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Modal damping

ian ward
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Hello forum,

I wonder if you could supply me with some guidance. We are looking to perform a seismic response spectrum analysis with different levels of damping in different parts of the model. Is it possible to do this in Autopipe, say through the use of composite modal damping? If Autopipe cannot calculate the composite modal damping values directly, we may wish to calculate these ourselves and specify the damping level for each mode. Does Autopipe allow different damping ratios to be specified  for different modes in a response spectrum analysis? In case it is relevant, I should also say that we would like to use the MSRS option to input different motions at different supports.

Many thanks in advance......

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    Greetings, 

    What exactly are you trying to model?

    What kind of load and how is it being applied?

    Why the requirement for variable dampening across the model and across modes?

    At this time, June 2021, AutoPIPE assumes the same damping ratio for all modes except when N411 damping is used. In that case the damping varies with the modal frequency between 2% and 5%.

    Suggest to read AutoPIPE help on N411 feature to see if it satisfices your requirements. 

    Regards,
    Mike Dattilio
    Design Analysis Engineering Group
    Senior Analyst Bentley Systems Inc
    =============================================================================

  • 0 Offline in reply to Mike Dattilio

    Thanks for your response Mike.

    We are modelling a support frame which supports some large diameter pipework, which in turn supports some small diameter pipework.

    It is a seismic, response spectrum analysis. Currently with the same spectrum being applied at all supports, but we will probably move to MSRS.

    The design document that we are using specifies one level of damping for the frame/large diameter pipework, and a lower level of damping for the small diameter pipework. This is why we are investigating if different levels of damping can be applied to different parts of the model, or whether we can look at the mode shapes and allocate the damping ratio to each mode depending on which part of the model is responding.

    Regards

    Ian

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  • 0 Offline in reply to Mike Dattilio

    Thanks for your response Mike.

    We are modelling a support frame which supports some large diameter pipework, which in turn supports some small diameter pipework.

    It is a seismic, response spectrum analysis. Currently with the same spectrum being applied at all supports, but we will probably move to MSRS.

    The design document that we are using specifies one level of damping for the frame/large diameter pipework, and a lower level of damping for the small diameter pipework. This is why we are investigating if different levels of damping can be applied to different parts of the model, or whether we can look at the mode shapes and allocate the damping ratio to each mode depending on which part of the model is responding.

    Regards

    Ian

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