# Modal analysis for a model with nonlinear contact elements and non linear material

Rana Nasser
Member

in Structures

Hi All,

I need to do modal analysis for a model consists of 10 meter of soil rested on a masonry wall. The soil material is defined as a Mohr-Coulomb material with cohesion value = 75500 pa and the contact between the soil and the masonry wall is a frictional contact element with friction factor =0.33. Is there an APDL command or any other way to do this analysis?

Thanks

## Comments

Hi Rana,

I don't think ANSYS will do a Modal analysis with a nonlinear material. Also, a Modal analysis will convert a nonlinear contact into a linear contact according to a table in the help section.

Duplicate your system, create a linear material with the same elastic modulus as the M-C material, change your Frictionless contact to No Separation. Delete the acceleration load and convert this system to Modal.

Regards,

Peter

I have already did this, but is the mode shapes of this model with the bonded contact element and the linear material represents the actual behavior of the real structure?

Hi Rana,

Modal analysis tells you the frequency and shape of one of many possible modes that are inherent in the structure. Modal analysis is limited to linear systems, so no nonlinear materials or nonlinear contacts or large deflection effects.

Actual behavior of a real structure might have many modes combined into a complex shape of vibration. Transient Structural analysis shows how a real structure will respond to any general input force or acceleration, periodic or not and can accommodate all nonlinearities.

Harmonic Response analysis shows how a real linear structure will respond to a harmonic (periodic) excitation at a particular place at a particular frequency. If you are very careful, you can excite a real linear structure so that is is almost purely vibrating with Mode 1, but if you move the excitation load to a different place, you will get more modes contributing to the vibration. Harmonic Response cannot have nonlinear materials, nonlinear contacts or large deflection effects.

Regards,

Peter

I've found the slide. The author here concluded that the glued interface between soil and a steel wall has better performance when subjected to

dynamic loading, but I still wondering the same question about themodal analysis!The results which will be extracted from this analysis will be used to calculate the dynamic characteristics of the simulated structure, so this result must be as close as possible to the reality. any insights?!

Modal analysis computes the shape and frequency of mode 1 (and all modes) and displays a magnitude of displacement that has

no meaningfor the displacement of a real structure because there isno load appliedto the structure in a modal analysis.To obtain displacement magnitudes of real structures, you have to apply a load and see the response. You can only apply a load in a Transient Structural or a Harmonic Response analysis where zero load = zero response, in other words, you have to apply a load. In Modal Analysis, you are

not permitted to apply a load.Does this insight stop your wondering about modal analysis and the dynamic response of real structures subject to real loads?

Kind regards,

Peter

Thanks Peter for your explanation,

In my research I'm using experimental work results to validate the ansys model. These results are the mode shapes and its frequencies, and damping ratio of the structure. The mode shapes and frequencies that will be extracted from ansys modal analysis will be compared with those which extracted from the experimental work, then the error in frequency will be calculated for the matched modes in shape. The next step is to enhance the ansys model to reduce the error in frequency between experimental results and numerical results, so I'm trying to get the most exact behavior of the structure including the effect of the confinement of the surrounding soil. Because of this I'm trying to do modal analysis of the structure and the surrounding soil ( non linear material) which has a frictional contact with the structure. Do you think if I used the modal analysis of the same structure with linear soil and bonded contact element between the soil and the walls of the structure this will be a good simulation to the structure? and will give quite accurate results?

Rana,

In your experimental work, how are you getting the mode shapes without applying any loads? If indeed you are loading your structure a certain way, why not perform a similar Transient structural or harmonic analysis as Peter suggests?

Regards,

Sandeep