Catthorpe re-modelling

[M4 Cardiff]

The design life of bridges is 120-125 years but a proportion of the bridges built in the 1960s are turning out to have a shorter life than planned, usually for one or other of a small number of reasons.

The old bridge itself was not in the way of the proposed interchange. If you look at a diagram such as this one, enlarged if necessary, you will see that the old bridge fits in a narrow gap between the replacement bridge that has now been built, and the proposed A14 westbound to M1 northbound bridge. I imagine that if the old bridge had had a long forecast life, the new junction would have been designed to incorporate it.

Thanks, it makes sense now.

[M4Simon]

What is the 'design life' of these bridges? surely that means that a lot will need replacing over the coming years. I know that replacement on the M1 seems to be tied in with widening works, but there's still plenty of M1 that has not been widened, and other old motorway bits that havn't either.

Current standards are for either 100 or 120 years (can't remember exactly and don't have time to look it up). There are a number of bridges of that era that are experiencing problems. From my memories of bridge strengthening (dating back nearly 20 years) the standard of the day had a less onerous requirement for resistance to shear forces, which resulted in some failing their assessments. This is certainly true of the Langleybury Lane bridge over the M25 Hunton Bridge Spur, which was strengthened in around 1992 for this reason.

(There are two main forces that a bridge deck needs to resist. One is the tendancy to bend (prop up a plank on a few bridges and watch the sag.) Bending tends to stretch the bottom side of the bridge and compresses the top part of the deck. The other is shear force, which is a slicing action through the deck. Bridges failing in bending tend to show cracking towards the middle of the deck and give warning of failure. Bridges failing in shear are are likely to give less warning, and will tend to fail towards the supports where the shear forces are highest. When you design a bridge you do so to ensure that if it fails, it fails by a mechanism that gives warning rather than a sudden failure.)

I don't know what the story is at Catthorpe, but the structural form appears to be a conventional reinforced concrete deck. If it is failing because of a mix of inadequate design standards (probably shear failure) together with deteroration (usually due to deicing salts), the structure will be beyond economic repair. Replacement with a viaduct that provides for another planned scheme is the sensible thing to do and shows a degree of forethought that is often missing.

Cheers,

Simon

[M4Simon]

The design life of bridges is 120-125 years but a proportion of the bridges built in the 1960s are turning out to have a shorter life than planned, usually for one or other of a small number of reasons.

The old bridge itself was not in the way of the proposed interchange. If you look at a diagram such as this one, enlarged if necessary, you will see that the old bridge fits in a narrow gap between the replacement bridge that has now been built, and the proposed A14 westbound to M1 northbound bridge. I imagine that if the old bridge had had a long forecast life, the new junction would have been designed to incorporate it.

Apologies for the double post. I think the old viaduct was in the wrong place, not so much because of the alignment across the M1 itself, but in how it affects the alignment of the proposed M6 south to M1 slip road - essentially the western end of the old viaduct points too far to the south.

Simon

[kit]

According to this site, Sir Owen Williams was aware that using reinforcing steel would limit the lifespan of the original M1 bridges, but the design was chosen for expediency due to the large number of bridges that had to be constructed in the 19 months they had available.

It would be interesting to see the whole life cost of a concrete arch bridge vs. a reinforced concrete beam bridge, though.

[MrEd]

According to this site, Sir Owen Williams was aware that using reinforcing steel would limit the lifespan of the original M1 bridges, but the design was chosen for expediency due to the large number of bridges that had to be constructed in the 19 months they had available.

It would be interesting to see the whole life cost of a concrete arch bridge vs. a reinforced concrete beam bridge, though.

IIRC the bridge at Catthorpe was built after the original contract, which didn't go that far...

[Steven]

According to this site, Sir Owen Williams was aware that using reinforcing steel would limit the lifespan of the original M1 bridges, but the design was chosen for expediency due to the large number of bridges that had to be constructed in the 19 months they had available.

It would be interesting to see the whole life cost of a concrete arch bridge vs. a reinforced concrete beam bridge, though.

IIRC the bridge at Catthorpe was built after the original contract, which didn't go that far...

It was, although the bridge at Catthorpe was built as part of the M1 contract, not the M6 works a few years later.

[Jonathan B4027]

The design life of bridges is 120-125 years but a proportion of the bridges built in the 1960s are turning out to have a shorter life than planned, usually for one or other of a small number of reasons.

Didn't the replacement of the bridge on the old M4 at Aust (Ingst Lane ??) cause carnage because it overran ??

[Glom]

The design life of bridges is 120-125 years but a proportion of the bridges built in the 1960s are turning out to have a shorter life than planned, usually for one or other of a small number of reasons.

Is that because absolutely everything and everything that was built in this country in the 1960s was a complete abomination of design?

[Chris5156]

The design life of bridges is 120-125 years but a proportion of the bridges built in the 1960s are turning out to have a shorter life than planned, usually for one or other of a small number of reasons.

Is that because absolutely everything and everything that was built in this country in the 1960s was a complete abomination of design?

I think that's a bit harsh. Who's to say that, in another 40-50 years, today's buildings won't be badly out of fashion?

In any case, structural failure is possible in any structure of any age. The new Thelwall Viaduct, which opened in the early 1990s, has already had to go through some extensive repair works.

[wrinkly]

In the 1960s bridges were being built in numbers not seen since the 19th century, and techniques were used that designers and builders were not experienced in. Most of the bridges built then are doing fine despite carrying far more traffic than originally envisaged. No doubt a lot of early railway bridges were replaced within a few decades too (e.g. West Country wooden viaducts!).

[jonny1990]

The old bridge itself was not in the way of the proposed interchange. If you look at a diagram such as this one, enlarged if necessary, you will see that the old bridge fits in a narrow gap between the replacement bridge that has now been built, and the proposed A14 westbound to M1 northbound bridge. I imagine that if the old bridge had had a long forecast life, the new junction would have been designed to incorporate it.

In that diagram, I notice what appears to be access between local roads and the M1/M6/A14 in every direction - is this for emergency access only or something (there doesn't look to be room for sliproads, and I was under the impression that local access was to be removed from the junction entirely)?

[M4Simon]

The old bridge itself was not in the way of the proposed interchange. If you look at a diagram such as this one, enlarged if necessary, you will see that the old bridge fits in a narrow gap between the replacement bridge that has now been built, and the proposed A14 westbound to M1 northbound bridge. I imagine that if the old bridge had had a long forecast life, the new junction would have been designed to incorporate it.

In that diagram, I notice what appears to be access between local roads and the M1/M6/A14 in every direction - is this for emergency access only or something (there doesn't look to be room for sliproads, and I was under the impression that local access was to be removed from the junction entirely)?

Judging by the alignment of them, they are for emergency / maintenance purposes only.

Simon

[wrinkly]

Other major interchanges have such emergency links too. For example some at M60/M62/M602 can be seen here. I think one use is to reduce the "dead" mileage travelled by gritters in winter.

[nowster]

Other major interchanges have such emergency links too. For example some at M60/M62/M602 can be seen here. I think one use is to reduce the "dead" mileage travelled by gritters in winter.

The "closed" slip road here http://g.co/maps/2nssu was built when the junction was being remodelled. The C/D lane there was built during the widening scheme. It was used as a bypass for through traffic whilst the anticlockwise carriageway was being rebuilt.

[kit]

Other major interchanges have such emergency links too. For example some at M60/M62/M602 can be seen here. I think one use is to reduce the "dead" mileage travelled by gritters in winter.

Doesn't seem much point though, as it only allows you to rejoin the motorway mainline. I wonder if the street lights are lit or not?

[wrinkly]

I was referring more to the smaller links which have right-angle junctions with the main slip roads.

[Matthew]

Video clips of the demolition... 5 min video and 2 min video, plus some of the pictures within the Flickr photostream.

[cooldude400]

That video really is interesting to watch . It's amazing how they do a tough job in such an organised way.

[Big L]

Worryingly, I'm sure I saw a (covered up) permanent-looking NSL sign after the new bridge on the M6 this morning. I'll have another look tomorrow.

We haven't either accidentally (or deliberately) re-alligned the motorway with a bend too sharp for 70mph have we ? Permanent camera enforcement, as-per M60 J25 to come ?

[c2R]

Worryingly, I'm sure I saw a (covered up) permanent-looking NSL sign after the new bridge on the M6 this morning. I'll have another look tomorrow.

We haven't either accidentally (or deliberately) re-alligned the motorway with a bend too sharp for 70mph have we ? Permanent camera enforcement, as-per M60 J25 to come ?

Or possibly further roadworks to come, or someone's not been reading their sign guidance and thinks they need to apply NSL repeaters on a lit stretch of motorway, like it was a trunk road...