The descriptive name associated with this approach, the method of sections, stems from one particular way of looking at how subassemblies are separated from the remainder of the truss. An imaginary section line is passed through the truss, piding it into two segments. It actuality there is no implication that the section line must be straight. The method of joints could quite easily be thought of in terms of passing a section line around each joint[see Figure 4-6(b)]. Both can be interchangeably used in analyzing trusses. The method of joints is often considered preferable for use when all the bar forces in a truss must be determined. The method of sections is particularly useful when only a limited number of forces must be found.
4.3.2构件受力:概述
    这一节讨论已知几何特性和受载情况的桁架中杆件受力的性质及其分布的传统分析方法。已有分析方法所遵从的基本原理是，任何结构，或任何结构的任意部分必须处于平衡状态。这个原理是普通桁架分析方法的关键。分析一个杆件繁多的桁架，第一步是把隔离结构的基本单元分离出来，分析作用在该单元上的力。如果已知其中一部分力，通常就可以运用基本的静力学公式算出其他的力，因为这个单元必须处于平衡状态。这些公式只不过是在形式上表达了一种概念，即在平衡状态下任何一组力，包括外加力与所产生的内力的总合力一定为零。论文网
    所研究隔离部分的范围不受限制。可以考虑由几根杆件和几个连接点组成的一整段，也可以只注意单个连接点或杆件。为了得到一个结构中存在于某一特定位置上各种杆件的内力。需要至少在该点处把结构加以分解。图4-6(b)~(e) 展示了图4-6(a)中所示桁架各种杆件 土木工程英文文献和中文翻译(7):http://www.youerw.com/fanyi/lunwen_34445.html