Rigid or Flexible

[rook1]

how do i determine if a structure s rigid or flexible?
i know as defined, natural frequency of 1Hz sets the boundary.. but, how do i compute for natural frequency?
would height over length less than 4 be generally assumed as rigid?
would buildings 10 storey or higher be assumed flexible?
would plywood diaphragm mean its flexible?
help much appreciated!

 

[UcfSE]

You have a lot of reading to do. The answers to your questions can easily take up a few semesters of class. We obviously cannot do that for you here.

Frequency of 1Hz or less beign flexible is a criterion defined by the ASCE with respect to wind loads. I would not use it as a general rule of thumb per se, but a structure with a natural frequency less than 1 Hz is probably flexible by most standards. In general, the natural frequency of a building will depend on its stiffness and its mass. The damping present in the building will also influence the dynamic response considerably.


Try checking reading some books on structural dynamics. Dynamics of Structures by Chopra is a good text from which to learn.

 

[MSDw]

There is no such a thing called: Flexible or Rigid building !!! i never heard of such classification. and if it existed it should depend on some basic reference to compare with!.

 

[rday]

MSDw,

ASCE 07-02

Section 6.2

"Building or other structures, Rigid. A building or other structure whose fundamental frequency is greater than or equal to 1 Hz."

Of course it defines flexible as well.

Rik

 

[MSDw]

You say this building is rigid RELATIVE to the supporting soilstructure ( or substructure) . Take for example a moment-frame building with N = one storey you can approximate its frequency with 10*N = 10 Hz, now if the soil supporting it was stiffer ( higher frequency than 10 Hz) then the building is flexible RELATIVE to the soil and the converse is true.

One point is to be away enough from the soil frequency.

Maybe you need the building frequency in as for a servicibility limit state such as comfort and vibration limitations. For that i think you deal with the elements inside in addition to the building as a whole.