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Re: "Progress" vs. "Tried & True" - A Few More Thoughts
Author: OldPoleBurner
Date: 08-02-2010 - 18:59
Expansion joints were not a significant issue on the Bay Bridge, nor would they be on any all steel bridge, for that matter; simply due to the fortuitous fact, that the expansion coefficient for steel rail is very close to that of steel bridge beams. Moreover, modern rail expansion joints, sometimes used with ribbon rail, usually do not impose any need for speed restrictions when properly anchored.
What did impose the 35mph restriction on bridge trains though, was the extreme unified mass of the train cars; ranging from over 100,000 to 160,000 pounds per car, depending the class of car.
The issue with this is a little known quirk of nature called "Harmonic Augmentation", which tends to amplify vibrations and stresses imposed by weight upon any structure. Harmonically augmented vibrations have a nasty habit of causing tiny flaws in the bridge materials to grow rapidly into cracks and breaks. It also fatigues metal and creates cracks and flaws.
To allow for this, you must either limit the weight of a moving mass, or limit its speed, or somehow de-synchronize the vibrations (hard to do if natural resonances of the cars are similar to that of bridge beams). In those days, the available technology pretty much dictated that it was speed that you limited - thus the 35mph limit. It also should be noted that for their weight, the articulated units were way under powered to get up much speed on the incline anyway. This under powering may have been deliberate though - what was the point of more power if you couldn't go any faster for other reasons anyway.
As for today, we routinely allow 80,000 pound trucks to do 50mph (likely more) on the bridge. Certainly, 68,000 pound BART cars could easily do just as well. But the Muni's old Boeing-Vertal "light rail" cars, or their successors? At 147,000 pounds apiece - I'm not so sure!
I have always wondered why it's called "Light Rail" anyway - neither the rail nor the cars are actually light!
OPB
What did impose the 35mph restriction on bridge trains though, was the extreme unified mass of the train cars; ranging from over 100,000 to 160,000 pounds per car, depending the class of car.
The issue with this is a little known quirk of nature called "Harmonic Augmentation", which tends to amplify vibrations and stresses imposed by weight upon any structure. Harmonically augmented vibrations have a nasty habit of causing tiny flaws in the bridge materials to grow rapidly into cracks and breaks. It also fatigues metal and creates cracks and flaws.
To allow for this, you must either limit the weight of a moving mass, or limit its speed, or somehow de-synchronize the vibrations (hard to do if natural resonances of the cars are similar to that of bridge beams). In those days, the available technology pretty much dictated that it was speed that you limited - thus the 35mph limit. It also should be noted that for their weight, the articulated units were way under powered to get up much speed on the incline anyway. This under powering may have been deliberate though - what was the point of more power if you couldn't go any faster for other reasons anyway.
As for today, we routinely allow 80,000 pound trucks to do 50mph (likely more) on the bridge. Certainly, 68,000 pound BART cars could easily do just as well. But the Muni's old Boeing-Vertal "light rail" cars, or their successors? At 147,000 pounds apiece - I'm not so sure!
I have always wondered why it's called "Light Rail" anyway - neither the rail nor the cars are actually light!
OPB
