Fluid Viscous Dampers internal force

03-Analytical/modelling capabilities
Abbas Mirfattah
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Joined: 21 Feb 2021, 23:19
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Re: Fluid Viscous Dampers internal force

Post by Abbas Mirfattah »

Thanks for your reply.

I double checked my model. The problem was the links representing the foundations, the link elements were not defined in the element connectivity module. Although, the model runs now (attached). However, the  issue still exists. I have done a lot of trials, and I found that as soon as I introduce the global Rayleigh damping model of 5%.the internal forces of dampers start to deviate from that of connecting frame elements (i.e. horizontal steel UCs in this model).The problem is that I have a twice larger axial load in connecting steel members than that of the damper. Please advise how I can have a 5% global damping with dampers in the same system without encountering this problem.

Thanks 
Abbas Mirfattah
Posts: 9
Joined: 21 Feb 2021, 23:19
Contact:

Re: Fluid Viscous Dampers internal force

Post by Abbas Mirfattah »

I also changed the global damping to none and the issue still exist.
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seismosoft
Posts: 1000
Joined: 06 Jul 2007, 04:55

Re: Fluid Viscous Dampers internal force

Post by seismosoft »

As we explained in a previous post and through our emails, it is not unreasonable that there is a difference in the internal forces. The differences are equal to the damping forces intorduced by Rayleight damping. Furthermore, the damper also gives forces that are equilibrated by the adjacent elements. It seems that it is more a modelling issue, rather than a problem in the use of the programs, hence it is probably better that you discuss your problems with your supervisor.
If however you feel that there is some sort of discrepancy in the results that you get, let us know, but you need to be specific, e.g. the equilibrated forces in one particular node do not make sense. We are a small company, we simply do not have the capacity to check all the models of all our academic user.
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seismosoft
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Joined: 06 Jul 2007, 04:55

Re: Fluid Viscous Dampers internal force

Post by seismosoft »

Note that it is better to have a DSP+FRC convergence criterion, rather than a force-based only criterion.
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ruipinho
Posts: 159
Joined: 07 Dec 2013, 14:37

Re: Fluid Viscous Dampers internal force

Post by ruipinho »

Personally, I tend to avoid the inclusion of equivalent viscous damping in my models, which has been extensively shown to be problematic and to potentially lead to important underestimation of the response of structures, especially when these are pushed highly into the nonlinear range.

There is really not a strong physical reason to include such additional damping, especially, I repeat, if one is more interested in the nonlinear inelastic response range, rather than in the linear elastic one.

And if you one really feels the need to include it (e.g. for a higher numerical stability of the analysis), then care should be taken in selecting the right damping model (I do not consider Rayleigh damping to be the best option) and a lower damping coefficient (the often used 5% value is, imo, way too high (1% could be more than enough), even more so if then applied in tandem with the initial elastic stiffness of the structure).

Rui
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