> For the complete documentation index, see [llms.txt](https://manual-1-3.karamba3d.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://manual-1-3.karamba3d.com/chinese_1_3_3/2-getting-started/2-getting-started-1/setting-up-a-structural-analysis/2.2.1-define-the-model-elements.md).

# 2.2.1: 定义模型元素

一般而言，直线是构成梁、桁架和弹性元件的基础。壳体和板材则主要基于Grasshopper（GH）网格。图2.2.1.1显示了如何借助一个简支梁的表达式构建模型，并借助书写板将其显示出来。首先借助**“LineToBeam（由线到梁）”**运算器输入**“Line（线）”**元素，再由“Line（线）”创建“Beam（梁）”等较为复杂的构件。Karamba3D 默认所有几何输入均以米为单位。在处理体量较大、形体较为复杂的结构时，及时命名新建的模型元素将大大有利于后期数据的管理。如图2.2.1.1所示，新构建的梁被命名为“bart”。

{% hint style="info" %}
在Karamba3D中，各构件标识符不一定是唯一的，这一特质便于用户对设计构件进行分组管理。
{% endhint %}

![图2.2.1.1：仅含几何体的结构模型](/files/-M9XuUlcNeNoQAWkYQkI)

**“Assemble（模型构建）”**运算器收集模型中的信息。当接入书写板时，书写板上会显示模型的基本特征：其中**“c.Length”**代表长度 —— 亦即结构边界框的对角线长度。而在定义梁时，通常由两个节点定义一个元素。在用户未对材料进行定义的情况下，Karamba3D会自动生成一个默认材料，该材料应用于默认梁的断面。如果存在壳体，则会出现第二个默认断面。该模型无任何荷载，但存在默认荷载工况。荷载工况对应一组外力对模型产生影响的状况。例如：风可以从若干方向冲击一个结构，但不会同时从所有方向吹向一个结构，因此，每个风向都将有其相对应的荷载工况计算。

{% hint style="info" %}
荷载工况对应一组外力对模型产生影响的状况。例如：风可以从若干方向冲击一个结构，但不会同时从所有方向吹向一个结构，因此，每个风向都将有其相对应的荷载工况计算。
{% endhint %}


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