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Period of rigid non building structure

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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?

 

Thanks,

Joe Grill