Natural frequencies Abaqus

[Henrik97]

Hello, i am working on a old reinforced concrete bridge in Abaqus, and are fairly inexperienced in the software. I need to do a frequency analysis in order to calibrate the model based on experimental sensor data. However the natural frequencies i am obtaining in Abaqus are way too high compared to analyses done in Etabs and the sensor data. First mode for sensors, Etabs and Abaqus are approximately 8, 13 and 30 Hz respectively (Etabs is ok, because a young's modulus of new concrete was used, and this has to be lowered). The problem is: the geometry, material properties are the same for Etabs and Abaqus, but the results are not. In order to come somewhat closer to Etabs results i would have to use a very coarse mesh. Any suggestions? Could my boundary conditions, interactions and/or constraints be the problem?

 

[FEA way]

Of course, the models should be defined in a possibly identical way to achieve good agreement of the results. In such cases, the difference often lies in boundary conditions but there can be other reasons too. Double-check both analyses, make sure that they are correctly defined and look for significant differences in both models (also units, material properties, connections and so on). There might also be a difference in how these solvers treat more advanced cases (like with contact and other interactions). Their documentations should help with that.

 

[GregLocock]

8, 13 and 30 Hz

I don't believe a viable structure could have a stiffness of 38% of nominal. Concrete gets stiffer with age

https://www.researchgate.net/figure...-age-considering-the-effect-of_fig4_237688151

although of course if t cracks then things change, but you'd need to explicitly model the cracking.

Cheers

Greg Locock


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