Calibrate model to give desired frequency

[alannakate]

I have a simplified model made in SAP2000 of a small structure that I constructed.

The structure consists of 3 levels above ground level. Each storey is 400mm in height so the total height is 1200mm. The columns are continuous from the base to the top of the structure and are made of steel tubing. The top level has plate masses added to it.

My SAP model has the floor masses lumped at nodes and the elements connecting the nodes has the properties of the steel tubing (square, 20mm outside diameter, 2.5mm thick, density of 66126.4 N/m^3). I have divided each element into 5 sections.

The floor masses are: First Level (above the ground) = 35.51N + 1.43N, Next Level = 35.51N + 1.43N, Top level = 302.6N + 1.43N.

I am trying to change the stiffness of the model to get a first mode frequency (first bending mode) of 8.38Hz. I am changing the stiffness by changing the property modifiers. I start with all the modification factors set to 4 since the structure has 4 columns and then I start changing the moment of inertia about the 2 and 3 axis to get a frequency of 8.38Hz.

For some reason when I increase the moment of inertia factors I can't get 8.38Hz, my frequencies go from being positive to negative and jump around quickly.

Am I doing something wrong? How can I calibrate my model by changing the stiffness to get the frequency I want? and what does a negative frequency in SAP mean?

I have attached the model so you can see what I have done.

Thanks!

 

[alannakate]

Nevermind, I figured it out!

 

[Ryuk]

hello
i have the same problem, how did you resolve this?

 

[alannakate]

I redrew my model so that instead of a lumped mass system it was more similar to the real life structure. I used four columns and plate elements for my floors. Then I allowed the mass of the columns to be distributed along their length while all other masses got lumped at the nodes on each corner of the plate elements (see attachment). Then the frequency was much easier to calibrate and didn't jump all over the place.

 

[johnbridge231]

That's very good news.

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html