hybrid columns in 2-story building

[jm69]

We need to create a model of 2-story bldg. using technics of identification of systems to calibrate natural frequencies. Using SAP2000 program we create a FEM model with frames and shells, but we are still far away the convergence to the frequencies obtained from experimental methods.
My question is, if the building was built with structural steel W12x58 encased in a square concrete column, how would you combine this 2 elements to create a unique element (as frame/wire)to produce the stiffness matrix?
I hope someone can give a tip.

 

[GregLocock]

I'd use the method of composite equivalent sections to obtain a revised estimate of EI for each beam. However, you could just overlay concrete and steel elements between the same nodes, if you are only interested in low frequency behaviour. This is a bit dangerous.

Most likely you have the boundary conditions wrong, or the wrong mechanism is being analysed. How much do your frequencies differ from the real ones? Have you checked the mass of the structure?



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[jm69]

Thanks GregLocock,
But I have already checked the structure mass, even frequencies from building are 3.14hz in NS, 3.25 EW, 5.56 torsion and 7.89 vertical, but from the model we get 1.55 NS, 1.72 EW, 1.91 torsion and 3.63hz vertical.
Since we have lower frequencies, it seems our model needs be more rigid, but all structural elements were taken account: 15-inches thick shear walls, steel beams with rigid connection to column, and a stiffed foundation.
What kind of boundary conditions could be considered in order to make a rigider model, or do I have the wrong picture?

 

[GregLocock]

Wow, it's not often people get a soft model.

have you checked your units for E?

Roughly speaking you are low by a factor of 3 or so in stiffness terms, that is an awful lot.

Are you sure you have no pin joints? Are all your members connected?

Have you looked at the mode shaes - are they reasonable?



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

I notice your torsional result is the worst of the lot - how did you get J for your columns? and the concrete?

How did you model your joint efficiencies in torsion?


Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.