Learning Forum

[Keenan Rodewald]

I am performing a response spectrum analysis on a system with many modes, each with it's own damping ratio. 

 

The spec I am using provides SRS curves for many damping values. How would I choose which to use?

[Avnish Pandey]

Hi,

When multiple Shock Response Spectrum (SRS) curves are provided for different damping values, the appropriate curve to use depends on the damping characteristics of the system you are analyzing and the specific requirements of your analysis.

Determine what you are trying to achieve with the SRS analysis. If you’re looking to ensure the survivability of a component, you might choose a curve with a lower damping value to represent a more severe environment. Conversly, if you're interested in performance during operation, a curve with a damping value closer to the system's actual damping might be more appropriate.

Some industries have standard damping values for SRS analysis. Check if there are any guidelines or standards that dictate which damping values should be used.

[Chandra Sekaran]

• •  

You will need to use APDL commands to input multiple curves with different damping ratios and to specify the damping ratio for each mode

The spec I am using provides SRS curves for many damping values. How would I choose which to use?

You should input all the spectral curves along with their damping ratio values using SV,damp,sv1,sv2,...  Refer to FREQ and SV commands to understand the format.

I am performing a response spectrum analysis on a system with many modes, each with it's own damping ratio. 

You can use MDAMP command to input the damping of each natural frequency

[Keenan Rodewald]

Thanks Avnish

The standard recommends testing the system to determine damping. My confusion is that there will be different damping values for each system mode. 

Maybe a more concrete example would be helful. If there are 3 SDOF mass-spring dampers connected in series, and the leftmost is connected to ground. Say the first body has damping ratio = 10%, the second = 5% and the third 1%.

 

If I am trying to find the repsonse of the third body, which damping value would be used for input. Using 1% does not seem correct since this body is isolated from ground motion by 2 highly damped bodies.