Seismosoft | Earthquake Engineering Solutions

Dear Seismosoft,

Now I am carrying out some nonlinear dynamic time-history analysis of multi-storey frame. Before running nonlinear dynamic time-history analysis, I run the Eigenvalue analysis and Static analysis (non-variable loading) in order to check whether the applied mass and gravity loading is right. But there are some problems with me.
1.When running Eigenvalue analysis, I set 6 directions in "Global Mass Directions", then the results shows the first period of structure is less than the second and the third period, it is unacceptable. But when I set 3 translational directions in "Global Mass Directions", then the results become normal and it is close to the results which was calculated by sap2000. Which should I adopt?
2.Generally the mass which need to define in dynamic time-history analysis is applied at the beam-column joints, but when I use this approach to model mass and I run the Static analysis, I check "carry out stess recovery" and "Automatically Transform Masses to Gravity Loads", but the moment of beam is obvious wrong. I read the Help System, then I use the material specific weight and sectional additional mass to account for the mass and the gravity, and I run the Eigenvalue analysis and Static analysis,the results is normal. Is this approach right in nonlinear dynamic time-history analysis? And if I use the material specific weight and sectional additional mass to account for the mass, does the program transform the distributed mass into equivalent point lumped mass?

Best regards,
Wenjun

Hi,

1) read the Help System or search this Forum for "eigenvalue order" and you will realise that there is nothing wrong with the results you are obtaining with the 6 mass dofs activated, since it is the participation mass, rather than the output order, that should dictate the relative significance of the modes you are obtaining from the analysis

2) yes, if you are interested in obtaining correct beam internal force values in your dynamic analysis, then you should certainly consider the use of distributed rather than lumped mass, accepting however that by doing so the analysis will become slower, since the mass matrix is no longer simply a diagonal matrix (the program does not transform the distributed mass (in your case defined through material specific weight and sectional addition mass) into lumped mass, but rather explicitly considers the distributed mass terms). Of course, in your specific case, the analyses will then be even slower because you are activating 'stress-recovery', in order to obtain precise beam internal forces.

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