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To: supercat

I don’t think you would want to bother using simple-support rollers on both ends. They all have to be restricted in both horizontal directions or else the truss will simply fall off its supports. It’s common practice because it works.

I don’t see how a “sudden quick movement” would necessarily cause a catastrophic failure. I’d be more concerned that the debris held the roller supports in place to act almost like a 2nd fixed joint, building up stress within the structure and possibly being the breaking point for those weak points in the structure.

As someone (some structural person, I think) said on a TV broadcast, it’s probably several factors that melded together that caused this, anyway.


55 posted on 08/10/2007 6:49:15 AM PDT by the OlLine Rebel (Common sense is an uncommon virtue.)
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To: the OlLine Rebel
I don’t think you would want to bother using simple-support rollers on both ends. They all have to be restricted in both horizontal directions or else the truss will simply fall off its supports. It’s common practice because it works.

Current practice is to have one fixed point that never moves. What I would suggest would be instead to have one point held in position by a positioning mechanism which could periodically (e.g. once a day) move the entire bridge back and forth a few inches, stopping at a slightly different position each day. For 23.9+ hours/day, a brake would hold the mechanism in place. But each day the brake would unlock and a motor would drive the bridge.

I don't think 'sudden quick movements' are themselves apt to cause damage. On the other hand, if a bearing is getting sufficiently worn that it tends to stick for awhile and then yield, that suggests that it's nearing the point where it's going to stick and not yield.

On a bridge with a single fixed point, a bearing that has frozen up may not be detected until the next major thermal event (very hot day, etc.). Even if there is instrumentation to measure movement, the fact that the bearing doesn't move when the temperature remains constant would hardly be alarming. The seized bearing may not be noticeable until it starts causing major stresses on the bridge, and those stress levels would be likely to increase before there's any opportunity to deal with the seized bearing.

If the 'fixed' bearing were replaced with a motorized one, then in the event that the bridge started to experience stresses as a result of a seized bearing elsewhere, the motorized bearing could be moved so as to relieve such stresses (it wouldn't need much power, since the motor would be trying to move the bearing in the direction it already wanted to go). The only danger would be if two bearings seized up simultaneously; that risk should be reduced by the fact that an alarm would sound when the first bearing seized up.

59 posted on 08/10/2007 3:25:29 PM PDT by supercat (Sony delenda est.)
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