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Consider the planar truss shown in Figure 4-4(b). In this case, j = 6, so must be 2(6) - 3, or 9. This is the minimum number of bars required for stability. Since the truss actually has this number of bars, it is a stable configuration. Generally speaking, fewer bars than are given by this expression will result in an unstable structure, more may indicate a structure with redundant members. The expression, however, is not foolproof and should not be used as a replacement for a careful visual inspection of the truss. The expression is more correctly an indicator of whether or not the internal forces in a structure can be calculated by the equations of statics alone than a predictor of stability. However, it is a useful tool for stability assessments, since it is not possible to calculate the forces in an unstable structure by the equations of statics.
Another aspect of stability is illustrated in Figure 4-5. Thus configurations may be used to stabilize structures with respect to laterally acting loads .Doing so via the use of rigid members is straight In certain cases, however, cables may be used in lieu of rigid members when the cables are subjected to tension forces only. The stability considerations thus far have assumed that all truss members can carry both tension and compression forces equally well. Cable members do not meet this assumption, since they buckle out of the way when subjected to compressive forces. As illustrated, whether tension or compression is developed in a diagonal depends on its orientation. When the loading comes from either direction, either tension or compression may be developed in a diagonal. A structure with a single diagonal cable could be unstable, a fact not predicted by the previous expressions relating the number ofjoints and members. If cables were to be used, a cross-cable system would be required wherein one cable takes up the horizontal force while the other buckles harmlessly out of the way.
4-3-2 Member Forces: General
This section considers classical analytical methods for determining the nature and distribution of forces in the members of a truss that has known geometrical characteristics and carries known loads. The principle underlying the analytical techniques to be developed is that any structure, or any elemental portion of any structure, must be in a state of equilibrium. This principle is the key to common truss analysis techniques. The first step in analyzing a truss of many members is to isolate an elemental portion of the structure and consider the system of forces acting on the element. If some of these forces are known, it is usually possible to calculate the others by use of the basic equations of statics, since it is known that the element must be in equilibrium. These equations are merely formal statements of the notion that any set of forces, including both those that are externally applied and those internally developed, must form a system whose net result is zero.
The extent of the portion of the structure selected for study is not restricted. A whole segment consisting of several members and joints could be considered, or attention could be limited.
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第2篇 结构构件的分析和设计
第2篇的9章将系统地讨论当前建筑物中使用的所有主要结构构件。我们将在每章的开头给出该构件的定义，并定性地讨论其特性，随后将对其结构特性详加分析，并进一步讨论设计时应用的理论。
第3篇中我们将更加详细地介绍设计。设计的一般性决策程序可以分为三个必须进行的阶段。第一、二阶段着手于探讨大体的设计意图、建筑物具体的形态特征、论文网不同结构体系（材料、系统、结构等级、间距、几何形状等）的类型和属性相互间的联系。第三阶段是在前期研究的基础上集中对构件和连接件进行设计。在第2篇中，设计这个词指的是第三阶段的内容。全面掌握拱、缆索、梁、柱、桁架、框架、板、壳体、膜等构件的分析和设计，对于了解第一、第二设计阶段是十分必要的。也正是由于这个原因，我们应优先讨论构件的分析和设计，尽管在整个设计过程中将它放在最后阶段。