# Natural Frequency in 1 direction

Hello,

I was wondering if there was a way of finding the natural frequency of a 3D structure in only the x or y direction. Or would I be required to build a 2D slice of the model in each plane and find the natural frequency of the structure that way?

Hello @christopher.ho,

do you want to model a volumetric structure or a beam or shell-model in 3D?

In the latter case you could use a ‘Nodal Displacements’-component to get the x- and y-components of the natural vibration modes.

Since Karamba3D 1.3.3. does not contain volume Elements, you would have to approximate the behavior of a volumetric structure via e.g. a truss lattice.

–Clemens

Hi Clemens,
I have the following 3d model and was looking to find the frequency in the xz and yz plane of the model.

All my results however seem to be in the xy plane:

Hence my question on would it only be possible to split each frame out and find the natural frequency of each frame?

As an alternative for seismic design. The frequency of a structure is 1/18/sqrt(deflection) when the structure is loaded with it’s self weight in the direction of deflection.
I was wondering if there is an easier way of finding the displaced centre of gravity of the structure for the purpose of this maths rather than inserting a new node connected to the structure with an element of zero mass and restrained vertically?

Hello Christopher,
you can deal with each frame individually. The question is whether this makes sense.
Due to the wind-bracing you introduce in the plane of the walls and roof, the structure is sufficiently stiffened out.
The vibration modes you get are localized to individual elements. You can control them by changing the distance between the frames.
I do not quite understand the last paragraph of your question. The center of gravity of the structure comes out as a result of the ‘Assemble’-component. I would not use a rule of thumb for the natural frequency when it is possible to calculate its exact value.
–Clemens

Hi Clemens,

For the design according to the New Zealand standard, we would normally ignore any localized vibration modes.

The rule of thumb is a simplified form of the equation in the standard in order to determine the frequency in each direction for calculating the the design coefficient used in the equivalent static method.

From,
Chris

Hi @christopher.ho,
you could turn some of the elements into trusses by setting ‘Bending’ to false in the ‘Option’ section of the ‘Line to Beam’-component. This should remove the local modes.
–Clemens