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Genoa Bridge

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Spud McLaren
1292866.  Tue Aug 14, 2018 4:38 pm Reply with quote

PDR wrote:
In my current role as “QI roving engineer-at-large (by proxy)” I wondered if people were interested in this. The collapse of the Genoa A10 bridge is on the news here:

https://www.bbc.co.uk/news/live/world-europe-45182675

But the following google streetview image is rather depressing:

https://goo.gl/maps/zvXyWk1PNrF2

This image shows clear signs of rather severe alkali-silica corrosion (aka “concrete cancer” – google it for details). The google image dates from August 2016, so the structural elements of the bridge have been shot for at least two years.

Someone needs to be held accountable for this.

 
crissdee
1292870.  Tue Aug 14, 2018 5:22 pm Reply with quote

As a side issue. The reporter from the BBC was harassing the Italian PM mere hours after the disaster, asking him why the bridge collapsed. How TF would he know at that stage?

 
barbados
1292877.  Wed Aug 15, 2018 1:18 am Reply with quote

I think it is very soon to apportion blame (as in failings of aa particular process)at the moment. But on the way home from work yesterday this pretty much took up the news.
One thing that I'm sure had been mentioned was the bridge had been closed (not recently, more towards the end of the last century - bear in mind we are only 18 years into this one) but was reopened when passed fit, following an inspection. Part of the reopening process was that it had regular inspections, and these were carried out twice yearly. There had apparently been "some concerns" at the last inspection but it was decided that it was safe to continue.
There was also talk of the bridge taking a lightning strike before collapsing.

I'm pretty sure that, because the inspections were part of keeping the bridge open, there would be a degree of caution in the passing, as in there would be thorough checking rather than skipping bits that were ok last time out. I also wonder how much of the lightning strike theory was down to there being a storm and hearing the crash as it collapses then having the witnesses puting a couple and a few together to come up with some?

 
dr.bob
1292891.  Wed Aug 15, 2018 6:10 am Reply with quote

PDR wrote:
But the following google streetview image is rather depressing:

https://goo.gl/maps/zvXyWk1PNrF2

This image shows clear signs of rather severe alkali-silica corrosion (aka “concrete cancer” – google it for details).


That's not severe concrete cancer. This is severe concrete cancer (appropriately enough in Australia)



On the Belongil Creek Bridge which is still standing, despite being "a high usage bridge into Byron Bay".

PDR wrote:
The google image dates from August 2016, so the structural elements of the bridge have been shot for at least two years.


Despite the fact that this is not the element of the bridge that collapsed (the part that did can be seen here with a remarkable lack of any signs of alkali-silica corrosion), or that restructuring work had been carried out on the bridge in 2016, or that work was being carried out on the bridge at the time of the collapse, it seems that PDR has managed to locate the source of the problem as poor maintenance based on a google streetview image.

Someone call the Italian government. It'll save them a huge amount of money now that they don't have to carry out an exhaustive investigation into the causes of the disaster.

 
dr.bob
1292892.  Wed Aug 15, 2018 6:10 am Reply with quote

barbados wrote:
I also wonder how much of the lightning strike theory was down to there being a storm and hearing the crash as it collapses then having the witnesses puting a couple and a few together to come up with some?


Hard to tell at this early stage. However, some of the people who reported the lightning strike claim to have seen it with their own eyes.

 
barbados
1292972.  Wed Aug 15, 2018 4:17 pm Reply with quote

dr.bob wrote:

Despite the fact that this is not the element of the bridge that collapsed (the part that did can be seen here with a remarkable lack of any signs of alkali-silica corrosion), or that restructuring work had been carried out on the bridge in 2016, or that work was being carried out on the bridge at the time of the collapse, it seems that PDR has managed to locate the source of the problem as poor maintenance based on a google streetview image.

Someone call the Italian government. It'll save them a huge amount of money now that they don't have to carry out an exhaustive investigation into the causes of the disaster.

It would appear today that the bridge has been under constant maintenance, and one of the options being investigated is the method that the stays are constructed.
Given the choice of who to side with over what are possible causes I’d pick engineer’s explanation over a sysadmin most days of the week.

 
Spud McLaren
1292983.  Wed Aug 15, 2018 5:31 pm Reply with quote

PDR wrote:
Apologies, DrBob is right – I looked at the wrong bridge support. The support in the picture I reference does exhibit ASR (aka “concrete cancer”) but the severity can’t be judged from the photo alone and I was wrong to make assumptions. Looking at the photo of the *correct* bridge support (the one that failed) it looks like it had received the protective coating which is used to prevent (and treat) ASR, so my outburst was unjustified. Again, apologies.

 
bobwilson
1292988.  Wed Aug 15, 2018 7:13 pm Reply with quote

I’m also not a structural engineer and my knowledge of concrete is limited to having spent a couple of months working as a front-desk person for a (concrete) garage manufacturer about 30 years ago. Based solely on that experience (with the necessary caveats) – the Belongil Creek Bridge and the Genoa Bridge look to me as if they’re structurally completely different.

The former looks as if it was built with reinforced concrete, whereas the latter looks as if it was built with pressed concrete. Reinforced concrete has steel bars running through it which gives it its’ strength; this type of concrete is also often used for concrete fencing posts and bollards which (at least in the UK) can be frequently seen still – sort of – standing even though there’s little left of the original form – mainly, if memory serves me, because it’s cheaper.

Pressed concrete is a different animal – there structural strength is inherent in the concrete itself. Although steel bars are SOMETIMES used in pressed concrete (for additional strength), that isn’t always the case – and even when it is used, the characteristics it exhibits are different to when it’s used in “ordinary” reinforced concrete.

Visually, the decay process shows up in the two different types of concrete exactly as shown in the photographs – ie with reinforced concrete the concrete falls away over time as water and acids intrude leaving the steel bars showing; whereas with pressed concrete cracks appear consistently on the outer surfaces but tend not to penetrate too deeply.

However, I suspect that there’s a whole lot more to explaining the collapse than can be gleaned by a few weeks of pre-sales experience on relatively trivial building projects combined with a glance at a couple of photographs, so I think I’ll leave it to the professionals to produce their reports before I even contemplate this.

 
barbados
1293012.  Wed Aug 15, 2018 10:56 pm Reply with quote

According to the "expert" on the news last night, the bridge is constructed from prestressed concrete, the same as Hammersmith flyover. It is different to other prestressed concrete bridges because the stays are lagged in concrete, masking any corrosion. In the event of a stay failing the bridge becomes unstable. Couple that with the severe weather and you end up with a potential disaster.

The bridge was seen as a disaster from construction, not because the method is unstable but because the maintenance costs are ongoing, and end up costing more than the bridge itself.
The local official interviewed likened bridges to cars saying "the more use use a bridge, like a car, the more you need to maintain it. There comes a time when the maintenance costs are greater than the cost of replacement so the only sensible thing to do is replace it. This bridge had reached that point" (I'm paraphrasing there)
It would seem that plans were a foot to replace the bridge before a disaster occured, it looks as though the disaster beat them.

 
Jenny
1293038.  Thu Aug 16, 2018 7:38 am Reply with quote

Barbados - as soon as Woodsman heard that the stays were encased in concrete he identified that as the probable cause of failure because it masks any problems.

 
barbados
1293048.  Thu Aug 16, 2018 8:50 am Reply with quote

Yes, it makes sense. The “expert” was quick to point out that the design (prestressed concrete) is fundamentally a good one for bridges, as long as you can see the stays

 
Dix
1293080.  Thu Aug 16, 2018 2:25 pm Reply with quote

bobwilson wrote:
I’m also not a structural engineer and my knowledge of concrete is limited to having spent a couple of months working as a front-desk person for a (concrete) garage manufacturer about 30 years ago. Based solely on that experience (with the necessary caveats) – the Belongil Creek Bridge and the Genoa Bridge look to me as if they’re structurally completely different.

The former looks as if it was built with reinforced concrete, whereas the latter looks as if it was built with pressed concrete. Reinforced concrete has steel bars running through it which gives it its’ strength; this type of concrete is also often used for concrete fencing posts and bollards which (at least in the UK) can be frequently seen still – sort of – standing even though there’s little left of the original form – mainly, if memory serves me, because it’s cheaper.

Being half (or maybe a quarter) structural engineer - I passed the building materials course which had a heavy concrete component*, including the practical excercises and a couple of other courses before I ran screaming away towards IT - I'd like to say that there is no way in hell anyone can build a bridge like the Genoa one without steel reinforcement.

Concrete on its own has no tensile strength. It is very strong in compression but the slightest pull and it breaks. Putting a load on a horizontal beam supported only at the ends will snap it. And those towers will need to be able to take a considerable wind load (sideways movement).

You can just about get away with a Roman-style arched bridge (witness the Pantheon in Rome) but that's it.

Pressed concrete is for driveways.

*SWIDT :-)

 
brunel
1293094.  Thu Aug 16, 2018 5:30 pm Reply with quote

barbados wrote:
According to the "expert" on the news last night, the bridge is constructed from prestressed concrete, the same as Hammersmith flyover. It is different to other prestressed concrete bridges because the stays are lagged in concrete, masking any corrosion. In the event of a stay failing the bridge becomes unstable. Couple that with the severe weather and you end up with a potential disaster.

The bridge was seen as a disaster from construction, not because the method is unstable but because the maintenance costs are ongoing, and end up costing more than the bridge itself.
The local official interviewed likened bridges to cars saying "the more use use a bridge, like a car, the more you need to maintain it. There comes a time when the maintenance costs are greater than the cost of replacement so the only sensible thing to do is replace it. This bridge had reached that point" (I'm paraphrasing there)
It would seem that plans were a foot to replace the bridge before a disaster occured, it looks as though the disaster beat them.

The Hammersmith flyover is a bit of a strange situation, as the major refurbishment that took place ahead of the Olympics effectively converted it from a prestressed to a post tensioned structure.

The other problem is that, to some extent, this particular bridge seems to have been something of an experimental design - the 1960s seeing a number of such structures being built to test what was possible with certain concepts.

This particular structural form seems to have been fairly particular to Morandi himself, and it seems that it was not the only time that this particular design has caused issued - there are reports that the Wadi el Kuf bridge in Libya, which is of a similar age and construction, was temporarily shut in 2017 because of concerns over cracks developing in the concrete.

That said, there is an argument that not all of the issues came from the design itself - the indication is that the bridge was being subjected to far higher traffic loads than it was ever intended to carry, suggesting that it was being pushed beyond the intended design envelope.

 
Dix
1293119.  Fri Aug 17, 2018 6:56 am Reply with quote

Jenny wrote:
Barbados - as soon as Woodsman heard that the stays were encased in concrete he identified that as the probable cause of failure because it masks any problems.

Woodsman is now being supported by an expert. The concrete casing reduces the effect of stress from vibrations but makes it very hard to monitor the condition of the steel within. So problems with corrosion might not be detected in time.
Moreover, the bridge is (was) a minimal construction as was the fashion in the 1960's - just strong enough to do its job. Such a construction needs to have top-notch maintenance. Current practice is to construct (say) 10-20% stronger than actually needed so that there is some wiggle-room and time for warning signs to be spotted before they become fatal. If one of the stays were to fail, the pylon would not be strong enough to take the extra (skewed) load of the remaining three stays. That's the theory being proposed: a stay failing and bringing the pylon down.

ing.dk again - you'll need google translate but there are some good diagrams of the structure you may want to look at

 
dr.bob
1293144.  Fri Aug 17, 2018 8:58 am Reply with quote

Dix wrote:
Concrete on its own has no tensile strength. It is very strong in compression but the slightest pull and it breaks. Putting a load on a horizontal beam supported only at the ends will snap it. And those towers will need to be able to take a considerable wind load (sideways movement).


I read an newspaper interview with an Italian engineer who was talking about the Genoa bridge and suggested the microfractures in the concrete were to blame for the pylon collapsing. Having done about 10 seconds googling to check that story out, I was left wondering if that could be true, since microfractures (AFAIK) come from tensile load on the concrete, and that's exactly what the steel reinforcement would mitigate against.

If there were no steel reinforcement, I would imagine they'd've found out about that many years previously!

 

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