Strength of screw threads

I'm an EE so this is outside my area, but....
While looking at <http://en.wikipedia.org/wiki/Hyatt_Regency_walkway_collapse with
a friend [his PhD is chemistry, and he writes crypto code, so no help THERE..] we go onto thread strength.
My recollection was the rule was "With good threads, the rod shall break {in pure tension} before the threads fail" but I do not know where that came from. I seem to recall that was based on a nut depth GT/EQ to one diameter as well....
But I've never seen such written down. How right/wrong am I?
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You are right, there is such (or should I say several similar) rule(s) of thumb. Like all such rules, they are quite useful in the circumstances where they are valid and a disaster waiting to happen in the circumstances where they are not.
Standard nuts for ordinary screws and bolts are, for example, a little thinner than the nominal diameter. A standard 1/2" nut is 7/16 thick. This is a good, conservative, well designed situation.
The general answer to where a specific threaded connection will break involves the characteristics of the nut and bolt materials, the threaded depth, and the particular thread characteristics.
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