Designing arms against buckling

[Rick TYler]

Originally Posted by Paul Copioli

Would a bridge with solid plate sides instead of trusses hold more load? Probably, depending on geometry conditions. Would it weigh too much? Yep.

Would it be better or worse for dynamic conditions (think Tecumseh Narrows bridge)? Much worse. Mass is a major player for dynamic performance.

Tacoma Narrows Bridge, and I'm not sure your comparison is apt, although the rest of your analysis is a great contribution to the hive mind. The Tacoma Narrows Bridge (the first one, not the one that stands today...) had a very thin bridge deck without deep stiffeners under the deck. High winds set up harmonic vibrations which caused the bridge to shake itself to pieces. It wasn't an issue of buckling under load, it was a failure to tune the bridge for a low enough or high enough resonant frequency (and I sure hope that's the right term -- I am NOT a civil engineer) to prevent it from vibrating like a guitar string.

Did your post imply that extra mass would be worse for dynamic conditions? I am trying to understand this, if this is what you meant. Tacoma Narrows Bridge #2 is substantially "beefier" than the first one, including a much deeper bridge deck.

- Rick TYler
(An old guy with a sticky shift KEy)