W3 and w4 option in direct transient analysis

In direct transient analysis it is recommended that we need to convert structural damping to viscous damping using w3 option in param or Tstep load collector, but I'm unable to get exact reason why the conversion is needed and what will be the effect on results if we don't convert it.

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v2022.3

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Rajashri_Saha

Hi Shiva,

W3 can be used to convert the overall structural damping to equivalent viscous damping for transient analysis. For estimating W3, you can try: 2*pi*[min_F, max_F], which are the smallest and largest non-rigid body mode frequencies from Normal Modes analysis.

W4 can be used in transient analysis to convert element Structural Damping GE matrix to equivalent viscous damping. This is because complex matrices are not handled in Transient response. In frequency response you can directly add structural damping GE matrix as complex stiffness term.

If you are not using this then viscous damping will be ignored and this will make a difference in result.

Thanks

Rajashri

Shiva Prasad

Hi rajashri ,Thank you for the response.

Can you elaborate more I'm unable to conclude anything from this.

Rajashri_Saha

Hi rajashri ,Thank you for the response.

Can you elaborate more I'm unable to conclude anything from this.

Hi Shiva,

You can verify the result by having two runs. one with W3, W4 and one without that.

Let me know if you find any difference.

Thanks

Rajashri

Alberto Campagna

Hello Shiva,

when you define global structural damping with G or GE the solver will add it to the global stiffness matrix of your model and create a complex stiffness matrix. Now, this is not allowed in transient analysis so this structural damping has to be converted into viscous damping and added to the viscous damping matrix, usually referred as B. For the conversion to be effective you need to add W3 (for G) and W4 (for GE). If you don't specify these values, the damping will not be applied.

If you want to have an insight on this you can read this article with focus on the transient chapter:

https://community.altair.com/nav_to.do?uri=kb_knowledge.do?sys_id=9e86ff4297338ed0da583b47f053af83%26sysparm_view=knowledgearticle

Hope this helps.

Alberto