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Author Topic: Masonry road bridges: Why low weight limits?  (Read 1476 times)

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Offline wlindley

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Masonry road bridges: Why low weight limits?
« on: November 24, 2020, 04:02:16 PM »
In 1850, for example, why can I build a masonry barge aqueduct with limit of 1000t, a masonry rail viaduct or elevated track with load limit 175t, but the roadway masonry bridge has a weight limit of 8t?
This makes it basically impossible to send many common road convoy combinations across a river or canal at all, while canals and railroads easily can; surely that is not realistic? 

Offline Spenk009

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Re: Masonry road bridges: Why low weight limits?
« Reply #1 on: November 27, 2020, 12:57:24 PM »
Adding to that: the wooden trestle elevated track can hold 150t of weight whereas the brick one can only support 90t (masonry=175t).

About the masonry road bridge, I do believe it's intended to be quite low as it is very cheap to build (187c) whereas any modern option is 500c or more. The exception here is the iron girder bridge (250c) that can only take 11t of weight. Maybe if inflation is implemented, it will correspond accordingly as it is available for 160 years (1750 - 1910), but it is intended to be from a time before automobiles and its intention as such may just be to prohibit said vehicles.

Would people be interested in a doubling or tripling of price for a corresponding factored maximum weight increase? It's not based on any factual findings, but rather a gameplay improvement. The alternative could be updated graphics showing a smaller bridge and a stronger more expensive alternative (the original blend files may be lost though).

Offline Matthew

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Re: Masonry road bridges: Why low weight limits?
« Reply #2 on: November 30, 2020, 03:24:39 PM »
The masonry road bridge has an axle limit of 12t and a weight limit of 8t.

Surely this is nonsensical. Is it not likely that the two values are just the wrong way round?


Axle limits are of course a property of the road, not the bridge - d'oh!
« Last Edit: November 30, 2020, 04:00:42 PM by Matthew »

Offline jamespetts

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Re: Masonry road bridges: Why low weight limits?
« Reply #3 on: November 30, 2020, 03:35:45 PM »
The masonry road bridge has an axle limit of 12t and a weight limit of 8t.

Surely this is nonsensical. Is it not likely that the two values are just the wrong way round?

The axle load is set not by the bridge but by the road surface on top of the bridge, so this varies depending on the road in question.

As to the maximum weight of the masonry bridge generally, does anyone have any data on the range of weight limits realistically achievable for 18th century (and earlier) masonry road bridges?

Offline wlindley

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Re: Masonry road bridges: Why low weight limits?
« Reply #4 on: November 30, 2020, 05:30:58 PM »
The masonry Lothian Bridge in Scotland was built in 1831, today being part of the A68 alignment, with a modern speed limit of 30mph (48km/h) but with no weight limit, accommodating all currently roadworthy lorries (you can see several rather large ones on the bridge approaches in the Google Street Maps views).

As for early iron road bridges: The Cound Arbour Bridge in Shropshire is still in use as a road bridge, with a modern concrete roadway surface that cnsists "of a cast iron structure, supporting a 150mm thick concrete deck slab that has been added at a later stage, so therefore does not act integrally with the iron, and acts only as a dead load."  The concrete alone (assuming 8 ft × 30ft × 150mm) would weigh 18,000 pounds (using this concrete calculator).

Also of interest, Weight restrictions on bridges in Northern Ireland including King James Bridge (circa early 1600s) a stone structure with a modern weight limit  of 7½ tonnes.

 In general, the weight limit for a bridge type should be roughly the same whether its load was a street or a railway.

Offline jamespetts

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Re: Masonry road bridges: Why low weight limits?
« Reply #5 on: November 30, 2020, 05:46:39 PM »
Thank you for that information. The Northern Irish bridge is the most relevant, since the other bridges are much later than the 18th century era in which our masonry bridges are allowed to be built, and its weight limit is consistent with the weight limit that we have already (and is significantly in excess of any road traffic in the era).

I am not sure that it is right that the same basic type of bridge should have the same weight limit regardless of way type, as a railway bridge (for example) might very well be built to a much higher standard than a road bridge, especially in the era when the only road traffic would either be foot traffic or something that could be pulled by horses.

Offline Freahk

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Re: Masonry road bridges: Why low weight limits?
« Reply #6 on: November 30, 2020, 06:29:02 PM »
I am not sure that it is right that the same basic type of bridge should have the same weight limit regardless of way type, as a railway bridge (for example) might very well be built to a much higher standard than a road bridge
I agree, but on the other hand, these _could_ be built to higher standards if needed.

We have the same issue with many types of objects. In case of tracks, there are many tracks of basically the same type (ballast, concrete sleepers, vignole rail)
There are many different objects of that type, representing different standards (lb/yd in this case), which are colored slightly different .
« Last Edit: November 30, 2020, 08:12:22 PM by Freahk »

Offline jamespetts

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Re: Masonry road bridges: Why low weight limits?
« Reply #7 on: December 01, 2020, 11:30:27 AM »
I agree, but on the other hand, these _could_ be built to higher standards if needed.

We have the same issue with many types of objects. In case of tracks, there are many tracks of basically the same type (ballast, concrete sleepers, vignole rail)
There are many different objects of that type, representing different standards (lb/yd in this case), which are colored slightly different .

In what circumstances would a player in the 18th or early 19th centuries need to build a road bridge with a higher weight limit than 8t?

Offline Matthew

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Re: Masonry road bridges: Why low weight limits?
« Reply #8 on: December 01, 2020, 12:12:05 PM »
In what circumstances would a player in the 18th or early 19th centuries need to build a road bridge with a higher weight limit than 8t?

I don't this is the issue that wlindley has raised, if I follow him rightly.

I think the question is, "why is the masonry road bridge still limited to 18th-century engineering requirements in the mid-19th and 20th centuries?"

So perhaps the problem could be solved by having two masonry road bridges, one with 18th-century weight limits, replaced later by one with higher cost and weight limits?

Offline Ranran

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Re: Masonry road bridges: Why low weight limits?
« Reply #9 on: December 01, 2020, 12:28:49 PM »
The stage wagon that can carry 6 crates of pieces goods is too slow to pull with one horse. However, slow horses greatly interfere with the transportation of other companies. If you connect two horses, it will exceed 8t.
Therefore, there was no choice but to avoid the bridge and detour or cut the river. (´・ω・`)

If the player is unaware of the existence of a bridge that cannot be passed, it may be bypassing a very long distance.
Only 6t ones can run at 10km/h. We cannot set our own capacity limits as we do in the real world. In the real world, you can order the cargo to be loaded so that the total weight does not exceed 8t. The schedule can specify a lower limit, not an upper limit. Therefore, no transporting weight limit can be imposed.

Online freddyhayward

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Re: Masonry road bridges: Why low weight limits?
« Reply #10 on: December 01, 2020, 12:34:24 PM »
In what circumstances would a player in the 18th or early 19th centuries need to build a road bridge with a higher weight limit than 8t?
as few as three pairs of pack horses loaded with piece goods can exceed 8t.

Offline jamespetts

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Re: Masonry road bridges: Why low weight limits?
« Reply #11 on: December 01, 2020, 12:48:08 PM »
Interesting - thank you both for your analysis. There are thus two issues here: (1) ought the masonry road bridge from the 18th century have a higher weight limit than 8t; and (2) ought there to be another, later masonry road bridge from the later 19th century with a higher weight limit?

As to (1), this will require some evidence that masonry road bridges of this era in fact did have higher weight limits, I think.

As to (2), this would require the players to have a reason in this era to build a masonry, rather than brick, road bridge. I am not sure that there is one (which is why I did not include a later masonry bridge in the first place); but can anyone think of any genuine economic reason, within the parameters of the economics simulated in Simutrans-Extended, for a player building such a bridge?

Offline Freahk

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Re: Masonry road bridges: Why low weight limits?
« Reply #12 on: December 01, 2020, 01:00:21 PM »
As mentioned above, there are some cases in the 19th or even the 18th centuary.
Just to add yet another one: In 1857 the first steam road vehicle will be available, which will exceed those 8t with only a single trailer (except from the livestock trailer) when loaded.
Practically this means you cannot use those steam road vehicles at that time.
On short distances it's not economically worth it and on long distances, there will practically always be a river or something else to cross.

A brick road bridge is not available at that time!

Offline jamespetts

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Re: Masonry road bridges: Why low weight limits?
« Reply #13 on: December 01, 2020, 01:04:36 PM »
As mentioned above, there are some cases in the 19th or even the 18th centuary.

The post above with the weight limits showed a 7.5t weight limit for the only 18th century or earlier masonry bridge. Later masonry bridges postdate the introduction of brick bridges, and are likely to have been built as masonry bridges only because of the relative inexpense of using specifically local stone - the locality related costs of building materials not being simulated, this aspect of economics cannot be simulated in Simutrans-Extended, and it is thus difficult to see any reason to have a later, stronger masonry bridge. If there were mid-18th century or earlier masonry bridge with a higher weight limit, that would be more relevant.

Quote
Just to add yet another one: In 1857 the first steam road vehicle will be available, which will exceed those 8t with only a single trailer (except from the livestock trailer) when loaded.

Brick bridges should be available by this date.

Offline Freahk

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Re: Masonry road bridges: Why low weight limits?
« Reply #14 on: December 01, 2020, 01:31:01 PM »
But they aren't available by this date. At least not on the road.

Anyways, if people were able to construct a 175t masonry railway bridge, why wouldn't they be able to construct a, let's say, 20t masonry road bridge at the same time?

I guess my point above was not clear enough: There are some ingame cases where vehicles in the 18th and 19th centuary exceed those 8t of the heavyest available bridge.
I am not sure how this was managed in the real-world, assuming there were no such strong bridges, but ingame we cannot handle this well.
« Last Edit: December 01, 2020, 01:42:05 PM by Freahk »

Offline jamespetts

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Re: Masonry road bridges: Why low weight limits?
« Reply #15 on: December 01, 2020, 01:49:01 PM »
But they aren't available by this date. At least not on the road

I had not realised this. I have now added a new brick road bridge available from 1815, based on the brick rail bridge available on the same date, using the same graphics.

Hopefully, this will deal with the issues.

Online freddyhayward

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Re: Masonry road bridges: Why low weight limits?
« Reply #16 on: December 01, 2020, 02:23:58 PM »
If there were mid-18th century or earlier masonry bridge with a higher weight limit, that would be more relevant.
The Old London Bridge was a stone road bridge capable of concurrently supporting over 200 buildings and hundreds of vehicles. Clearly, stone bridges supporting a wide range of weights existed during this period, and it does not make sense to limit it to 8t.

Offline jamespetts

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Re: Masonry road bridges: Why low weight limits?
« Reply #17 on: December 01, 2020, 03:11:26 PM »
The Old London Bridge was a stone road bridge capable of concurrently supporting over 200 buildings and hundreds of vehicles. Clearly, stone bridges supporting a wide range of weights existed during this period, and it does not make sense to limit it to 8t.

Do we have any data on what the actual weight limit of contemporaneous bridges was? Do not forget that the old London Bridge did, in fact, have structural problems.

Offline Vladki

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Re: Masonry road bridges: Why low weight limits?
« Reply #18 on: December 01, 2020, 07:07:37 PM »
In what circumstances would a player in the 18th or early 19th centuries need to build a road bridge with a higher weight limit than 8t?
Player probably won't, but in reality, people did. Reasons? Over-engineering so that the bridge would last "forever". Especially to withstand floods. Also I think that the weight of vehicles is not that high in comparison to the weight of the bridge itself. If cheap bridge was needed, a wooden one would be built which would of course support only horse drawn carts, or pack horses. In player terms wooden bridge - cheap to build, low weight and speed limit, expensive to maintain (rebuild every now and then due to floods, rotten wood, etc.), and stone bridge - expensive to build, but cheap to maintain, high weight and speed limit (a kind of investment to the future).

Examples from Bohemia:

https://en.wikipedia.org/wiki/Charles_Bridge#20th_century_to_present
Charles Bridge in Prague. Founded 1357. Reconstructions in 1784 and 1890 were only partial, to fix the damage caused by floods. So we can assume that at least some pillars were 18th century or older in 1900. There was a horse tram line over it, electrified in 1905, but it was withdrawn in 1908 due to badly affecting the bridge. In 1932, bus line was established over the bridge. Since 1965 it is pedestrian zone.

https://en.wikipedia.org/wiki/P%C3%ADsek_Stone_Bridge
Even older than Charles Bridge in Prague.  6 arches are supposedly original. There was a bus line in operation over the bridge until 1994 when big reconstruction started and the bridge is only for pedestrians since then.

One problem is that this kind of bridges does not have one easily settable max weight. You could say maybe how much each arch can support, but the total depends on the lenght (number of arches). They behave more like elevated road than a bridge. But I think that even ancient stone bridge shall be strong enough to support horse trams, 20th century buses and small to medium sized trucks.

Offline jonbridg

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Re: Masonry road bridges: Why low weight limits?
« Reply #19 on: December 02, 2020, 12:57:01 PM »
Player probably won't, but in reality, people did. Reasons? Over-engineering so that the bridge would last "forever". Especially to withstand floods.
I think this is important. We're trying to assign one bridge representing one specific loading case to hundreds of different real-life situations. Simutrans does not accurately simulate river width, depth or current; quality of construction, stone, mortar or foundations; ground stability; arch height or span; asymmetric or cyclic loading; or any other factors which affect the strength of a masonry arch bridge.

So you're probably better off looking at what was technically possible. Cathedral masons had been supporting very high static loads (or bell towers!) with arches for centuries. But while cathedral-sized bridges were proposed they were never built because traffic was never high enough (by weight or quantity) to justify the cost.

Gameplay wise, as the strongest bridge available (until 1815 now), it should be able support all traffic until that date, including the heaviest carts. Packhorse trains are a tricky one as in reality they could be split into smaller groups when travelling over weak bridges.

Offline wlindley

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Re: Masonry road bridges: Why low weight limits?
« Reply #20 on: December 02, 2020, 02:50:55 PM »
With the latest commit of an earlier brick road bridge, the gameplay seems entirely perfect right now.  With the eventual overall rebalancing, there are probably enough variables available (maximum height and span) so that players might be able to build a few very tall or long bridges at relatively great expense.  Much appreciate the good discussion!

Offline prissi

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Re: Masonry road bridges: Why low weight limits?
« Reply #21 on: December 21, 2020, 12:57:08 PM »
As addenum to the Prague bridges:

The "Steinerne Brück" from 1136 in Regensburg in Grrmany had a tram waz lkine and several Bus lines on it and was until 1938 the only brigde crossing the Danube river at this place. Given the maximum weigth of buses, truck and trams,, the weight limit was close to the maximum weight limit of modern bridges.

Or from 1714 Puente Nuevo in Ronda (Spain) which was a major traffic route even for large trucks. At least in 1985 the main road was still over this bridge, which I vividly remmber from my childhood times. Given that a cubic meter of rock is close to 2t a weight limit of 8t seems awfully small indeed. Also I wonder which bricks should have a higher weight limit than masonery, and not the other way round, als least if the right rocks are selected and cutted.

Offline jamespetts

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Re: Masonry road bridges: Why low weight limits?
« Reply #22 on: December 21, 2020, 02:55:53 PM »
In reality, there would have been a great variety of masonry bridges. Without better data to calibrate this variety (and the relative costs of each), it is likely to be very difficult adequately to balance these to take account of all possibilities.

Offline Vladki

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Re: Masonry road bridges: Why low weight limits?
« Reply #23 on: December 21, 2020, 05:18:37 PM »
Imho brick bridges became more popular due to lower cost, than higher acceptable load. So I think they should both have similar weight limits, but a noticeable difference in price. There are a few railway bridges from 1940's in Czechoslovakia, which have masonry pillars, but concrete arches. (They originally planned to build steel bridge, but steel was in high demand during the war, so the project was changed.) Train line from Brno to Havlickuv Brod:

https://cs.wikipedia.org/wiki/%C5%BDelezni%C4%8Dn%C3%AD_viadukt_v_Meziho%C5%99%C3%AD#/media/Soubor:Doln%C3%AD_Lou%C4%8Dky,_Meziho%C5%99%C3%AD,_%C5%BEelezni%C4%8Dn%C3%AD_viadukt_(2013-08-01;_01).jpg

https://cs.wikipedia.org/wiki/Most_M%C3%ADru_(Doln%C3%AD_Lou%C4%8Dky)#/media/Soubor:Doln%C3%AD_Lou%C4%8Dky,_most_M%C3%ADru_(2013-08-01;_06).jpg

https://cs.wikipedia.org/wiki/%C5%BDelezni%C4%8Dn%C3%AD_nehoda_u_%C5%98ikon%C3%ADna#/media/Soubor:Lubn%C3%A9,_Kutiny,_v%C4%9Bt%C5%A1%C3%AD_%C5%BEelezni%C4%8Dn%C3%AD_viadukt_(2013-09-07;_02).jpg

Offline Matthew

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Re: Masonry road bridges: Why low weight limits?
« Reply #24 on: December 22, 2020, 05:27:00 PM »
I spent quite a lot of time today investigating 18th century bridges and eventually hit gold in D.F. Harrison 'Bridges and Economic Development, 1300-1800' (The Economic History Review: N.S.LIXV:2: 1992), pp.240-261. I will sometimes use [square brackets] to make clear where I am adding my views to Harrison's.

The main theme is continuity. There were many bridges in 18th century England and they mostly dated to medieval times. If there was a bridge in 1750, it was almost certainly there in 1300; many new bridges were built after 1750 but this did not change the basic pattern of the national road network, which remained unchanged from medieval times [until the mid-20th century motorways and by-passes, I guess]. All major and minor rivers were bridged at short intervals except for the lower reaches of the very largest rivers, such as the Thames below London Bridge. [In Sim-Ex terms, this probably means everything represented as an in-game river (rather than estuarine sea tiles) should be bridgeable in 1750.] Those lower reaches were generally linked by ferries until about 1790, when they began to be bridged. Fords were rare and used essentially as relief routes if a bridge was damaged or by people who couldn't afford the bridge tolls.

Focusing on the topic of this thread: "Contrary to the picture of ramshackle and narrow timber structures that is sometimes painted, medieval bridges were often of high quality... Arched stone bridges are able to bear very great weights, so it is likely that medieval bridges could bear heavy axle weights except in exceptional circumstances" (pp.246, 253). This includes bearing carts. For example, all but one medieval bridge over the Thames is known to have been able to carry carts (and that exception is due to lack of data); the statistics are similar for other major rivers. There were bridges that were too narrow to take carts, but they were only found over minor rivers, such as the river Brue at Bruton, shown here:

(Photo: Wikimedia Commons/Martin Southwood, CC-BY-SA)
[In pak128.Britain-Ex this would be a stream, I would think.] People did not build bridges to packhorse standards because they had been replaced by carts by 1300 or so. [Pack transport was still very important in some parts of the world up until the 20th century, but the British Isles was not one of those places.] The limit on bridge loads was generally imposed by the foundations, not by the bridge itself, but Harrison did not know of any research into this.

Building and maintaining good bridges was, by medieval standards, staggeringly expensive. Harrison cites repairs to the Tyne bridge at Newcastle in the 14th century which cost £1,500 (p.252)! [£1.7m in today's sterling, but of course in a much, much smaller economy. Harrison's data shows you could have built ten churches for that amount of money and that is the most appropriate comparison.] The people putting up the money were generally not thinking about earthly profits: bridges were often financed by godly donors who hoped that the bridge would both express their gratitude to God and earn the gratitude of their neighbours: "Bridge building was a pious and charitable act" (p.255). [So don't think of medieval bridges like modern bridges; think of them like cathedrals. If you couldn't afford to build a cathedral or a monastery, a bridge was the next best thing. Like cathedrals, they were built to the highest standards possible.] So England's bridge network remained stagnant between 1530 and 1750 not just because the technology remained the same, but also because the medieval legacy was so good that there was no need to improve on it. Outside of the estuarine exception discussed above, the improvements made to bridges in the late 18th century generally focused on widening bridges (to increase capacity) rather than strengthening them.

The article suggests that we have probably got the relationship between masonry bridges and wooden trestles the wrong way round. If I read the road .dat files correctly, we have a maintenance cost of 1500 for the former and 320 for the latter. But Harrison says: "Wooden bridges could be as adequate for their purpose as stone ones, but would usually require more frequent and expensive maintenance" (p.249). [That perhaps is a further explanation for the fact that] bridges over major rivers were all or overwhelmingly built of stone; even small streams were often bridged in stone. And we also limit timber bridges to 4 tiles, while allowing masonry bridges of any length. Historically, "[t]imber bridges were probably most often located on the lower reaches of major rivers... This choice of materials is presumably connected with the technical problems and expense of bridging a relatively wide and deep stretch of water" (p.251). The most common location for timber was therefore the Thames between Reading and London. Reading is also notable for having a road bridge with timber on stone piers, which was otherwise rare in England (so the pakset is right to omit them). There are only two other known clusters of timbered crossings, one in Middlesex (and therefore adjacent to the Thames cluster) and one on the Wye.

The other significant regional difference was the span between arches. Rivers in southern England tended to have wide floodplains and were therefore bridged with narrow spans. However, bridges north of the Humber needed to leave room for flash floods to pass through and were built with fewer piers. So if we ever get regional ways, then we could have alternative bridges with different pillar_distance settings.

Harrison's bibliography omits one important source cited elsewhere: Thomas S. Willan The Inland Trade: Studies in English Internal Trade in the Sixteenth and Seventeenth Centuries (Manchester: MUP 1976). This also appears to be very relevant to the other thread on the role of markets and I should be able to get hold of it from my local library once the pandemic is over.
« Last Edit: December 22, 2020, 05:50:05 PM by Matthew »

Offline Vladki

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Re: Masonry road bridges: Why low weight limits?
« Reply #25 on: December 22, 2020, 06:59:02 PM »
So to sum up:
Stone bridge: high weight allowance, high build cost, low maintenance.
Wooden bridge: low weight allowance, low build cost, high maintenance.

Regarding the length we have a problem. From what you wrote a stone bridge should be only for short lengths and shallow water, and wooden bridge for longer lengths and deep water. But the only limit on depth is for bridges without length limit. Or is the depth of water added to maximum height?

Offline jamespetts

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Re: Masonry road bridges: Why low weight limits?
« Reply #26 on: December 22, 2020, 07:07:50 PM »
That is very interesting - thank you for that. I note nothing specifically on the weight limits, and nothing apparently inconsistent with an 8t weight limit so far as I can discern. As to the costs, it is planned to make increased use of an inverse relationship between capital and maintenance costs (where this is informed by reality) when the rebalancing has been completed, but, as is well known, the critical balance features will need to be completed first.

In relation to length limits, I wonder whether we should re-conceive of the wooden bridges as akin to viaduct type bridges, that is, bridges with many supports that thus cannot be built on deep water but that can be built over unlimited lengths.