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Active Contributor
Posts: 32
Registered: ‎03-28-2012

Inconsistencies in damping methods for Footfall Analysis

598 Views, 4 Replies
03-25-2013 05:45 PM

Hi All,

 

I was wondering whether anybody could explain to me the large differences I am getting in my response factors (RFs) for Footfall Analysis when using constant 3% damping and Rayleigh damping set up as explained below.  

 

My contention is that for the range of fundamental frequencies I am considering (7.04Hz to 15.05Hz) that the Rayleigh damping (based on omega1 and omega 2 being conservatively assumed as the minimum and maximum fundamental frequencies listed above) gives approx. 3% damping for all modes here (see attached excel spreadsheet for my calculation of damping values for various omega,i frequencies) ie we have a low Beta value affecting the stiffness matrix.

 

Based on this thought process I have run the analysis on the same model changing only the approach to damping from 'Constant' to 'Rayleigh' and running the modal analysis portion of the Footfall Analysis up to 15.1Hz (a previous run showed that 15.05Hz was the first mode over th 15Hz cut of we are assuming).  The RF maps for these plots vary significantly (see maps attached)

 

I was wondering whether anyone could give me some insight into why this is occurring, I am going to make a spreadsheet based on the Concrete Centre/Arup method (which I am using for this analysis) to hopefully guide me towards the correct answer.

 

I have also attached the model...unfortunately the Rayleigh damping is not set up in this model becuase Robot crashed and lost all of this mornings work for me but the parameters are as per the excel spreadsheet.

*Amendment due to the ridiculously small 1.5MB filesize limit I have had to remove the meshing and results.

 

Many thanks,

 

Rich

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Active Contributor
Posts: 32
Registered: ‎03-28-2012

Re: Inconsistencies in damping methods for Footfall Analysis

03-27-2013 05:00 PM in reply to: richard.whitfield

Hi All,

 

Just commenting back on this as it has slipped to page 2.  Any idea of a timeframe for this one?

 

Thanks,

 

Richard

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Product Support
Posts: 4,445
Registered: ‎04-26-2010

Re: Inconsistencies in damping methods for Footfall Analysis

03-28-2013 01:01 AM in reply to: richard.whitfield

hopefully today.



Rafal Gaweda
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Product Support
Posts: 4,445
Registered: ‎04-26-2010

Re: Inconsistencies in damping methods for Footfall Analysis

03-29-2013 04:30 AM in reply to: Rafal.Gaweda

Have you sent us correct model?

This one has no masses considered in footfall.

 

reylaigh.jpg

 



Rafal Gaweda
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Product Support
Posts: 4,445
Registered: ‎04-26-2010

Re: Inconsistencies in damping methods for Footfall Analysis

03-29-2013 05:07 AM in reply to: Rafal.Gaweda

If I consider masses as shown below and correct Rayleigh data I get comparable results.

 

reylaigh1.jpg

 

damping.jpg



Rafal Gaweda
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