# Modify cross section to model Timber

Hello,

For my master thesis, I’m trying to model in a simple way a cut log of wood using Karamba3D. These logs are cut lengthwise (its section would be a circle with a part cut, as represented in the figure below) and this type of section is not available in the box “Cross section”.
There are 2 main things we would like to know in order to overcome this problem using “Modify CrossSection” :

• we would like to have a better understanding of the in inputs of the box “Modify CrossSection”. We already checked the Karamba3D manual, but we have uncertainty for multiple inputs. Also, we would like to know how the boxes “Analyze” and “Nvibes” (in algorithms) work and the section inputs they need.
• we realized that we must put a box “section” as input of “modify cross section”. What is the influence of this input on the results? It seems that the I section doesn’t work when O, and Trapezoid section does work.

Could you give us more information about how these functions work and the underlying theory that is used ?
Thank you very much for your help!

Sincerly,
Ivan Stevens from UCLouvain

Hello @Ivan_Stevens,

• Which are the inputs of the ‘Modify Cross Section’-component where you have uncertainties?
• The ‘Analyze’-component performs a first order theory calculation of the given structure; The ‘Natural Vibes’ solves the general eigenvalue problem involving the structure’s stiffness- and mass-matrix and outputs natural vibration modes and frequencies.
• You should be able to use any type of cross section as input to the ‘Modify Cross Section’-component. In your case it is probably best to use a circular cross section. This influence the way how the maximum stress in the cross section is determined and impacts e.g. the cross section optimization.
• In Karamba3D the beam calculations are based on the Bernoulli hypothesis: Sections remain plane and can rotate with respect to the normal of the deformed axis in the presence of shear. This is controlled via the properties ‘Ay’ and ‘Az’.
– Clemens