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Re: Stripping Treads

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On Oct 3, 2006, at 6:33 AM, Jordan Truesdell, PE wrote:

This brings up a reasonable question: Is it worth running five or six pull tests on a similar condition in the lab? You certainly could spend a good amount of dollars in time running down a theoretical solution, and still not being confident that the math matches the real world.
Sounds like classic paralysis by analysis. What's your criteria? What do you do if some of the tests pass and others don't? How do you really know what the engagement will be? Why 5 tests? or 6? WIll tests on new hardware match tests on hardware that's been in service for a while? What if the tests don't match the calculations? (If the tests come out with greater loads, do you dare push it? If they come out low does that mean that the rest of the anchors are too weak?) Most important, has your contractor always done it that way and never had any problems? Tests are only good if you know your test matches the reality, but I'd suspect you never really know that for anchor bolts.

What you have is a situation where the incomplete engagement is eating into your reserve. You don't actually know what your reserve is, only that there's usually enough, either in the specified loading or the threaded connection so that a properly made-up connection usually holds. This is really a feel-good calculation. Do it using the estimated engagement less 2 threads--one each for the imperfect threads on the nut and the anchor. Don't try to cut it too fine, because it really isn't all that accurate.

I've had people ask me similar questions about improper engagement, but for machinery, it's usually a no brainer, thank god. Fasteners are almost always preloaded and full threads are required. If it doesn't engage, you replace it on the spot, because fatigue is important. Moreover tolerances are tight enough, so that if the specified fastener doesn't engage, it's a sigh that there's been a screw-up somewhere else that needs fixing. When that's done, the poor engagement is usually not a problem.

 Christopher Wright P.E. |"They couldn't hit an elephant at
chrisw(--nospam--at)   | this distance" (last words of Gen.
.......................................| John Sedgwick, Spotsylvania 1864)

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