Pre-tensioning Cables

[Calastigro]

on further thought, pre-tensioning could lead to some whacky designs working, such as:
http://www.angelfire.com/ca4/dragonsden/randomstuff/nowork.gif" border="0">

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

[Micha]

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

[Calis]

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!

[Micha]

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

[baggio]

Andy 24 has started a new thread so that this one can continue with pretensioned cables. Visit the new thread, http://www.chroniclogic.com/cgi-bin/ikonboard/topic.cgi?forum=6&topic=63" target="_blank">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.

[Borogove]

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.

[Andy24]

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

And it would help the game a lot

[falkon2]

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

[Calis]

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 :)

[beaujob]

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).

[VRBones]

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.

[baggio]

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.

[mendel]

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.