Originally posted by Cy Shuster:
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How do they compensate for this in, say, suspension bridges (civil engineering here, not bellywork :-)?
And is there a significant amount of friction between neighboring coils? That seems hard to believe (but I've been surprised by physics before).
--Cy-- [/QB][/QUOTE]
We don't compensate for this. We calculate deflection (and in fact most bad designs result in excess deflection, hardly ever do we actually break something <grin>). Deflection is purely elastic, we carefully design to avoid the plastic region, and of course way short of the failure zone. (There are exceptions. Properly torqued bolts are intended to be loaded into the plastic zone. This is why we never reinstall a used bolt. Also why I NEVER use a dry fastener, I even have been known to lube wood screws. Whoops, I'm at risk of digressing.)
Friction between neighboring coils. Well, you guys are the piano wire experts, I'm reasoning from guitar experience.
Ever restring a guitar? Warning: does not apply to gut/nylon strings! The standard method, that goofy wrap knot, is of course unnecessary. Here's the easy way. Put the ball end on. Put the other end through the hole in the peg. Pull it through most of the way, just leaving a little sag in it. Holding the sag with one hand, kink the wire at the end of the hole in the peg. Now wind it tight by turning the peg. You're done. The kink holds it just long enough for the coil to peg and coil to coil friction to take over.
The fact that piano and guitar strings break when we overtighten suggests we are normally loading these strings very close to the tensile stress limit - maybe near plastic, maybe sometimes in it. Creep requires two criteria be satisfied though: high enough load, AND high enough temperature.
