purposes but it won't be critical for taking load. As the concrete decays it will crack off etc but its relatively slow process with no sudden changes in load baring cross section. You can see that in the photo of the compression struts.

On an inverted truss the support is under the deck and the spans are under tension. Because concrete is so poor under tension they have to run tension stays along the bottom spans to take the load. these are preloaded to control the deflection of the bridge under self weight and to ensure the concrete surrounding them never goes into tension because it would just crack.

Now steel corroding forms an oxide layer with different material properties to the base material, it has a higher youngs modulus, different yield stress but can take load, its also very brittle and hard. When nothing moves the oxide layer grows at a set rate depending on the environmental conditions.

The fun starts when things start moving because the oxide layer is brittle and has a different youngs modulus to the base when the main structure flexes the oxide layer breaks not only radially through its thickness but also laterally as well giving flakes due to the shear stress created by the different youngs mods . The oxide layer also has a lower density which means there is an increase in volume. This gives a instantaneous reduction in load bearing cross section which with the same load gives higher stress and strains. The higher the strains the more the oxide layers will flake. The higher stress puts you into a different part of the SN curve for fatigue. Once the cross section reduces enough you start getting work hardening coming into play. Which is one of the ways Engineers find out that things are starting to get interesting we smack a ball bearing off the metal and measure the size of dent it makes. As the metal work hardens its UTS increases but that only goes so far. Because of the density change and volume increase this causes swelling in the tension runs which results in the outside length increasing. Back to concrete not liking tension this means the outside surface cracks letting in more oxygen and water.

While all these changes of the metal are ongoing the frequency response of the structure is changing usually very little change is seen until its to late but what does happen is peaks of various harmonics tend to cause hot spots of cross section reduction. So a civil engineer will look at a spot and think its some sort of bad concrete/build a frequency response structural will think that's the 5th harmonic peak doing that. For some reason civils tend to only look at the 1st and 2nd frequency modes. Anyway the bridge can sit and rust without load for ages because the load never gets high enough to crack and flake the oxide layer so the cross section reduction is gradual. The failure mode is slow and fairly obvious. Start putting it under load and moving things you get sudden cross section decreases and eventually you will get something hit critical crack intensity for a fast fracture fatigue failure or hit the UTS of the material that's left. Either way is a fast failure and will fail with the speed this bridge has.

That picture to me shows the bottom tension span on an inverted truss bridge. The black shadow of the deck can be seen at the top of the photo. You can even see the swelling in the strapping round the tension stay pack.

Maybe my use of tension stay is wrong in your world... Sorry but the head gaffer in the structural office I started out in as a graduate was one of the top marine engineers in the UK. He also dabbled with square riggers so nautical terms were pretty common in the office.

I freely admit I know cock all about concrete. Metal, frequency response, forces in bridges I do know about. My experience in life is most civils don't have a clue with metal or frequency response or materials subject to work hardening and corrosion. Unfortunately it tends to be civils who deal with concrete bridges mainly because the other flavours of engineers keep well away from them because usually you can't get to the metal to determine its condition.

So if that means I have no idea about it that's fine by me.
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Old 26th Aug 2018, 18:24
  #83 (permalink)  
 
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tescoapp, the spalling and corrosion shown in that photo should have been spotted and repaired long before it got into that state however money for infrastructure is not always available when needed. I know that in our bridge stock (some 2400 structures of varying types) any instance of spalling concrete (along with any other defects) is be recorded with extent and severity during each inspection and monitored on each subsequent inspection.
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Old 26th Aug 2018, 19:08
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From my experience you can't repair concrete members, you can beef up areas to compensate for drop off in load bearing strength. You can reseal to try and reduce the oxidisation and water ingress. But you can't replace a section in the same way as you can cut out and replace a metal structure to remove the rot. I am fully aware that replacing metal doesn't zero the fatigue history of the rest of the structure.

.
however money for infrastructure is not always available when needed
Ok we are into what's acceptable risk and acceptable collateral deaths due to running a structure/device above design loads and life. Thankfully I don't have to deal with that sort of engineering any more.

A change in law so that the accountants go to prison for man slaughter when something fails would go a long way to ensuring money was available when required. At the moment they pocket the bonuses and turn their backs when it all goes wrong.
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Old 26th Aug 2018, 21:15
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I agree that concrete repairs only offer a 'sticking plaster' solution however new materials and techniques do offer alternatives. I am aware of a project that involved the use of high pressure water to cut away the defective concrete and replace, corroded reinforcement cut out and replaced, carbon wrap the repaired beams. The structure is a 3 span reinforced concrete arch over a tidal river approximately 120m wide at this point so not an easy project by any means. Structures in the UK are generally the responsibility of the public sector (Highways England/ local authorities etc) rather than in the private sector so no bonuses there. The A10 bridge was, i believe, in the private sector so if things get tight (financially speaking) then routine maintenance is an easy thing to cut back on.
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Old 27th Aug 2018, 05:27
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I am pretty certain we are both on the same side when it comes to fit for use engineering structures. Local authorities being in charge of the condition monitoring though doesn't fill me with confidence.

Thanks for the info on using water cutting to excise the rot will go and have a paper search and see what they are doing these days. I still do a bit of project engineering on the side, more installation, company man for the customer than anything else nothing that will kill anyone if I screw up.

Still reckon the financial types in charge of money allocation being directly in line for jail if a structure fails due poor condition monitoring/ preventative maintenance would helps matters. Not dependant on if they received a bonus or not.
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Old 27th Aug 2018, 06:45
  #87 (permalink)  
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Originally Posted by tescoapp;10234090I

Still reckon the financial types in charge of money allocation being directly in line for jail .
Really?

"Here is my budget request"

" you can have 75%"

"Well it's not my fault of you got your priorities wrong, after all, how many people do you employ and how much over time did you pay?"

Catch a slippery eel is easier.
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Old 27th Aug 2018, 10:44
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Tescoapp, some local authorities do employ consultants in that role because they no longer have the in house skills themselves. Others retain an engineering team who do have the knowledge and skills necessary for such work. The project i referred to was designed and supervised in house.
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Old 27th Aug 2018, 17:15
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Originally Posted by tescoapp View Post
From my experience you can't repair concrete members, you can beef up areas to compensate for drop off in load bearing strength. You can reseal to try and reduce the oxidisation and water ingress. But you can't replace a section in the same way as you can cut out and replace a metal structure to remove the rot. I am fully aware that replacing metal doesn't zero the fatigue history of the rest of the structure.

.

Ok we are into what's acceptable risk and acceptable collateral deaths due to running a structure/device above design loads and life. Thankfully I don't have to deal with that sort of engineering any more.

A change in law so that the accountants go to prison for man slaughter when something fails would go a long way to ensuring money was available when required. At the moment they pocket the bonuses and turn their backs when it all goes wrong.
Back in the day I had a friend who was making a very good living indeed by being a 'foreign' consultant to sign off projects in Sov Block countries. The procedure was simple: ship the bloke in, get him to sign off all paperwork, pay him silly amounts of money and say goodbye. If the project failed and blame was being placed - probably with prison sentences to follow swiftly - then the so-said perpetrator was thousands of miles away and untouchable.
He worked at this government-sponsored scam for a couple of years, and had enough saved to set up his own well-founded consultancy - a proper one this time.
Saw him ten years later, and he reported that (a) they were doing very nicely, thank you, and (b) none of 'his' projects had failed. So that was all right...

But what would he have felt like if there had been a collapse similar to the A10 one?
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Old 7th Sep 2018, 05:15
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Will it change anything though if the rest of the infrastructure is in the same condition after being on the books of the same people?


https://www.bbc.com/news/world-europe-45439173

Also a nice simulation of what the failure mode was.

I have zero clue if its correct or not

https://www.nytimes.com/interactive/...ly-bridge.html
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Old 8th Sep 2018, 15:53
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Interesting hint in this video – see the crack on the pylon filmed shortly before collapse:

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Old 9th Sep 2018, 10:29
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Taiwan

This enterprising builder used empty tin cans instead of on concrete to save some cash, nearly worked.
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Old 10th Sep 2018, 09:01
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Interestingly the film of the crack is clearly filmed by the driver whilst in motion, one wonders whether the crack down by the Polizia on holding mobile phones when driving will result in a prosecution for the driver? The only 'winners' in all of this will be the lawyers despite all of the political posturing going on. That whole stretch of road from the airport to the centre of Genova is appalling, there should have been a more suitable road built years ago. The entry slip-roads are typically about 30m and end in a tunnel entrance if you can't squeeze your way in to the live carriageway you are in the wall. The arrangement of grill sections over cuttings in many places results in absolute lane confusion as the patches of sunlight resemble white lines and people who do not know the road then deviate and cut across the lanes. I avoided wherever possible that route but at least once per month had to pass by to get to the centre of Genova from our house, luckily the coast road had not long reopened after a landslip at Arenzano. I am waiting to see the rebuilt bridge in operation by October 2019.
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Old 12th Sep 2018, 09:57
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https://www.newcivilengineer.com/lat...034343.article

Just for interest
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Old 14th Sep 2018, 18:56
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One of the seventy-two cables supporting the bridge from the French mainland to the l'île de Ré has been found to have ruptured. HGVs over 40 tonnes are banned and a 50kph speed limit introduced.

https://medias.laprovence.com/FOPwai...ntiledere2.jpg
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Old 14th Sep 2018, 19:34
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Sorry but where are the cables?
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Old 14th Sep 2018, 19:36
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Originally Posted by uffington sb View Post
Sorry but where are the cables?
Inside the box sections, six per box.
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