Chronic Logic

on further thought, pre-tensioning could lead to some whacky designs working, such as:

maybe thats why they don't have pre-tensioning....

even with pretentioning I don't think that design would work.

the bridge would sagg and tear the cables right from the steel

pre-tensioning is a great idea. CL, implement it right this minute!

I dont think pretension would make it easier, it is just one more option you would have. Some of my cables snap off already, they would be gone faster with added tension.

Dont forget: adding tension reduces the amount of ´real´ load the cable can hold!

easing into gravity is a great idea.  real bridges have scaffolding for just that reason.

Andy 24 has started a new thread so that this one can continue with pretensioned cables. Visit the new thread, Easing into Gravity to continue gravity related discussions.

I agree with kvinge that easing into gravity is not a solution to making suspension bridges work, pretensing the cables is needed, and I hope to see this thread address some of the ways this can be implemented.

Based on the limited knowledge I have, this is not something that can be implemented with the current bridge format.  I think this is something that can be added, but will require a change in the way the cables are stored, and will result in a new version of the pxb file.

It should also be possible to convert version 1 pxb file to version 2, without causing some radical change in the P* engine. After reading the header, P* can determine if it is dealing with the newer file format, or the old format, and add a pretension of 0 to all the old bridges.

Being able to pretension the cables wouldn't make them any stronger - they are still going to break under the same stress.

One of the main reasons for wanting pretensioning is so that you can control the initial sag that occurs when the simulation starts, to avoid the excessive stresses that this puts on the structure.  A simple solution to this would be for the physics engine to gradually ramp up gravity for the first couple of seconds, so that it can settle into a stable position more easily.  This wouldn't be totally realistic - but then bridges don't spontaneously pop into existence either.

It shouldn't be to hgard to ease into gravity.

And it would help the game a lot

Hmm.. I think the best solution would be to adopt the system MSWord uses for .doc.. you can load .docs from earlier versions, but not later versions.

This might also provide the opportunity for CL to fix some of the physics bugs and/or modify physical properties without destroying the old game somewhat.

it would be nice if the gravity slowly gained until it reached normal that way bridges wouldn't bounce up and down for a few minutes

Slowly increasing gravity probably can solve the initial settling problem. But it will not solve the problem of node breaking, since they will still be bend a lot when the bridge (slowly) drops down.

It would be ok if we can get flexible nodes (discussed elsewhere), too. But I would prefer 'real', adjustable pretension :)

I think having a stiffer kind of cable would be indispensible.   Rather than stretching out so much, this kind of cable should just be able to absorb a higher amount of stress without expansion of any kind.  This is analagous to heavy steel.  You can put a very high amount of compression on heavy steel that simply does not register as a deformation of the truss.  I think this same principle would alleviate bridge flop, especially for a catenary suspension bridge (or parabolic as the case may be).

I'm with pulseJet on this one. It's not just cables, it is steel structures and everything going from no stress (due to gravity) to instant stress. If you gradually increased gravity you would see the load applied more evenly and the bridge deform into position instead of overshooting the mark and oscillating like it does at present.

Although pre-tensioning would work, you would need to do this for EVERY section of cable, steel, internal strut etc. until you approximated all the stresses on a stable bridge. Think of trying to replicate the various shades of read/blue you see now on a bridge that has 'stabilised'. That isn't going to be an easy task.

The PM has spoken. Take note CL ;). I will however disagree slightly. Yes, some nodes might still break, but not all the ones that currently break. This is because the dampining of the structure does not currently prevent overshoot. That is why current bridge designs must "settle". If gravity were eased into the simultation, then there wouldn't be an overshoot, and some joints that had previously failed would not experience as great a load.

Pretensioned cables, while a great idea, and something I want to see implemented, are not the end all. I think that the ease into gravity is more improtant to more than just cables, and should be implemented first.

There are 2 ways to go:
1) Use less elastic cables.
2) make your bridge more elastic.

The latter is practicable right now. I solved one of my problems with my lame contest bridge design that way.
I used to have it so the cable tower was built on the lower anchor and had the first deck joint integrated in its steel structure. This resulted in the second deck piece taking a lot of strain, because one joint was fixed in the heavy-steel-supported tower, the other was cabled and connected to sagging rest of the bridge. The stress was noticeably reduced when I built the tower past the first deck joint (removing the angle in the tower, too!) and suspended the first deck joint from that same or an auxiliary tower, I don't remember which. What I do remember is that this setup made the first joint sag a little, too, and that allowed it to distribute some of that stress farther to the outside, resulting in a more even stress distribution and a better bridge.
(Actually, this is a "current version" idea, so I'm a little bit off-topic here :-).

What you could do would be to build the deck at an angle, so it's allowed to sag a little, thus stressing the cables. The problem with this is that the deck gets stressed, too - and that should not be happening with a sus bridge...
If the members were still pin-jointed as they were in BB, the sag would not induce much extra stress, so instead of demanding prestressed cables, we could instead demand a no-diagonals beam (demanded elsewhere on this forum as well, if I recall). I dub this "swinging bar".

Swinging bars could be easily implemented as a new material type without changing the current file format, and it could even use the same colors now used for light and heavy steel.

Anything would be nice, but its my desire to build golden-gate-style bridges. This cannot be done very well w/out a method to pre-stress cables. brett talked about another method: Have a new material - High tension cable, but I don't agree it should cost three times as much, that seems a little excessive.

Lowering gravity will not eliminate "overshoot" or bouncing, because if damping was to eat all potential energy, the bridge would be "stuck" by friction and suddenly give when the train passed. If the bridge moves at all when sttling, it will overshoot and bounce back. Doing so at lesser gravity just makes the forces smaller, so the static load and the dynamic "bounce" force don't differ as much as they do now.

I'm on vacation... well, visiting the folks out of town anyway, so I son't be able to put a lot into this discussion, but I would like to mention, that I too have solved problems with the deck breaking by suspending it completely, and not using anchor points in the water to physically attach to the deck.

Until this post, I never gave much thought as to why it works. I also find myself using that technique subconsiously now. I've been intentionally building in a way so as not to physically support the deck. In order to create stronger support towers, I've been sagging the deck by 10m to allow me to build taller towers. (See the short links bug)

Anyway, just wanted to give mendel some feedaback on his post, and I'll try to make more of an appearence when I get back in town.

i like the idea to ease into gravity.

Please impliment it, CL!

OK so lets say that you design a bridge that does have the above pre-tensioned cables.

At some point the cable strength just wouldn't be enough.  You can only pull it so tight before it still breaks.

I don't think that situation would be an issue if there was a good way to handle cable stregth.  Even with that God mode grab thing you can whip the train around so fast that even 'the hand of God" can't hold it...

I think pre-tensioning would be good.

sopwath

Quote: from Calastigro on 9:10 pm on Oct. 25, 2001
i like the idea to ease into gravity.

Please impliment it, CL!Has my vote... even so I think the pre-tensioned cables would be needed for some structures.  I can't think of any easy way to implement it though.

My vote is for having a settle button in the editor that briefly turns on gravity to determine how much give to give (eeek) the cables.

I love Borogove's idea of gradually increasing gravity... with very large bridges 95% of the challenge is making them stay up at all, adding the train usually doesn't make much difference.  Gradually increasing gravity would remove the initial shock-of-childbirth that bridges currently have to endure, and would make bridge-costs more realistic.
Pre-tensioned cables would be great, a lot of real bridges are made like that - often to make concrete bridges look elegant ;)  The classic example being the two-cantilever bridge that is impossibly narrow in the centre.
From a coding point of view its a snap: cables already have an initial-length and a current-length, they just happen to be set equal at game start currently.
The hard part is user interface design, representing which cables are pre-tensioned might be tricky.
Perhaps a "select tension" option for cables, adjustable between say 50% and 100%.  (50% would only lay down half as much cable as 100%, so it would be well-tensioned.)
Colour could be used to represent the percentage-tension, and perhaps show the exact percentage as the mouse is hovered over it.
Whether 50% cable is half price or not I leave to CL :cheesy:

Quote: from mendel on 5:08 am on Oct. 31, 2001
Lowering gravity will not eliminate "overshoot" or bouncing, because if damping was to eat all potential energy, the bridge would be "stuck" by friction and suddenly give when the train passed.Damping doesn't work to eliminate potential energy, it works to eliminate kinetic energy. Damping, in a mechanical sense, is a function of velocity. It will not change the final position of the system, only how long it takes to get there.

There are only four possible states for a mechanical system. It can be overdamped, underdamped, undamped and critically damped. The bridges in P* are an underdamped system. If the bridges were critically damped, they would not have any overshoot, and they would reach steady state in a minimal amount of time. If they were overdamped, they wouldn't overshoot (and bounce) either, but the time to reach ss would be longer. If the coefficients of damping were too great, it may take hours to reach ss. The opposite of course would be an undamped system. In this system, the bridge would continue to bounce forever.

P* is an underdamped system because the bridge will bounce, and it will reach ss at sometime.

You are correct that as an underdamped system, slowly introducing gravity will not elliminate a problem with bounce. However, if the bridge is allowed to reach ss before gravity is increased additionally, then the impact of that bounce on the overall design will be minimal.

If the current system causes the bridge to overshoot its ss position by 1m when the level is first tested, then by easing into gravity, it may only overshoot by .01m. This makes a tremendous difference when waiting for the bridge to settle, and an even greater difference when you start adding mass (in the form of a train).

Of course, a poorly designed bridge is still going to collapse, but perhaps the deflection in the bridge will be less from the train than by the start of the simulation. A level that only requires 2 cars or so would gain from such a change.

Quote: from beaujob on 10:10 pm on Oct. 31, 2001
My vote is for having a settle button in the editorI think this is a tremendous idea. It is certainly needed if there is going to be a way to tense the cables. In such a mode, I think CL needs to change the damping coefficient to be critical.  This way, you won't need to wait for a settle period before you tense the cables.  An underdamped system is still reasonable for the actual simulation.

Wow! I really touched off a powder keg! I don't see how the ability to ease into gravity can be compared to pre-stressing cables. No matter how sloooow gravity comes on, unless you can pre-tension cables, suspension bridges will never work as well as they should.

Would there be any way to pre-tension the cables so the bridge wouldn't flop around and get damaged when you test it? I know this was talked about in the bridge contructor forum, somebody suggested a slider that allows you to choose a tension. I don't know how easy it would be to impliment, or what the ramifications of such an implimentation would be. Flopping cables kind of get annoying. Real bridges almost always have pre-tensioned cables which allows them to better support the weight of the bridge. Does anybody else feel similarly?

It would definitely make things a LOT easier in some situations, but i'd definitely like the feature.
Some present levels would be made easy, but new levels could reflect the change.

I agree, pretensioning is a must.

Might make for some funny slingshot effects also ;)

I agree! I have been annoyed by this all along, but didn't know what the correct terminology (pretensioning) was.

This would also allow for some bridges with cables supporting more cables which could look really cool.

I can envision a whole bridge made of a web of cables with just a few stiff supports at strategic places. AWESOME!

I don't think this would be too hard to make happen (speaking with little expertise) but it would add so much to the realism of the game. Of a list of five things I would like in a new version, this would be toward the top for me.

You guys are missing the point entirely.  There's a reason cables are cheap--because they flop around and are somewhat difficult to use.

Just take a look at your bridge for Map Pack 1 Level 16, and think about what made it difficult.  It took me over an hour, even though I already had plenty of practice with arches.  It's difficult because the bridge is so large that it sways and must be very carefully cabled and reinforced.  I had one verson of MPACK116 that actually stood up with no broken links after four passes 80% of the time (it depended on when you started the train), but it would start swaying and tear itself apart if you just let it stand there for ten minutes without even running the train.

If you could make the tension on cables tighter, it would make things ridiculously easy.  After I completed all 48 levels that come with Pontifex, I started going back and making them cheaper.  When you're cutting costs, even smaller bridges can use a lot of cable, and the challenge is to make stable designs that don't oscillate and tear themselves apart--a challenge imposed mostly by cable slack.

If you could increase the cable tension, this aspect of the game would be totally removed, and almost every level would be much less challenging.  Without slack, the bridge doesn't oscillate, and it quickly stabilizes itself after the train passes, so there would really be no point in levels requiring four train passes--you would know immediately as soon as the train made it across once that every other run would be successful because the bridge wouldn't be swaying.

OTOH, a good addition might be high-tension cable at a higher price.  It would have to be at least three times as much as regular cable, though.

From a realism front, though, Bridges aren't built in 0G as they are in pontifex.  they don't suddenly appear and flop around.  the cables ARE pre-tensioned.  Being able to adjust the tension would enable you to take on larger, more difficult gaps, make more advanced bridges, and lower the cost of current bridges.

That said, with some level redesigning, the game would be much improved.

From a coding standpoint, however, pre-tensioning would be a bitch to code.  But then again, phizicks engines are bitches, and ChronicLogic seems pretty adept at writing those...

In conclusion, OG WANT PRE-TENSIONING!