Pre-tensioning Cables

[JohnK]

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.

[mendel]

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.

[baggio]

I'm on vacation... http://www.pontifex2.com/iB_html/non-cgi/emoticons/smile.gif" border="0" valign="absmiddle" alt=':)'> 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.

[Calastigro]

i like the idea to ease into gravity.

Please impliment it, CL!

[sopwath]

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

[baggio]

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.

[beaujob]

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.

[mat-c]

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:

[baggio]

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.

[JohnK]

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.

[JohnK]

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?

[alcor]

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.

[elrond]

I agree, pretensioning is a must.

Might make for some funny slingshot effects also ;)

[pasqualz]

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

[JohnK]

This would also allow for some bridges with cables supporting more cables which could look really cool. http://www.pontifex2.com/iB_html/non-cgi/emoticons/cool.gif" border="0" valign="absmiddle" alt=':cool:'>