桁架A truss：由直杆组成的铰接体系assemblage of straight members connected at their ends by hinged joints
基本假设:Basic assumptions
1、各结点为理想铰；all hinges are frictionless
2、各杆轴线为直线,所有用铰联结的杆交于铰心；The centroidal axes are straight lines,those meeting at a hinge all intersect at the centre of hinge.
3、荷载只作用于结点上。All loads are applied on truss only at its joints.
导出特性：各杆截面上只受轴力作用，而没有弯矩和剪力，这样的杆称为二力杆。All members are subjected only to axial forces, but not subjected to bending moments and shear forces, these members are called two-force members.
桁架结构的分类Classification of trusses：
一、平面（二维）桁架（plane trusses）
所有组成桁架的杆件以及荷载的作用线都在同一平面内.
All the members and the applied loads lie in the a single plane.
二、按外型分类Classification according to their forms
平行弦桁架
Parallel chord trusses
2. 三角形桁架Triangular trusses
3. 抛物线桁架（折线桁架）Parabolic trusses
4. 梯形桁架Trapezoidal trusses