ME-PE with a (dumb?) question about trusses

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jean15paul_PE

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I hope it's ok for an ME to come in the Civil forum and ask dumb questions. :)

I was trying to explain what a truss is to a non-engineer the other day and it got me thinking. We did a lot of truss problems in my Statics and my Mechanics of Materials classes in school. It even was used pretty commonly in my Machine Design capstone course. My understanding of a truss is (by definition?) a structure made up of 2-force members with pinned ends. Therefore each member can only take compression and tension... no shear, no bending.

That being said, I don't think I've ever seen a truss structure in the real world with pinned ends. I'm not a Civil, so I've never worked on a truss. But all of the truss-like bridges, cranes, roofs, frames, etc that I've seen are connected with welds, or multiple rivets/nails, or those shear plate things used on wood beams. (What are those called?) All of those connections could carry a moment. That would make it a statically indeterminate structure, since each member can carry tension/compression, shear, and bending? How are these trusses handled in the real world? Are the members assumed to be only tension-compression? Or do you have to treat the structure as statically indeterminate and include moments?

Thanks for putting up with my questions? :)

A little background about me. I'm a mechanical engineer who has specialized in structural design and analysis of mechanical structures, both FEA and classical hand calcs... mostly stress analysis, but also some dynamic analysis also.
 
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vhab49_PE

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In real world conditions, you are right, they aren't truly pinned, nor are they truly fixed, which creates a much more difficult analysis. I would say that 99% of the time we still analyze them as simple pin-pin connections. Saying that, I analyze a LOT of old trusses, but have never designed one from scratch.
 
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