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Author Topic: Total bridge weight allowance not calculated correctly  (Read 4000 times)

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Offline Vladki cz

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Total bridge weight allowance not calculated correctly
« on: July 24, 2019, 11:02:53 PM »
First question is - if the bridge ramps are counted as bridge tiles or not. Both when deciding the allowed length and also in weight allowance calculations.

In this save http://list.extended.simutrans.org/debug-saves/owerveight2.sve (pak128.britain-ex), there is a 5-tiles long, 309 t heavy train approaching two bridges with 300 t allowance. The first is 6-tiles long, so for a while the whole train is on the bridge. The train is correctly slowed down, and a warning is show. However the next bridge is only 3-tiles long (including ramps), so there's always only part of the train on the bridge, and thus it should not be considered overweight, and yet is slows down in the same way as on the longer bridge.

It is either bug in implementation, or bug in documentation, or I did not understand the documentation correctly.


Offline DrSuperGood

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Re: Total bridge weight allowance not calculated correctly
« Reply #1 on: July 25, 2019, 06:25:45 AM »
It was my understanding that overweight trains should not be allowed to use bridges at any speed. Hence the 5 tile 309 t train should fail with no route when trying to route over the 6 tile 300 t bridge since it is 6 t over the designed bridge carry weight so cannot use the bridge safely at any speed. It should correctly route over the 3 tile 300 t bridge at full speed as long as any 3 sequential tiles of the train do not add up to more than 300 t, with it failing to no route if any 3 sequential tiles of the train weigh more than 300 t.

This is because bridge carry weight is the maximum weight loading that can be safely applied to a bridge. If it is exceeded, even by a small amount, the bridge would risk collapse as you are overloading it beyond its designed carry weight. Sure in real life a 300 t bridge can likely cope with a 309 t train driving over it, but that is only due to pushing into the engineered safety margins which exist to cope with structural fatigue or human stupidity. If the bridge could reliably handle 309 t it would state so rather than 300 t, after all the civil engineers who design bridges do not pull these numbers out of thin air or as a joke to inconvenience users of the bridge.

If such overloading is done one can easily end up with a tragedy like the sinking of MV Sewol in 2014. Which to quote Wikipedia...
Quote
Overloading and improperly secured cargo were also seen as direct causes.[191] Sewol was carrying 3,608 tons of cargo, more than three times the limit of 987 tons.[192]
Yes one can physically overload bridges and ships beyond their designed weight and they appear to work but doing so will eventually lead to such tragedies which is why one must not do so.

Offline Vladki cz

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Re: Total bridge weight allowance not calculated correctly
« Reply #2 on: July 25, 2019, 06:54:49 AM »
There is an option in simuconf tab, where you can choose if overweight is absolutely forbidden, or if small overweight is allowed with reduced speed.

But that does not change the bug that the 3 tile Bridge should be passed at full speed.

Offline jamespetts gb

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Re: Total bridge weight allowance not calculated correctly
« Reply #3 on: August 10, 2019, 01:17:44 PM »
I believe that I have now fixed this: I should be grateful if you could re-test with the next nightly build.
Incidentally, there are simuconf.tab settings for allowing vehicles that are over the wieght limit to be able to use bridges at a lower speed. This is intentional and realistic, as one must take into account the dynamic component of loading stress. Reducing the speed reduces this dynamic component and allows vehicles that would otherwise be too heavy to traverse such a bridge.