SAP2000 modelling: Imaginary Part of the FRF plot does not show the mode shape correctly

[mahi93]

Hi Guys!
I modelled a pin-pin supported beam, defined steady-state load case and load pattern, impose a 1 N steady state load on a point, and analyze my beam. then by plotting the values of bode plot, and multiplying the magnitude by sin of the phase, I have now 3 plots of imaginary parts for 3 points in the left, center and right part of my beam. the value of the calculated imaginary part at each natural frequency was plotted to define the mode shape. however, these mode shapes are not correct at all. I wonder what \i might did wrong that this happened. worth mentioning that the deform shapes and joint displacements in modal load case are exactly correct.
it would be really helpful if someone tell me what can be wrong (the calculations or the modeling?), etc.
Many Thanks
Mahnoosh

 

[GregLocock]

I suspect you need to show us what you are talking about. How do you know what the correct mode shape is?

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[mahi93]

Dear Greg Locock,
the structure I modelled is a simple pin-pin supported beam. And I analysed it in a 2-d analyse option; so, the mode shapes are known. But when I extracted the phases and amplitudes from 'show plot function' feature of SAP2000, and I calculated the mode shapes from them, they are not compatible with analytical solutions.

 

[GregLocock]

I realise your work is top secret but without actually seeing the data i don't think anyone can help.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?