Hello!
I am setting up a seismic modal response spectrum model following your youtube tutorial. I have some questions...!
Following the modal analysis I was struggling to get 90% mass participation, even using 200 modes, is this normal? Seems a large number compared to the example you were showing in the tutorial. I wondered if it was a symptom of a mistake? Additionally my minimum frequency is less than 5Hz, and this does not change with increased mode numbers – see table extracts below.
I am struggling to understand the difference between consistent, lumped with rotations and lumped without rotations – how much does matter which I choose and how will it affect my results?
I am not sure whether I should be choosing Modal or Seismic analysis? What difference will this make to my results. It does not seem to affect the mass participation, yet I read somewhere that if I choose the seismic option it should increase the number of modes automatically to reach 90% participation. This does not seem to happen?
Which iteration should I choose? Subspace iteration is the default, I saw on the tutotial that Block Lanczos is faster, yet it was not used in the example – why is this? And what is the difference between Block Lanczos and Lanczos?
If I have missing eigenvalues but I choose ‘Yes’ to continue does this resolve the issue? Is this anything to worry about?
I have chosen Newmark Groups 1 and 2, I assume the groups refer to X and Y? I do not need to include Z direction for this building according to Cl 4.3.3.5.2 (1).
Does it matter if the main mode is the same for X and Y?
I have set up my slabs as shell elements, would I gain any benefit from setting them up as rigid diaphragms? The youtube tutorial has unmeshed slabs in at each level - are these rigid links? Does this not mean the masses are not taken into account? Extract from tutorial:
Thanks,
Rosie