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)

[Paul Copioli]

Rick,

I knew it started with a T, but I always screw the name up (the video will NEVER leave my memory).

Every part of my message dealt with buckling, except the last sentence which mentioned dynamic conditions. I should have elaborated on what I meant by dynamic conditions, but I wanted to keep my message short. The point I was trying to make was that when dealing with buckling, mass is not a contributor to the critical load. However, when dealing with dynamic conditions (aka natural frequency, resonant frequency, etc.), mass is a big player. After re-reading the message, I can see how someone would make the buckling connection to the last sent ace. I apologize for the confusion.

-Paul

[Gdeaver]

Don't forget torsional stress. Allot of load analysis formulas ignore torsional stress because they are analyzing a load bearing beam in 1 or 2 dimensions. The arm will be subjected to torsional stress. We're using a 2" square tube with a 1" tube foamed and epoxied inside to account for these stresses. (fiberglass). Tube in tube can help also butted tubing can help.