Why do most newer international suspension bridges not use the triangular cables used on the Severn and Humber bridges?

[RJDG14]

I believe the Severn Bridge was the first suspension bridge anywhere in the world to use triangularly spaced rather than vertical suspension cables, and the Humber Bridge was built with the same cable design. At the same time, nearly all of the post-1966 (when the Severn Bridge opened) suspension bridges in other countries still seem to use the traditional vertical cable design that's found on bridges in the UK such as the Clifton Suspension Bridge and the Forth Road Bridge. I've compared timelapse footage of the Severn and Humber Bridges to footage of newer suspension bridges in other countries that use the traditional cable design (along with older suspension bridges in the UK that also do), and the Severn and Humber Bridges have noticeably less vertical motion. The bridge simulation software I've tested also shows the same thing. This makes me ask why other countries generally didn't adopt the new suspension bridge standard that had been developed in Britain during the 1960s.

One potential setback would have been if the architects/engineers of the Severn Bridge had patented the triangular cable layout, which may have acted as a deterrent to architects in other countries, but even then it would have probably expired in the late 1970s or early 1980s, assuming that the design had been patented in the late 1950s or early 1960s, and most suspension bridges built internationally since then still use vertical cables. I'm also unsure if UK registered architecture patents would apply internationally.

[M4Simon]

I believe the Severn Bridge was the first suspension bridge anywhere in the world to use triangularly spaced rather than vertical suspension cables, and the Humber Bridge was built with the same cable design. At the same time, nearly all of the post-1966 (when the Severn Bridge opened) suspension bridges in other countries still seem to use the traditional vertical cable design that's found on bridges in the UK such as the Clifton Suspension Bridge and the Forth Road Bridge. I've compared timelapse footage of the Severn and Humber Bridges to footage of newer suspension bridges in other countries that use the traditional cable design (along with older suspension bridges in the UK that also do), and the Severn and Humber Bridges have noticeably less vertical motion. The bridge simulation software I've tested also shows the same thing. This makes me ask why other countries generally didn't adopt the new suspension bridge standard that had been developed in Britain during the 1960s.

One potential setback would have been if the architects/engineers of the Severn Bridge had patented the triangular cable layout, which may have acted as a deterrent to architects in other countries, but even then it would have probably expired in the late 1970s or early 1980s, assuming that the design had been patented in the late 1950s or early 1960s, and most suspension bridges built internationally since then still use vertical cables. I'm also unsure if UK registered architecture patents would apply internationally.

The Severn Bridge was a product of research into suspension bridge design. It is a few years newer than the Forth Road Bridge, but was designed by the same team. If you look at cross-sections of the deck of each bridge, you will see that the Forth Bridge has a lot of steel bracing to create a stiffer deck in order to resist oscillation of the deck due to wind. The Severn Bridge and Humber bridges both have lighter aeordynamic decks, and the triangular arrangement of the cables acts to damp any oscillation of the deck. It is an alternative solution to the same problem.

For an example of what happens if the bridge structure is not stiff enough to resist oscillation due to wind, have a look at the Tacoma Narrows Bridge disaster. If I recall correctly, that bridge was about the same length as the Severn Bridge, but had a much narrower deck and lacked enough stiffness in the bridge to resist oscillation due to wind. Google will take you to a video of the bridge being torn up in a moderate breeze.

You can see sections of the Severn Bridge deck here: https://www.walesonline.co.uk/news/wale ... d-14210964
There is a section through the Forth Road Bridge here: https://www.theforthbridges.org/forth-r ... nd-figures

Simon

[c2R]

There's definitely been a design move away from either and to using multiple stays linked directly to the central tower (e.g. New Severn Bridge, Dartford River Crossing, Oresund Bridge) - which allows for cables to be replaced more easily.

That said, and interestingly for such a modern bridge, the Great Belt Bridge was built to the older design using vertical cables

[M4Simon]

There's definitely been a design move away from either and to using multiple stays linked directly to the central tower (e.g. New Severn Bridge, Dartford River Crossing, Oresund Bridge) - which allows for cables to be replaced more easily.

That said, and interestingly for such a modern bridge, the Great Belt Bridge was built to the older design using vertical cables

Indeed, if you lose one of the main cables on a suspension bridge due to corrosion or sufficient individual strands break, it is very difficult to replace the cable, whereas on a cable stayed bridge with multiple main cables fixed to the towers, it is easier to replace them one at a time. Having said that, I think the lines of a true suspension bridge look much better than a cable stayed bridge - compare the original and second Severn bridges.

The Wye bridge was originally supported by a single main cable that ran up and over two towers along the centre line of the bridge deck. In the major strengthening programme in the early 80s (I think), the towers were raised in height and the single main cable was replaced by two pairs of separate cables running over the tower. This page describes that strengthening work. The drawings show the current configuration, and the photo on the right hand side shows the single main cable during the strengthening work. I have seen a photo showing both cables in place, I think with the old cable having just been released from the deck. I've looked (briefly) but I can't find it at the moment.

Simon
[edit - typo fixed]

[Bryn666]

There's definitely been a design move away from either and to using multiple stays linked directly to the central tower (e.g. New Severn Bridge, Dartford River Crossing, Oresund Bridge) - which allows for cables to be replaced more easily.

That said, and interestingly for such a modern bridge, the Great Belt Bridge was built to the older design using vertical cables

Indeed, if you lose one of the main cables on a suspension bridge due to corrosion or sufficient individual strands break, it is very difficult to replace the cable, whereas on a cable datyed bridge with multiple main cables fixed to the towers, it is easier to replace them one at a time. Having said that, I think the lines of a true suspension bridge look much better than a cable stayed bridge - compare the original and second Severn bridges.

The Wye bridge was originally supported by a single main cable that ran up and over two towers along the centre line of the bridge deck. In the major strengthening programme in the early 80s (I think), the towers were raised in height and the single main cable was replaced by two pairs of separate cables running over the tower. This page describes that strengthening work. The drawings show the current configuration, and the photo on the right hand side shows the single main cable during the strengthening work. I have seen a photo showing both cables in place, I think with the old cable having just been released from the deck. I've looked (briefly) but I can't find it at the moment.

Simon

This is basically the reason the Forth Road Bridge has been bypassed with a new bridge - it'd just be a logistical impossibility to keep traffic moving whilst replacing cables on a live bridge. The argument about additional road capacity was stifled by the need to just keep some kind of crossing open.

Conversely the SSC was a capacity scheme - otherwise they'd have never built the M49 either.

[Helvellyn]

There's definitely been a design move away from either and to using multiple stays linked directly to the central tower (e.g. New Severn Bridge, Dartford River Crossing, Oresund Bridge) - which allows for cables to be replaced more easily.

That said, and interestingly for such a modern bridge, the Great Belt Bridge was built to the older design using vertical cables

Indeed, if you lose one of the main cables on a suspension bridge due to corrosion or sufficient individual strands break, it is very difficult to replace the cable, whereas on a cable datyed bridge with multiple main cables fixed to the towers, it is easier to replace them one at a time. Having said that, I think the lines of a true suspension bridge look much better than a cable stayed bridge - compare the original and second Severn bridges.

The Wye bridge was originally supported by a single main cable that ran up and over two towers along the centre line of the bridge deck. In the major strengthening programme in the early 80s (I think), the towers were raised in height and the single main cable was replaced by two pairs of separate cables running over the tower. This page describes that strengthening work. The drawings show the current configuration, and the photo on the right hand side shows the single main cable during the strengthening work. I have seen a photo showing both cables in place, I think with the old cable having just been released from the deck. I've looked (briefly) but I can't find it at the moment.

Simon

Makes me wonder how practical a suspension bridge is where instead of a single main cable (well a pair of them) you have several. Obviously they couldn't be twisted together (because that's no different from a single cable made of many strands), so from a distance it would look more like a strip than a cable.

Mind you I'm sure someone's already thought of that and if it would be practical would already have been done.

[M4Simon]

There's definitely been a design move away from either and to using multiple stays linked directly to the central tower (e.g. New Severn Bridge, Dartford River Crossing, Oresund Bridge) - which allows for cables to be replaced more easily.

That said, and interestingly for such a modern bridge, the Great Belt Bridge was built to the older design using vertical cables

Indeed, if you lose one of the main cables on a suspension bridge due to corrosion or sufficient individual strands break, it is very difficult to replace the cable, whereas on a cable datyed bridge with multiple main cables fixed to the towers, it is easier to replace them one at a time. Having said that, I think the lines of a true suspension bridge look much better than a cable stayed bridge - compare the original and second Severn bridges.

The Wye bridge was originally supported by a single main cable that ran up and over two towers along the centre line of the bridge deck. In the major strengthening programme in the early 80s (I think), the towers were raised in height and the single main cable was replaced by two pairs of separate cables running over the tower. This page describes that strengthening work. The drawings show the current configuration, and the photo on the right hand side shows the single main cable during the strengthening work. I have seen a photo showing both cables in place, I think with the old cable having just been released from the deck. I've looked (briefly) but I can't find it at the moment.

Simon

Makes me wonder how practical a suspension bridge is where instead of a single main cable (well a pair of them) you have several. Obviously they couldn't be twisted together (because that's no different from a single cable made of many strands), so from a distance it would look more like a strip than a cable.

Mind you I'm sure someone's already thought of that and if it would be practical would already have been done.

Each main cable on the Severn Bridge is 511mm in diameter (just over half a metre) and they have a cross-sectional area of just over 0.2 sqm. If you spluit them into separate strands, each 1/10 of the diameter, each strand would have a cross-sectional area of 0.002 sqm - in other words you'd need to lay 100 similar cables side by side, over a width of 5.1m. Then do the same for the other cable. Then you'd need to be able to fix the hangers so each one applies the same load equally to all 100 cables. If you don't you'd end up with some doing a lot more work than others, with failure of the overstressed cables causing the remaining cables to take more and more load until they also fail. As you've guesed above, it isn't practical.

What you can do is, for example, give each cable a separate fixing point on the main towers and connect each cable to a separate point on the bridge deck. What you then have is a cable-stayed bridge like the Queensferry Crossing. We don't appear to have any decent photographs on the SABRE Wiki, but you can find them elsewhere on the internet. Another well known example is the Viaduc du Millau.

Simon

[Nwallace]

The other advantage of a cable stayed bridge over a suspension bridge is you don't need to drill huge holes for the anchor point.


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[Pendlemac]

Makes me wonder how practical a suspension bridge is where instead of a single main cable (well a pair of them) you have several. Obviously they couldn't be twisted together (because that's no different from a single cable made of many strands), so from a distance it would look more like a strip than a cable.

Mind you I'm sure someone's already thought of that and if it would be practical would already have been done.

What you are describing is the layout of the older type chain suspension bridge.

Here in the UK we have the Telford Suspension Bridge at Conwy, the Menai Bridge, the Union Bridge, the Clifton Suspension Bridge and several others.

[KeithW]

Makes me wonder how practical a suspension bridge is where instead of a single main cable (well a pair of them) you have several. Obviously they couldn't be twisted together (because that's no different from a single cable made of many strands), so from a distance it would look more like a strip than a cable.

Mind you I'm sure someone's already thought of that and if it would be practical would already have been done.

Well there's a combination bridge of that type - The Brooklyn Bridge. It has both cable stayed supports and vertical wires from the spun cables. It has been in use since 1883 so they must have done something right

[KeithW]

What you are describing is the layout of the older type chain suspension bridge.

Here in the UK we have the Telford Suspension Bridge at Conwy, the Menai Bridge, the Union Bridge, the Clifton Suspension Bridge and several others.

Trouble is all those bridges have a single point of failure in the form of the eyebolt that connects the chain segments. This was discovered the hard way with the Silver Bridge that crossed the Ohio River.

[MJG]

There's definitely been a design move away from either and to using multiple stays linked directly to the central tower (e.g. New Severn Bridge, Dartford River Crossing, Oresund Bridge) - which allows for cables to be replaced more easily.

That said, and interestingly for such a modern bridge, the Great Belt Bridge was built to the older design using vertical cables

It is not about being older or newer design. Even if the suspension and cable-stayed bridges look pretty similar, technically they are very different, and they are considered different bridge types.



Currently, suspension bridges can be made having longer spans than cable-stayed ones. The longest main span of a cable-stayed bridge is 1000+ meters while the longest suspension bridges reach 2000 meters.

The main girder of the suspension bridge hangs on vertical or nearly-vertical suspenders connected to the main cable. The girder needs to be as lightweight as possible. Usually it works like a wing of an aircraft: It is elastic, and it vibrates substantially. Because of the vibrations, railways built on long suspension bridges are rare.

In the cable-stayed bridges, the suspension cables carry directly the girder. That structure makes a compression force to exist. And concrete is a very good material for compression. That is why many cable-stayed bridges have the main girder made of steel-concrete composite structures: Concrete where there is compression, and steel to handle tension, bending and torsion.

Therefore, the stiffness of the main girder of a cable-stayed bridge can be made much better than that of the suspension bridge (and the bridge can have a curved shape). Thus, there are such bridges carrying a railway.

The Öresund bridge between Denmark and Sweden has cable-stayed parts, and the bridge carries a railway. Other way round, the main span of Great Belt Bridge is a suspension bridge, and the railway runs in a subsea tunnel between Sprogø and Sjælland.

[KeithW]

Therefore, the stiffness of the main girder of a cable-stayed bridge can be made much better than that of the suspension bridge (and the bridge can have a curved shape). Thus, there are such bridges carrying a railway.

The Öresund bridge between Denmark and Sweden has cable-stayed parts, and the bridge carries a railway. Other way round, the main span of Great Belt Bridge is a suspension bridge, and the railway runs in a subsea tunnel between Sprogø and Sjælland.

They found problems with suspension bridges for railways on the first outing. The Stockton and Darlington Railway originally crossed the Tees on a suspension bridge.



It was not a success, they had to drastically reduce the number of wagons and prop the thing up in mid stream.

More here
https://www.thenorthernecho.co.uk/histo ... on-bridge/

[Bryn666]

Therefore, the stiffness of the main girder of a cable-stayed bridge can be made much better than that of the suspension bridge (and the bridge can have a curved shape). Thus, there are such bridges carrying a railway.

The Öresund bridge between Denmark and Sweden has cable-stayed parts, and the bridge carries a railway. Other way round, the main span of Great Belt Bridge is a suspension bridge, and the railway runs in a subsea tunnel between Sprogø and Sjælland.

They found problems with suspension bridges for railways on the first outing. The Stockton and Darlington Railway originally crossed the Tees on a suspension bridge.



It was not a success, they had to drastically reduce the number of wagons and prop the thing up in mid stream.

More here
https://www.thenorthernecho.co.uk/histo ... on-bridge/

Suspension bridges are stuffed whenever there is a load that spreads itself in a rhythmic pattern. Many French soldiers learned this the hard way when the suspension bridge at Angers gave way under their marching and dumped them 250 of them to their deaths in the Loire in 1850. This is why the suspension bridges over the Thames tell soldiers to break step.

Of course, this same load shift is what stuffed up the Millennium Bridge - although given the advances in bridge design since the 19th Century you can only really conclude potentially incompetent designers more interested in creating a signature structure than dealing with basic physics were at play there.

[RichardA626]

I remember one of my Dad's friends mentioned that he sometimes had to break step while crossing bridges during his national service.

[KeithW]

Suspension bridges are stuffed whenever there is a load that spreads itself in a rhythmic pattern. Many French soldiers learned this the hard way when the suspension bridge at Angers gave way under their marching and dumped them 250 of them to their deaths in the Loire in 1850. This is why the suspension bridges over the Thames tell soldiers to break step.

Of course, this same load shift is what stuffed up the Millennium Bridge - although given the advances in bridge design since the 19th Century you can only really conclude potentially incompetent designers more interested in creating a signature structure than dealing with basic physics were at play there.

In the case of the Tees Bridge the issue was that it sagged under load so from mid span to the other side the train was effectively climbing a steep incline. While famous as the first public passenger railway the Stockton and Darlington was really built to haul coal from the Durham coalfield to Port Stockton which is now Middlesbrough.

[Owain]

What a fascinating thread!

As far as I was concerned, the Severn Bridge was the local landmark of my childhood (who needs Tintern Abbey? - it's just a ruin!).

During the final year of primary school I made a model of the bridge out of wooden sticks, paper and wool. Yes, wool! I understood the concept of suspension, because the wool was used for the cables to suspend the carriageway.

The project caused quite a lot of interest from some of my classmates, who evidently didn't get it because several asked why I didn't just glue the road deck to the towers.

Looking back as a SABRE member, some 35 years later, I realise now that painting the road as S4 with no central reservation might not have been the best design....

However, I thought it was pretty good until I read this thread: I'd never really registered that the cables are attached in triangular formation, and that didn't feature in the design, although I had noticed it because when I read the thread title I knew what it was referring too.

Ah, that lost career in civil engineering...

[KeithW]

What a fascinating thread!

As far as I was concerned, the Severn Bridge was the local landmark of my childhood (who needs Tintern Abbey? - it's just a ruin!).

During the final year of primary school I made a model of the bridge out of wooden sticks, paper and wool. Yes, wool! I understood the concept of suspension, because the wool was used for the cables to suspend the carriageway.

The project caused quite a lot of interest from some of my classmates, who evidently didn't get it because several asked why I didn't just glue the road deck to the towers.

Looking back as a SABRE member, some 35 years later, I realise now that painting the road as S4 with no central reservation might not have been the best design....

However, I thought it was pretty good until I read this thread: I'd never really registered that the cables are attached in triangular formation, and that didn't feature in the design, although I had noticed it because when I read the thread title I knew what it was referring too.

Ah, that lost career in civil engineering...

S4 was understandable , many major roads including dual carriageways had S4 bridges

This section of the A1085 Trunk road (which was never trunk) was a narrow S4 but now is S2 with dual use cyclepaths / footpaths
https://www.google.co.uk/maps/@54.57158 ... 8192?hl=en

Newport bridge is still S4 but now has a 40 limit.
https://www.google.co.uk/maps/@54.57334 ... 8192?hl=en

Before the A174 Parkway and new A19 were opened the best road on Teesside was the old A19 north of Billingham.
https://www.google.co.uk/maps/@54.61697 ... 8192?hl=en

[owen b]

What a fascinating thread!

As far as I was concerned, the Severn Bridge was the local landmark of my childhood (who needs Tintern Abbey? - it's just a ruin!).

During the final year of primary school I made a model of the bridge out of wooden sticks, paper and wool. Yes, wool! I understood the concept of suspension, because the wool was used for the cables to suspend the carriageway.

The project caused quite a lot of interest from some of my classmates, who evidently didn't get it because several asked why I didn't just glue the road deck to the towers.

Looking back as a SABRE member, some 35 years later, I realise now that painting the road as S4 with no central reservation might not have been the best design....

However, I thought it was pretty good until I read this thread: I'd never really registered that the cables are attached in triangular formation, and that didn't feature in the design, although I had noticed it because when I read the thread title I knew what it was referring too.

Ah, that lost career in civil engineering...

S4 was understandable , many major roads including dual carriageways had S4 bridges

This section of the A1085 Trunk road (which was never trunk) was a narrow S4 but now is S2 with dual use cyclepaths / footpaths
https://www.google.co.uk/maps/@54.57158 ... 8192?hl=en

Newport bridge is still S4 but now has a 40 limit.
https://www.google.co.uk/maps/@54.57334 ... 8192?hl=en

Before the A174 Parkway and new A19 were opened the best road on Teesside was the old A19 north of Billingham.
https://www.google.co.uk/maps/@54.61697 ... 8192?hl=en

The Tyne bridge : https://www.google.co.uk/maps/@54.96763 ... 384!8i8192
The Wearmouth bridge : https://www.google.co.uk/maps/@54.91014 ... 384!8i8192
Both S4.

[MJG]

Suspension bridges are stuffed whenever there is a load that spreads itself in a rhythmic pattern. Many French soldiers learned this the hard way when the suspension bridge at Angers gave way under their marching and dumped them 250 of them to their deaths in the Loire in 1850. This is why the suspension bridges over the Thames tell soldiers to break step.

Of course, this same load shift is what stuffed up the Millennium Bridge - although given the advances in bridge design since the 19th Century you can only really conclude potentially incompetent designers more interested in creating a signature structure than dealing with basic physics were at play there.

Every structure has its natural frequency. It there is an external force impacting the structure at the frequency close to the natural one, the structure might come into resonance. It may lead to a quick or slow collapse of the structure. The disaster of the Tacoma Narrows Bridge is said to be a good example of this. (I know that the scientists still argue on whether it was resonance or an aerodynamic phenomenon having similar impacts. This is more or less an academic dispute on terminology, because there is no waterproof definition what resonance exactly is.)

Suspension bridges are prone to another serious problem: material fatigue. Repeating vibrations slowly lead to a loss of the strength of steel, even if it is not about resonance. If traffic volumes are higher than what were projected at the planning, and the weight of vehicles grow, the lifetime of a suspension bridge might turn surprisingly short.

Reverting to the topic "International Roads": Finland build six pretty notable suspension road bridges in the 1960s over the waters where girder bridges were not feasible in those years. They were mid-size, the main span varying from 115 to 250 meters. The one on the main road transport corridor route 4/E75 lost a lot of its capacity within a few decades because of steel fatigue, and it had to replaced. The bridge was demolished in 2010, at the age of 58 years only.



The Kirjalansalmi bridge in the SW Finland will face the same destiny. Its main span width is 250 meters. The road 180 is a heavily used industrial route, and the bridge is at the end of the lifecycle. The construction of a new cable-stayed bridge will likely begin in 2023. The era of suspension bridges seem to be behind in Finland: All long bridges since 1980s have either cable-stayed bridges or traditional box-girder ones.