> For the complete documentation index, see [llms.txt](https://docs.openbrim.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.openbrim.org/faq/analysis-and-design/faq-rigid-material-and-section-definition.md).

# Rigid Material and Section Definition

To establish a proper definition of a rigid material, users must exercise caution regarding two key parameters:

* Ensuring that the **unit weight** of the rigid material is **set to 0** is absolutely critical. Failure to do so could lead to the generation of excessive forces due to the self-weight of the rigid section.
* To prevent the creation of an **ill-conditioned matrix**, users must exercise caution when assigning stiffness values, particularly when using high values such as >10000 times E\*I or E\*A. It is advisable to define stiffness values that are only **100 or 1000 times** greater than those of the connected elements.

The below example serves, as an illustration, to grasp the logic behind defining rigid sections by comparing them with other sections within the project:

As seen below, the properties of "Rigid\_Section1" are 100 times greater than those of "Column1," which happens to be the largest section in the project. Hence, it is duplicated, and its sectional properties are multiplied by 100. Moreover, "RigidMaterial1" has a specified modulus of elasticity of 29000ksi, approximately 6.54 times higher than the modulus of elasticity of “FC\_5\_1/2ksi” concrete, which is 4428ksi. Consequently, the EI and EA values of the rigid section are 650 times larger than the largest section in the project. Additionally, the “**unit weight”** and **“Thermal Coeff.”** of "Rigid\_Material1" are explicitly set to 0.

{% hint style="info" %}
Duplicating a section might not be a good idea because it duplicates along with its shapes, and graphically, you will see a section in the FEA view at rigid section locations. Therefore, make sure that the section you use does not include any shapes.
{% endhint %}

![](https://openbrim.atlassian.net/wiki/download/attachments/2244214785/image-20230724-171701.png?api=v2)

![image-20240320-142753.png](https://openbrim.atlassian.net/wiki/download/attachments/2244214785/image-20240320-142753.png?api=v2)


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