This content has been marked as final.
Show 6 replies

Modal Damping vs Rayleigh Damping
Nur Kholish Majid Jul 10, 2009 4:27 PM (in response to Nur Kholish Majid)Another basic question is When I should apply damping ?
I wish somebody can give me real life example on this case....
Thank You !!!!
Modal Damping vs Rayleigh Damping
Brian Zias Jul 10, 2009 8:50 PM (in response to Nur Kholish Majid)Ahmad,
Damping is important whenever you are interested in the transient response of your structure. See this image. The xaxis is time, and the yaxis is a nodal displacement. A load is applied for 0.5s, then removed. Any excitation in the time domain will be followed by a decayed amplitude of response as energy is converted in the system.
The best (only?) way to determine damping behavior is emperically. The Simulation help as a decent explanation of the different models.
There are plenty of texts out there on damping (and Google) if you get into it. As a first pass, just enter modal damping with something on the magnitude of 0.01. But be ware that your dynamic response amplification is intimately related to damping ratio.
Brian
time hist.JPG 46.2 KB



Modal Damping vs Rayleigh Damping
Nur Kholish Majid Jul 11, 2009 1:39 AM (in response to Nur Kholish Majid)Hi Brian,
Thanks for answered my question.
What is the different between Modal Damping vs Rayleigh Damping ?

Modal Damping vs Rayleigh Damping
Nur Kholish Majid Jul 12, 2009 2:13 PM (in response to Nur Kholish Majid)Below is what I have found in Wikipedia :
Damping
Any real structure will dissipate energy (mainly through friction). This can be modelled by modifying the DAF:
DAF = 1 + e ? c?
where c=\frac{{\text {Damping Coefficient}}}{{\text{Critical Damping Coefficient}}} and is typically 2%10% depending on the type of construction:
* Bolted steel ~6%
* Reinforced concrete ~ 4%
* Welded steel ~ 2%
Generally damping would be ignored for transient events (for example, an impulse load such as a bomb blast), but would be important for nontransient events (such as wind loading or crowd loading).
Find more here : http://en.wikipedia.org/wiki/Structural_dynamics
But I still cant understand :
The different between Modal & Rayleigh Damping ?\
When I should use Modal and When I should use Rayleigh Damping ?
Thank You !!!

Re: Modal Damping vs Rayleigh Damping
Brian Zias Jul 15, 2009 12:53 PM (in response to Anna Wood)Anna has the right idea. That google search actually turns up a wealth of information, including a PDF describing how to come up with the Rayleigh alpha and beta coefficients.
In linear dynaics, modal is easier to use, since all you must do is input the damping ratio by mode. e.g. if you want to assume 1% damping of your structure, just put in 0.01 for all the modes you are solving for. Remember modal problems are sensitive to the number of modes solved for, always check your mass participation after solving.
If you get into a nonlinear dynamic problem, you'll see that the only damping formulation available is the Rayleigh, so you are forced to use that in NL studies.

Damping coeffficients.pdf 187.9 KB


