A432 bridge over M4 to be replaced

[Bryn666]

https://www.bbc.com/news/uk-england-bri ... 718217.amp

The bridge was closed in July apparently when cracks were found in it.

This is at least being replaced but this is exactly the maintenance timebomb that keeps being mentioned. We are probably going to see a lot more bridges need emergency replacement soon.

[SouthWest Philip]

https://www.bbc.com/news/uk-england-bri ... 718217.amp

The bridge was closed in July apparently when cracks were found in it.

This is at least being replaced but this is exactly the maintenance timebomb that keeps being mentioned. We are probably going to see a lot more bridges need emergency replacement soon.

The proposed timeline for demolition and replacement rather lacks urgency.

I can't help feel that once the decision to demolish has been made it should be possible to arrange a short notice weekend closure to remove it. And surely it shouldn't take two years to put a new bridge in place?

[ForestChav]

https://www.bbc.com/news/uk-england-bri ... 718217.amp

The bridge was closed in July apparently when cracks were found in it.

This is at least being replaced but this is exactly the maintenance timebomb that keeps being mentioned. We are probably going to see a lot more bridges need emergency replacement soon.

The proposed timeline for demolition and replacement rather lacks urgency.

I can't help feel that once the decision to demolish has been made it should be possible to arrange a short notice weekend closure to remove it. And surely it shouldn't take two years to put a new bridge in place?

I have to wonder the same. Now we had the same kind of thing up here a few years ago just before lockdown with the A52 and the reduced traffic probably allowed them to be able to repair without causing gridlock constantly over the whole city. But it could easily have been quicker to take the bridge deck off whilst building another offsite and shipping it into place... which is what they are doing here.

Let's hope they have assessed it correctly and that closing it to traffic will be sufficient, after all the bridge is spanning a motorway, and the last thing people want is for it to be spewing concrete off of it or worse still collapsing whilst the motorway is being used.

Have to agree with the sentiment that if it isn't going to reopen without being replaced the safest thing has to be an overnight closure to get rid of what is there...

[DB617]

https://www.bbc.com/news/uk-england-bri ... 718217.amp

The bridge was closed in July apparently when cracks were found in it.

This is at least being replaced but this is exactly the maintenance timebomb that keeps being mentioned. We are probably going to see a lot more bridges need emergency replacement soon.

The proposed timeline for demolition and replacement rather lacks urgency.

I can't help feel that once the decision to demolish has been made it should be possible to arrange a short notice weekend closure to remove it. And surely it shouldn't take two years to put a new bridge in place?

Yes, local people are rather upset, and I can't entirely blame them. NH and contractors have been patting themselves on the back lately about how quickly they can replace a bridge. I suspect there may be more than just concrete and steel at play here as utilities diversions have been mentioned as being required. There are precious few M4 crossings for utilities between S. Glos and North Bristol so it's unsurprising. One can see how programme delays quickly overtake site time when several different stakeholders and their chosen subbies are involved... Cross-agency work is just an unadulterated nightmare.

What is unique in this case is the level of bad timing. The M4 and M5 are both already restricted capacity at the moment causing severe delays especially on M32 and M4. One of the two remaining routes from NE Bristol to Yate requires drivers to take a u-turn under M32 J1 to access the B4058 Bristol Road, but that movement is completely backed up by the terrible design of the temporary tiger tail merge onto the M4 westbound at J19 (5 lanes into 3 narrow ones). The right-turn at Hambrook Junction that was replaced by said u-turn was closed by a previous South Glos council because the lights used to back up the motorway junction and A4174. It is a total mess of an overstretched network and unfortunately buses can't get through either. On the really rainy evening peaks here it has been very painful to travel any direction except for towards the city.

A few weeks ago the whole system collapsed because the new South Glos council decided to investigate a series of total failures of the signals at Hambrook and found that the previous (Tory) council's contractor had made a slash up job of the resignalling back in 2014 and had to pull through and re-terminate 2000 cables (so they say) in the midst of Storm Ciaran before the turnings could reopen. To be fair to the new contractor, I passed during the storm and they were under their little tents just keeping on keeping on.

Gee, now sure would be a bad time for Great Western Railway to be in its worst service state for generations.

[ravenbluemoon]

I do wonder if we are going to get to the stage where we have so many bridges strting to fail that we have to crane in "temporary" bridges like the Hogarth Flyover or a Bailey Bridge in order to keep places connected?

If the bridges are knackered and they're a danger to the public, get them down pronto, stick something up temporarily while they spend two years getting the new one sorted.

[SouthWest Philip]

If the bridges are knackered and they're a danger to the public, get them down pronto, stick something up temporarily while they spend two years getting the new one sorted.

A motorway overbridge replacement should, in theory, be quite straightforward. The earthworks are already in place. Build some new supports if required, modular construction for the deck. A few weeks at most?

Up the road, a series of bridges over the M5 have been/are being refurbished in a painfully slow fashion when it would surely have been quicker and cheaper to demolish and replace.

Railway bridges are routinely replaced over long weekend closures. With the will, it could be done that way for motorways too.

The Italians replaced a huge collapsed viaduct in Genoa in around a year. I fear it will take a similar tragedy on a key structure to lead to an increased sense of urgency in maintenance here too.

[darkcape]

Most bridge replacements require entirely new designs, it is rare bridges are replaced like-for-like, due to various reasons:

- original building technique no longer possible. Lots of bridges were built when the roads didn't exist, so deck soffits could be supported by scaffolding/formwork/falsework. Now the roads are open, you can't put scaffolding across 6 lanes of motorway to cast your new insitu deck slab, you may need to switch to a composite beam & slab solution etc
- design standards have changed
- material availability has changed
- construction techniques & regard for safety has changed.

To design a new bridge takes generally a minimum of a year, easily up to two years, because of the various disciplines & stakeholders that need to input. One bridge I worked on was delayed for 3 months whilst we waited for the British Horse Society to answer a query about the proposed bridleway on the deck.

[SteveA30]

The Rownhams M27 bridge replacement needed 3 weekend closures, and diversions.

viewtopic.php?t=39468&hilit=Rownhams+M27

I've been waiting for the rest to come up along the M3, M4 and M5.......

[Owain]

If the bridges are knackered and they're a danger to the public, get them down pronto, stick something up temporarily while they spend two years getting the new one sorted.

A motorway overbridge replacement should, in theory, be quite straightforward. The earthworks are already in place. Build some new supports if required, modular construction for the deck. A few weeks at most?

Up the road, a series of bridges over the M5 have been/are being refurbished in a painfully slow fashion when it would surely have been quicker and cheaper to demolish and replace.

Railway bridges are routinely replaced over long weekend closures. With the will, it could be done that way for motorways too.

The Italians replaced a huge collapsed viaduct in Genoa in around a year. I fear it will take a similar tragedy on a key structure to lead to an increased sense of urgency in maintenance here too.

To be precise, the total duration between the collapse of the Ponte Morandi (August 2018) and the opening of the Ponte San Giorgio (August 2020) was closer to two years. It took almost a year (to June 2019) to demolish what remained of the old structure, and the plans for its replacement were presented almost two years before its completion (September 2018).

That said, I agree that it was a tremendously impressive turnaround for such a vast structure, and there are surely lessons to be learnt from the fact that the old bridge was known to be in a poor state before its collapse.

EDIT: A bridge more similar to that under discussion here collapsed on the A14 near Ancona in 2017, killing two people and injuring two more.

[Bryn666]

If the bridges are knackered and they're a danger to the public, get them down pronto, stick something up temporarily while they spend two years getting the new one sorted.

A motorway overbridge replacement should, in theory, be quite straightforward. The earthworks are already in place. Build some new supports if required, modular construction for the deck. A few weeks at most?

Up the road, a series of bridges over the M5 have been/are being refurbished in a painfully slow fashion when it would surely have been quicker and cheaper to demolish and replace.

Railway bridges are routinely replaced over long weekend closures. With the will, it could be done that way for motorways too.

The Italians replaced a huge collapsed viaduct in Genoa in around a year. I fear it will take a similar tragedy on a key structure to lead to an increased sense of urgency in maintenance here too.

It varies - as others have said you have to keep a motorway running underneath as well which adds complexity and that's before you find the gas main running across it as the M67 bridge currently being replaced has.

Two years though is a ridiculously long time, we saw how long the A56 bridge took to replace at Runcorn recently too.

[DB617]

Another musing of mine is that if National Highways does not have a dedicated high level assessment project team trying to model/predict failure of concrete structures by now, I would view that as a pretty bad dereliction of duty. I am part of one at work in a different field. We're just at that stage in the life of the material.

For those of you who have played Cities Skylines, what we're experiencing seems to be a concrete 'death wave' - as most of our motorways and their bridges were built in one big boom, the structures will fail over about the same time span they took to construct, probably following a bell curve of service/ultimate lifetimes based on how well or poorly maintained they have been plus other factors like salting, how well the drainage worked, percentage of OGVs, etc etc. I am sure this is something that can be risk modelled. One thing that is certain is that we haven't reached the mean point in that bell curve yet and it's probably going to be a very steep climb.

[Vierwielen]

Another musing of mine is that if National Highways does not have a dedicated high level assessment project team trying to model/predict failure of concrete structures by now, I would view that as a pretty bad dereliction of duty. I am part of one at work in a different field. We're just at that stage in the life of the material.

For those of you who have played Cities Skylines, what we're experiencing seems to be a concrete 'death wave' - as most of our motorways and their bridges were built in one big boom, the structures will fail over about the same time span they took to construct, probably following a bell curve of service/ultimate lifetimes based on how well or poorly maintained they have been plus other factors like salting, how well the drainage worked, percentage of OGVs, etc etc. I am sure this is something that can be risk modelled. One thing that is certain is that we haven't reached the mean point in that bell curve yet and it's probably going to be a very steep climb.

Who owns the bridges over the motorways - National Highways or the owner of the local road? I know that many railway bridges over roads are owned by National Rail (or whatever they call themselves these days).

[jervi]

Another musing of mine is that if National Highways does not have a dedicated high level assessment project team trying to model/predict failure of concrete structures by now, I would view that as a pretty bad dereliction of duty. I am part of one at work in a different field. We're just at that stage in the life of the material.

For those of you who have played Cities Skylines, what we're experiencing seems to be a concrete 'death wave' - as most of our motorways and their bridges were built in one big boom, the structures will fail over about the same time span they took to construct, probably following a bell curve of service/ultimate lifetimes based on how well or poorly maintained they have been plus other factors like salting, how well the drainage worked, percentage of OGVs, etc etc. I am sure this is something that can be risk modelled. One thing that is certain is that we haven't reached the mean point in that bell curve yet and it's probably going to be a very steep climb.

Who owns the bridges over the motorways - National Highways or the owner of the local road? I know that many railway bridges over roads are owned by National Rail (or whatever they call themselves these days).

The bridge structure is owned and maintained by national highways. However the road, prow, or private access that runs over (or under) the trunk road is responsibility of the local highway authority, or private owner as appropriate.

[Phil]

Who owns the bridges over the motorways - National Highways or the owner of the local road? I know that many railway bridges over roads are owned by National Rail (or whatever they call themselves these days).

It largely depends on who was there first!

As most roads were in existence before railways, then the bridges built by the railway company to maintain the right of way would also end up owned by the railway (even though responsibility for the road surface would reside with the highway authority).

If a new road was built over an existing railway then things got more interesting. In theory the owner of the structure will be the roads authority but sometimes the railway authority will 'adopt' the bridge on completion. The latter is more common for bridges built to allow an existing railway to cross a new road (perhaps because as the railway would still need to maintain the track on the structure it seemed more logical for them to take responsibility for the entire structure) but where a new road bridges a railway then said bridge might well still be under the ownership of the highways authority. This was certainly the case with respect to the M20 bridges over the Strood - Maidstone railway - as a number of years ago Highways England decided they didn't need to tell the railway authority about their plans to dangle a mobile work platform over the side for a bridge examination with the result that a train went and crashed into it! https://www.gov.uk/raib-reports/collisi ... -aylesford

NOTE - The various 'Contact us' Bridge ID plates attached to bridges over / under railways have nothing to do with bridge ownership - they are there as a pure safety measure so train s can be stopped if something collides with the structure. They in no way confirm ownership of said structure!

[Bryn666]

To add to Phil's point above, people always go "why don't National Rail improve signs on bridges" and the answer is they can't.

Signs are the responsibility of the highway authority, hence why NR go out and put all the luminous and non-prescribed warnings all over the bridge that you can see from the moon (but never have any useful information on them), whilst a tatty 1970s warning triangle hangs uselessly off the parapets.

[AndyB]

One reason for the long lead-in, apart from the general statement that there is no such thing as a standard motorway bridge, is that to replace like-for-like would often be a terrible waste of money because modern techniques are easier, better and less ugly.

The classic example is M1(NI) J2 where the under bridges were replaced with D3M in the 2000s. The new bridge has two intermediate piers under each carriageway where the old D2M had about four because of improved techniques.

If, as they should, bridge replacements are being planned in advance instead of in emergency, then it’s all invisible. Only if cost-cutting means it’s an emergency or overheight and overweight vehicles damage a structure does the public notice that it takes time to work out what is an appropriate replacement.

[DB617]

One reason for the long lead-in, apart from the general statement that there is no such thing as a standard motorway bridge, is that to replace like-for-like would often be a terrible waste of money because modern techniques are easier, better and less ugly.

The classic example is M1(NI) J2 where the under bridges were replaced with D3M in the 2000s. The new bridge has two intermediate piers under each carriageway where the old D2M had about four because of improved techniques.

If, as they should, bridge replacements are being planned in advance instead of in emergency, then it’s all invisible. Only if cost-cutting means it’s an emergency or overheight and overweight vehicles damage a structure does the public notice that it takes time to work out what is an appropriate replacement.

I had not considered that. In this case gold-plating seems like a very good idea. It's an overstretched section of motorway which can't afford to be getting closed every y number of years due to bridge issues.

[AndyB]

Yes, the old rule of doing it properly or not at all.

There are so many things which can’t be ignored when designing a replacement bridge, and these are a few I can think of as a strict amateur:
1. Stronger building methods requiring fewer piers
2. Was the old overbridge underengineered in the first place
3. Extending hard shoulders under bridges
4. Adequacy of overbridge for present and predicted traffic levels
5. NMU provision
6. Would a more expensive design pay for itself in less disruption during construction?

As for Bailey bridges, the sappers know what they’re doing. All sorts of reasons why they’re not ideal for road users (restricted width, speed, approach angles, NMU provision), but they are strong and do the job until the new permanent bridge is ready.

[ajuk]

It's a good opertunity to put in a junction there. I've never really understood why an east Bristol M4 junction was never built.

[DB617]

It's a good opertunity to put in a junction there. I've never really understood why an east Bristol M4 junction was never built.

That bridge is far too close to J19. The weaving would be horrendous unless it was limited access. The proposal a few years ago was for a fork junction at Emerald Park but I believe it was at odds with the Lyde Green masterplan which has now been built out. Road transport is very out of vogue.