结构：由杆件、结点和支座组成的杆件体系Structure consists of members, joints and supports.
结构必须是在不考虑材料变形的条件下能保持几何形状和位置不变的杆件体系。
Structure must maintain its geometric shape and positions without consideration of the deformation of materials.
在不考虑材料变形的条件下，杆件体系可分为如下两种类型
If the deformation of materials is neglected, then framed systems can be classified into two categories：
几何不变体系 ( geometrically stable system )
几何可变体系( geometrically unstable system )
几何不变体系 ( geometrically stable system )
在任意荷载作用下，几何形状及位置均
保持不变的体系。（不考虑材料的变形）
Under the action of any loads, the system still maintain its shape and remains its location if the deformations of the members are neglected.
几何可变体系( geometrically unstable system )
在一般荷载作用下，几何形状及位置将发生改变的体系。（不考虑材料的变形） Under the action of any loads, the system will change its shape and its location if the deformations of the members are neglected.
体系组成分析的目的
The purpose of geometric Construction analysis：

1.判定体系是否几何
不变to estimate whether or not a system is geometrically stable；
2.研究几何不变体系的组成规则to discuss the geometric construction rules of stable systems；3. 区分静定和超静定的组成distinguish statically determinate structures and statically indeterminate structures 。