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Re: Pin Connected Members

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On May 29, 2008, at 4:59 PM, Garner, Robert wrote:
What is the purpose of these minimum dimensions?
Which minimums are you talking about? And what sort of calculations are you using to make the stresses 'calculate OK' As near as I can tell from some FEA work I've done, they're to keep you from adding material that isn't effective or complicating the stress calculation. The actual stress distribution in a pinned connection is very complicated to reproduce manually. It's easy to do with FEA except that the resulting profile shows very high peak and secondary stresses for which the AISC has no criteria and which the average designer has a tough time interpreting. As one example most of the load is carried within 1 hole diameter or less on each side of the hole; There's a temptation to decrease the net stress by using a wider bar: A wider bar makes the average stress--load/net area--go down but not the actual stress, since the added material isn't working. Conversely, since the stress concentration around the edge of the hole isn't accounted for in the AISC design methodology. What may seem like a suitable section on the basis of the average net stress ends up with an unacceptably high local stress, which you wouldn't see except perhaps with an FEA model.

Can we deviate from these minimum dimensions if the stresses calculate O.K?
The short answer to your question is no--the Code language uses 'shall' in specifying minimum dimensions, meaning they are mandatory. You'd be sticking your neck indefensibly far out by taking exception to the stated good practice for such connections.

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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