Mode 2 result

[saclovitzky]

Hello,

Pls. find attached model and screenshot.I'm just wondering why Mode 2 in Modal analysis is showing to be "twisting",normally the first 2 modes are always deflecting in orthogonal directions. Am I correct on this?

Any ideas will be greatly appreciated.

[Stephane.kapetanovic]

It seems normal. In that case, modes 2 and 3 are neighbors. To convince yourself of this, you can change the section of bars 3 4 24 25 to BR 300x600 and release ART-ART beams 30A32 35A37. After that you have the first two modes in the main axes.

[saclovitzky]

I've tried this on a different software and my assumption is correct ,the first two modes shapes are acting on the two principal axes,not twisting.

[Simau]

Hi @saclovitzky 

How do you interpret that for mode 2 the participating mass is zero, then is there really twisting?

[saclovitzky]

@Simau try it.

[saclovitzky]

@Simau it is still the same, when you convert the SW as mass, mode 2 shape is twisting?Any idea what cause it to twist rather than moving/deflecting to major axis?

[Simau]

@saclovitzky 

As i told you in msg 4, there is not really twisting as the mobilzed mass on mode 2 is  zero.

You said : I've tried this on a different software and my assumption is correct

Could you post here frequencies and mobilized mass for mode 1 2 3 you obtain with the other software

[saclovitzky]

@Simau as I mentioned before take the SW as mass source, there is really a twisting in mode 2.Did you try it yourself?

[Simau]

Yes i tried it

Look at screenshot in msg 4, what i say is that in spite of theorical tsiwting in mode 2, no mass it activated by this mode

[saclovitzky]

@Simau pls. see attached files. Despite converting SW as mass source,still the same thing.

[saclovitzky]

It looks like Mode 3 should be Mode 2 and vice versa!

[Simau]

@saclovitzky 

Mass was already converted in your first model (density was taken into acount automatically).

In the secon model you converted the mass but you must tick neglect density

Yes i saw that mode shap shows a twisting, but with no mass mobilized,  no efforts will be created.

May be the support team could give its opinion

[Simau]

If you complete your model with sismic cases you'll see below that there is no efforts neither reactions in twisting mode 2.

[saclovitzky]

We need help from the development team! 

[MartinTrs]

@saclovitzky Please elaborate - why do you think that the first two modes always have to be orthogonal?

The modes of modal analysis depend on stiffness of the structure in different directions. I could easily create a structure that has first 9 modes in the weaker direction (see the picture). 

In your case the stiffness difference in different directions is also large (in one number of columns is a lot smaller and the span of the beam is a lot larger than in the orthogonal direction)

[saclovitzky]

Generally,if you have a regular structure that should be the case.