Dundas unmasked

[Athy]

43 minutes ago, Pluto said:

Hump back bridges were actually comparatively expensive to build, while swing bridges were much cheaper. On the L&LC, originally almost all the bridges between Liverpool and Wigan were swing bridges for this reason.

That depends with what you compare them, but I take your point: ditto the many lift bridges on the South Oxford.

[Pluto]

2 hours ago, Athy said:

That depends with what you compare them, but I take your point: ditto the many lift bridges on the South Oxford.

I am making the statement after having looked at many historical costings for canal structures and the company papers that go with them. So comparing the various ways for a road to cross a canal, bridges in stone or brick were much more expensive than swing bridges. No need to be so condescending in your replies.

[1st ade]

4 hours ago, magpie patrick said:

Yes, they did build the bridges humpy to keep costs down - an extreme example below (Lockgate bridge - lower Frankton)

I remember it well - i learnt to drive round there in 1980. If you can do a hill start on Lockgate Bridge you can probably do it anywhere!

 

[Athy]

2 hours ago, Pluto said:

I am making the statement after having looked at many historical costings for canal structures and the company papers that go with them. So comparing the various ways for a road to cross a canal, bridges in stone or brick were much more expensive than swing bridges. No need to be so condescending in your replies.

Of course there isn't.

[Captain Pegg]

10 hours ago, Athy said:

If the original canal builders were so oblivious to economy, how come they built humpy bridges? Surely, being shorter*, they cost less to construct.

 

 

 

* than bridges which were not humpy, of course.

Arches were simply the state of the art technology of the day. The available materials - stone or brick - are characterised by having effectively no tensile strength. The key to a masonry arch is that it is purely acting in compression. The cost saving feature of canal bridges was usually to narrow the channel to reduce the span. Even then the shortest distance from one side to the other would never be a curve. Observe the mid-1800s bridges of the BCN, many are composite metal/masonry structures with flat soffits.

 

JP

[David Mack]

15 hours ago, magpie patrick said:

Yes, they did build the bridges humpy to keep costs down - an extreme example below (Lockgate bridge - lower Frankton)

I knew someone who took that bridge too fast when he had a part-used bag of plaster in the back of the van. As soon as he landed he was enveloped in a pink fog!

[Pluto]

9 hours ago, Captain Pegg said:

Arches were simply the state of the art technology of the day. The available materials - stone or brick - are characterised by having effectively no tensile strength. The key to a masonry arch is that it is purely acting in compression. The cost saving feature of canal bridges was usually to narrow the channel to reduce the span. Even then the shortest distance from one side to the other would never be a curve. Observe the mid-1800s bridges of the BCN, many are composite metal/masonry structures with flat soffits.

 

JP

You can't really call arch bridges 'state of the art technology' as they were just an idea which had been passed down from craftsman to craftsman over the centuries. By the end of the 18th century, European engineers and mathematicians were working on the technology behind bridges and arches, creating formulae which would allow their design to be more exact. Here, in Britain, we had to wait until almost the middle of the 19th century before we accepted the mathematics needed for such calculations. Continental engineers were also far in advance of us regarding the cements and mortars needed for bridge building. Our industries were reliant upon craftsmen, with little theoretical expertise, and though they allowed us to develop an industrial society before other European countries, our lack of a decent educational system then held us back. Ironically, the way technical education developed here was also a bit of a barrier to future developments, in that the intelligence of the craftsman has been downgraded by educationists for over 150 years.

[Captain Pegg]

3 minutes ago, Pluto said:

You can't really call arch bridges 'state of the art technology' as they were just an idea which had been passed down from craftsman to craftsman over the centuries. By the end of the 18th century, European engineers and mathematicians were working on the technology behind bridges and arches, creating formulae which would allow their design to be more exact. Here, in Britain, we had to wait until almost the middle of the 19th century before we accepted the mathematics needed for such calculations. Continental engineers were also far in advance of us regarding the cements and mortars needed for bridge building. Our industries were reliant upon craftsmen, with little theoretical expertise, and though they allowed us to develop an industrial society before other European countries, our lack of a decent educational system then held us back. Ironically, the way technical education developed here was also a bit of a barrier to future developments, in that the intelligence of the craftsman has been downgraded by educationists for over 150 years.

I didn't use "state of the art" to mean modern and at the forefront of the technology. That's a connotation that isn't present in the words that seems to have taken hold in modern language. Whatever is the common practice of the time is the state of the art.

 

JP

[David Schweizer]

The traditioal way of building arches from stone or brick was aleady well established by the time the canals were built, and required the involvement of specialist joiners who constructed centering framework over which the arch was built.  As most of the hump back bridges on any particular canal would have been small and of the same dimensions, I suspect that the centering framework could be moved from one site to another, after the bridge was completed, only requiring the re-constuction of the bottom framing. Most canal bridges are slightly wider at the bottom, which would have eased the ability to remove the framework without damaging either it, or the newly constructed bridge.

 

 

 

 

 

Edited April 11, 2019 by David Schweizer

[David Mack]

2 hours ago, David Schweizer said:

The traditioal way of building arches from stone or brick was aleady well established by the time the canals were built, and required the involvement of specialist joiners who constructed centering framework over which the arch was built.  As most of the hump back bridges on any particular canal would have been small and of the same dimensions, I suspect that the centering framework could be moved from one site to another, after the bridge was completed, only requiring the re-constuction of the bottom framing. Most canal bridges are slightly wider at the bottom, which would have eased the ability to remove the framework without damaging either it, or the newly constructed bridge.

 

 

 

 

 

Since arches were commonplace in bridges and buildings, and the structural requirements to support small arches are modest - the arch is self supporting once the first ring of masonry is completed - I would have expected that any competent joiner could construct centering, and that specialist joiners would not be required.

I agree with the rest of your post.

[Pluto]

The simple arch is easy to build, with difficulty arising more when the arch is not a simple semicircle. This first drawing comes from Meerwein's 1802 book on arches, and shows how he was looking at the failure of arches of various shapes. The second drawing shows that he was beginning to get to grips with the design of skew arches, something which would take a few more years to develop. The first skew arches had been built, with those dating from the late 1790s on the Naas of the Grand Canal in Ireland being the first in the UK, though they have not survived. The first in England could be that on the Rochdale Canal, though research is on-going on the subject. Mathematics in England was not really advanced enough for the technology to be fully understood before the 1830s.

[Heartland]

The cost of bridges clearly varied as to the span, Leeds & Liverpool bridges required more material than the narrow canals of the Midlands. Swing bridges were no doubt cheaper, but being initially made of wood perhaps more costly over time. The early swing bridges on the  original BCN were removed and replaced by more durable structures. Engineers would provide costs for making the bridges.  Such construction requirements were not just confined to the canals, bridges were also required for early stone block tramroads.

 

[David Schweizer]

5 hours ago, David Mack said:

Since arches were commonplace in bridges and buildings, and the structural requirements to support small arches are modest - the arch is self supporting once the first ring of masonry is completed - I would have expected that any competent joiner could construct centering, and that specialist joiners would not be required.

I agree with the rest of your post.

You may well be correct, but in the 19th century centering was considered to be a specialism, not performed by most Joiners who concentrated upon domestic architectural features such as doorways and staircases. The specialism was considered to be of sufficient importance to feature on the membership certificate for the Amalagamated Society of Carpenters and Joiners :-

 

 

 

[Pluto]

Amalagamated Society of Carpenters and Joiners: Unfortunately founded in 1860, so post-dating most canal building. At the time canals were being built, such trades usually seem to have been split into 'house carpenters' and 'ship carpenters', the former covering all forms of civil engineering work, though it is easy to simplify such titles and their meaning.

[Chris Williams]

On ‎10‎/‎04‎/‎2019 at 17:02, Pluto said:

Hump back bridges were actually comparatively expensive to build, while swing bridges were much cheaper. On the L&LC, originally almost all the bridges between Liverpool and Wigan were swing bridges for this reason.

On the Oxford Canal, a lot of construction - bridges, locks, etc, was done more cheaply below Banbury, because the money had run out.

Lift bridges, single bottom gates, all still existing.

Pic - bridge 233 balancing half open.

[David Schweizer]

11 hours ago, Pluto said:

Amalagamated Society of Carpenters and Joiners: Unfortunately founded in 1860, so post-dating most canal building. At the time canals were being built, such trades usually seem to have been split into 'house carpenters' and 'ship carpenters', the former covering all forms of civil engineering work, though it is easy to simplify such titles and their meaning.

The term Carpenter is a generic one which fails to identify specific trades and, for examople, omits the classification of Joiner which already in use by the mid 1600's, describing any tradesman who made items from wood using cut joints, it is that catagory into which centering would rest.

[Pluto]

But contemporary documents do differentiate between ships carpenters and house carpenters with, for example, the latter being described as involved with the construction of the Howland Great Dock at Rotherhithe circa 1705. It may be possible that the terms were used more with regard to a business owner, rather than the individual craftsman, though business owners could well be craftsmen as well.

[magpie patrick]

We've moved away from the topic, but moving only round the corner from where this topic started, the entrance bridge to the Coal Canal was, for most of it's life, a masonry hump backed bridge, but originally it was a swing bridge, as were many others on the route. Not all of them, it's doubtful if even then a swing bridge on the Warminster Road or the Wellsway would have been acceptable. 

 

The entrance bridge has come full circle, as it is now a lift bridge. The last bridge on the Canal, Terminus Bridge just before Paulton Basin, was also originally a swing bridge, then a masonry arch, and now only the base survives at towpath level - that gives us some interesting options come restoration

[David Schweizer]

1 hour ago, Pluto said:

But contemporary documents do differentiate between ships carpenters and house carpenters with, for example, the latter being described as involved with the construction of the Howland Great Dock at Rotherhithe circa 1705. It may be possible that the terms were used more with regard to a business owner, rather than the individual craftsman, though business owners could well be craftsmen as well.

The use of Carpenter in that example would be correct, although I have never come across the suffix of House in my researches.  I have found some shipwrights described as ships carpenters, although in strict terms they were different trades using different tools   Separate Guilds for both Carpenters and Joiners were well established in London by 1705, the princilpal difference between them being that, to join timbers, Joiners used and glue, wheras Carpenters used nails or pegs. Going back to the issue of Arch Centering, I have always seen them catagorised as Joiners, but I conceed that given the fact that bridges would have been built in an outside enviroment subject to the weather, glued joints would have been of little use, so in that scenario, the generic catagorization of them as Carpenters would be correct,

[Heartland]

Returning to Patricks original thread, the engineer for this aqueduct over the GWR railway tracks was just as important, in engineering reputation that is, as that who built Dundas, I suspect. And perhaps credit should be directed there. The railway was promoted by Acts of Parliament in 1845 and 1846 was called the Wilts, Somerset and Weymouth Railway and featured in the gauge question, which was being determined at the time.  Looking at the image provided by Mr Moss :

 

 

There is sufficient width between the arches for a broad gauge track, and the contractors working on this section to Bradford upon Avon 1848 and 1849 would or should have made it to that gauge. These dimensions would also have dictated the width of the aqueduct. Isambard Kingdom Brunel was the engineer and in his report to the Board in 1849 suggested that the line to Bradford was nearing completion.

 

Building the complete route encountered financial difficulties, a take over by the GWR also left an uncompleted line, even though contracts were let in 1854 to finish the remaining portion. The route finally opened in 1857.

 

Looking at the first edition ordnance survey the railway was confined to a narrow strip of land between the Avon and the canal.

 

 

 

[David Schweizer]

The line under Dundas Aqueduct was originally broad gauge, as the engraving produced in 1857 for the Illustrated Home magazine demonstrates. It was converted to standard gauge in 1874.

 

 

 

 

Edited April 12, 2019 by David Schweizer

[bizzard]

And I doubt they had nice flexible plywood to construct centering for a nice curve of the arch, which I expect involved clever shaping of solid timbers instead.

Edited April 12, 2019 by bizzard

[bizzard]

Not forgetting of course the GW railway bridge across the Thames at Maidenhead, the story of which most folk know. Built by Brunel in 1838, the flatest, gentlest curved arch in the world at the time, built like that because the Thames authority refused to have any supports planted in the fairway of the river. Thousands gathered to watch the centering knocked out expecting it to collapse, which of course it didn't.

[Chris Williams]

it's doubtful if even then a swing bridge on the Warminster Road or the Wellsway would have been acceptable. 

 

The A340 road crosses the Kennet Navigation on a swing bridge at Aldermaston.

[magpie patrick]

1 hour ago, Chris Williams said:

 

The A340 road crosses the Kennet Navigation on a swing bridge at Aldermaston.

This is true, and there are several quite busy roads crossing navigations on moveable bridges, and the A36 and the A367 (as the Warminster Road and the Wellsway are now numbered) are probably less busy than some which are carried on swing bridges today. 

 

What would matter, though, is the context of the time the decision was made - I have no knowledge of the history of the A340 in the early 18th century when Aldermaston Bridge was first installed - but it was 90 years earlier than the bridges on the Coal Canal, so trade everywhere (including on the Warminster Road) would have been less. Also the area around Bath hasn't kept up with the rest of the country in terms of population increase - Bath is only twice the size it was in 1801 whereas England as a whole is around 6 times the population, and urban areas are 15 times more populous, , so traffic here has grown less than in the rest of the country.  

 

The A36 Warminster Road was certainly very important in 1805 when the coal canal opened, 30 years later an 11 arch viaduct was built to carry it over the valley, just as the canal was also reaching the height of it's trade - it's difficult to imagine a road justifying  a structure on this scale would have been a suitable candidate for a swing bridge  

Edited April 12, 2019 by magpie patrick