ASCE 7 Seismic, Inverted Pendulum

[vmirat]

ASCE 7, paragraph 12.2.5.3 requires that the base moment on an inverted pendulum be applied uniformly up the column to half at the top. What is their rationale for this and how do they want you to apply it?

 

[JoshPlumSE]

There is no redundancy in an inverted pendulum..... Therefore, they ask you to design for a minimum moment at the top of the pendulum.

Plus, there is a dynamic effect (rotation at the top of the pendulum rather than pure translation) that isn't really accounted for in the static analysis procedure.

At least that's my take on the requirement.

 

[vmirat]

I kind of figured the same thing. I was thinking that they want to account for any possible partial fixity at the top that could cause additional moment. The thing that threw me was that the moment was linear to the top. I was thinking the moment diagram was all on one side of the beam, but then I thought they must mean that it varies linearly, goes through zero, and then goes negative. However, you still design for the maximum moment which is at the base, so I'm not so sure what value there is in applying half the base moment to the top of the column. Do you use this condition to figure drift? Does this moment at the top transfer to the structure supported above?

 

[TERIO]

I haven't read it, but the paper "The behavior of inverted pendulum structures during earthquakes" by George Housner might be worth getting your hands on. Here is a link to the abstract:

http://www.bssaonline.org/cgi/content/abstract/53/2/403