结构由一系列相连的用以支撑荷载的构件组成。显著的例子包括建筑、桥梁、塔、水箱和大坝等。建造这些结构中的任何一个的过程需要规划、分析、设计和建造。结构分析包括各种各样的数学程序以确定诸如当一个结构对荷载有响应时构件的力和不同结构位移的大小。根据结构的使用和位置来估计它的实际荷载经常是结构分析的一部分。
             Only two assumptions are made regarding（关于）the materials used in the structures of this chapter. First, the material has a linear stress-strain relationship（线性的应力-应变关系）. Second, there is no difference in the material behavior when stressed in tension vis-a-vis（与..相比）compression. The frames and trusses studied are plane structural systems（平面结构体系）. It will be assumed that there is adequate bracing perpendicular to（垂直于）the plane so that no member will fail due to an elastic instability（弹性失稳）. The very important consideration regarding such instability will be left for the specific（具体的）design course.
关于本章结构中所用的材料只作了两点假设。首先，材料具有线性的应力应变关系。其次，材料的性能在受拉和受压时没有区别。研究的框架和桁架是平面结构体系。假定垂直于平面的方向有足够的支撑，因而构件不会因为弹性失稳而失效。一个非常重要的关于这种失稳的考虑留待具体的设计过程。
    All structures are assumed to undergo only small deformations as they are loaded. As a consequence（因此）we assume no change in the position or direction of a force as a result of （由于）structural deflections（变位）. Finally, since linear elastic materials and small displacement are assumed, the principle of superposition will apply in all cases. Thus the displacements or internal forces that arise from two different forces systems applied one at a time（一次一个）may be added algebraically（几何相加）to determine the structure’s response when both system(s) are applied simultaneously.
          假定所有的结构在它们加荷时只经历小的变形。因此，我们假定当结构变位时荷载的位置与方向不变。最后，因为假定了线弹性材料和小位移，叠加原理将适用于所有的情况。这样当两种不同的力系同时施加时，可以由不同的力系一次施加一个引起的位移或内力几何相加来确定结构的响应。
           In the real sense（真正意义上）an exact analysis of a structure can never be carried out since estimates always have to be made of the loadings and the strength of the materials composing（构成）the structure. Furthermore, points of application（作用点）for the loadings must also be estimated. It is important, therefore, that the structural engineers develop（形成）the ability to model（模拟）or idealize（使..理想化）a structure so that he or she can perform a practical force analysis of the members.
           真正意义上对一个结构准确的分析是永远也不可能进行的，因为总是不得不估计荷载和构成结构的材料的强度。而且，必须估计荷载的作用点。因此，结构工程师有能力模拟一个结构或使其理想化很重要，这样，他或她能对构件进行实际的力的分析。
               Structural members are joined together in various ways depending on the intent（意图）of the designer. The two types of joints most often specified（规定的）are the pin connection and the fixed joint（节点）. A pin-connected joint allows some freedom for slight（轻微）rotation, whereas the fixed joint allows no relative rotation between the connected members. In reality, however, all connections exhibit（显现）some stiffness toward joint rotations, owing to friction（摩擦）and material behavior. When selecting a particular model for each support（支座）or joint, the engineer must be aware of how the assumptions will affect the actual performance（运行）of the member and whether the assumptions are reasonable for the structural design. In reality, all structural supports actually exert（产生）distributed surface loads（面荷载）on their contacting members.
结构构件根据设计者的意图采用不同的方式连在一起。最常规定的两种节点是铰接节点和固定节点。铰接节点允许有一些轻微的转动自由，而固定节点不允许相连的构件有相对的转动。但是，事实上由于摩擦和材料的特性使所有的连接对节点的转动显现出一些刚度。当为每一个支座或节点选择一个特定的模型时，工程师必须知道该假设将如何影响构件的实际运行，以及该假设是否对结构的设计是合理的。实际上，所有的结构支座在它们接触的构件上产生分布的面荷载。
     The resultants（合力） of these load distributions are often idealized as the concentrated forces（集中力）and moments, since the surface area （表面积）over which the distributed load acts is considerably smaller than the total surface area of the connecting members. The ability to reduce an actual structure to（将..简化为）an idealized form can only be gained by experience. In engineering practice, if it becomes doubtful（不明确）as to how to model a structure or transfer the loads to the members, it is best to consider several idealized structures and loadings and then design the actual structure so that it can resist（抵抗）the loadings in all the idealized models.