I modelled a building (Dual shear wall/frame building) by using ETABS and PERFORM 3D with the same meshing. However, I got the difference between natural time period of ETABS model and the one of PERFORM 3D model, (TETABS > TPERFORM 3D). For example: a 12-story building; TETABS=1.74s, TPERFORM3D = 1.5s.
I use fibre sections for whole length of shear wall members and 2 ends of column members.
The cause may be that the ETABS model is linear while the PERFORM 3D model is nonlinear. But I would like to understand more in-depth: which specific factors affect this?
If anyone have done with ETABS and PERFORM 3D, please tell me why there is difference between natural period of ETABS model and the one of PERFORM 3D model. And what should I do in that case?
Also check whether the modes are calculated using Eigen vectors or Ritz vectors. .
If you have reinforced concrete shear walls in your model, it is possible that this is an effect of having uncracked concrete shear walls (in-plane). Try to reduce the initial modulus of your concrete material model. You should also set a realistic modulus for your walls in the out-of-plane direction.
It depends on
1) material properties (E,G)
2) section properties (I,A)
3) Member length
If you introduce your members with fiber sections in Perform3D, it will include the EI of the reinforcement so your EI value will be higher compared to linear model of ETABS, theus the periods will be lower. Just introduce a simple frame model with three elements in Perform 3D with or without considering steel fibers, you will see the difference!
The periods are different becuase the stiffness of the elements and/oror the assigned mass are different. What type of elment are you using in your model?
The natural period come from an modal analysis. Modal Analysis is a linear-dynamic analysis. If you run a modal analysis you are using a linear behaviour in both softwares.
Likely there are some elements with differents FEM-assignment or some active option about postcracring approximation.
Regards.
Structures period depend on mass & stiffness
- T=2p (m/k)^(0.5)
- My thesis was as your research and I had your problem but when I attention to period equation I understand this difference depend to m or k
- Werb etabs period> perform period ..... etabs mass> perform mass or Etabs k
I modeled a 9-story concrete structure in SAP2000 with height of 33.7 m. SAP2000 calculate the first mode natural frequency about 4.5 sec that is too much for this height. I consumed beams and columns cracked with 0.35 and 0.7 factor in property modifier section.
I would be thankful if anybody can experience this long period?
You should not use fiber
fiber nonlinear characteristics into consideration, it is not right
Good question.. Please share me the best answer might you trust...
Regards…
Check the following reference Please.. it might's assist you in your question … Regards…
https://www.fema.gov/media-library-data/1393888487069-c071850c3050c34da135376baa478a1b/P-752_Unit4.pdf
I recommend you check the stiffness of the embedded beams in PERFORM-3D connecting coupling beams and shear walls. You need to watch out its stiffness, which is usually taken 20 times that of its connected coupling beam.
Also, you need to check the base reaction forces of the two models under gravity load only so that you can determine whether your problem is caused by stiffness or by mass of the structure.
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