Different between periods

[MA_M]

I have a 3D-RC frames. The period which was determine from the ETABS is 0.87, while in seismoStruct based on eigenvalue analysis, it was calculated 0.54. What is the reason for this difference? I didn't change the default of the program.

[huffte]

There are many reasons the two periods might differ, MA_M.

You would need to make certain that both models are treating mass the same. Do both models employ lumped mass, rather than consistent mass, matrices? Do both models have identical element densities and do both models automatically generate additional mass from the element densities? Do both models treat auto-generation of mass from dead loads identically?

Stiffness is of concern as well. Do both models use the same value for E (Young's Modulus)? Do both models treat the rigidity of joints equally? Do both models have identical adjustment factors on calculated EI?

Mass and stiffness. The two combined completely determine natural frequencies of the structure.

Try to imagine any possible difference, in addition to those I have mentioned here, in mass or stiffness between the two models.

Best of luck.

[MA_M]

Thank you for your answer. I create the model by Building modeller. So, the mass calculated based on additional mass and assign to all of the beams automatically. In the ETABS, I used end-offset (0.5), but in seismo, It was calculated automaticaly. The ETABS file analyzed based on 0.5 and 1 for cracking in the "assign menu - frame - properties modifiers - moment inertia 3", in beams and columns respectively. there is no more different. I attach the files. could you please check it for me? Someone say to me that this problem is usual for RC structure!

seismostruct file: https://ufile.io/zgz6p
etabs file: https://ufile.io/mph9r

[huffte]

Certainly, the reduction in I for some of the elements in ETABS has a significant impact. I believe that your Seismostruct model includes no such reductions. Also, did you verify that both models use the same value for E? The end offsets evidently are not identical as well. Finally, I would recommend hand calculations to estimate the total structure mass, followed by inspection of the reported masses in both the ETABS and Seismostruct models to locate any differences in the masses.