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Re: Period of rigid non building structure

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The cantilevered model should give you one boundary on the possible period range, but depending on the anchorage pattern of the skid, the anchors themselves, and the skid members and connections, the period could be substantially longer.  I'd suggest bounding your solution, getting the longer first mode period using FEA like Jordan suggests.  The two checks should get you in the ball park.


"Jordan Truesdell, PE" <seaint2(--nospam--at)> wrote:
Chris's idea of doing it from scratch (cantilevered beam) is a good one.  You could also throw it into your favorite FEM, or see if Blevins "Formulas for Natural Frequency and Mode Shape" has it.

Joseph R. Grill wrote:
I am looking at (the overturning only, per the clients request) an industrial stack that will be constructed of basically 24? dia. ¼? thick pipe.  The stack is about 25? tall and will be supported on a steel skid.  ASCE 7-05 has an equation 15.4-5 for a Rigid Nonbuilding Structure which must have a ?T? less than .06s.  If not, the way I read things is that if it is not rigid then V is calculated from 12.8-1 using Cs from 15.4.1 (2).  If all of this is correct, I guess that I could assume a rigid structure (since V will be larger using 15.4-5 and wind will probably govern overturning anyway) or calculate the period and see if it isn?t rigid and use the other equations that I have listed.  I don?t really have a problem assuming a rigid structure and using the larger V, but I would like to see what happens if it isn?t.  Now to my other question.  How can I calculate the period of a non-building structure such as this.  Basically a vertical pipe on a steel skid.  In all my references, I can?t find an example.  Can anyone out there help me out?
Joe Grill
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