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
