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. 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。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.
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.
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.
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。这些荷载分布的合力常常理想化为集中力和弯矩，因为分布荷载作用的表面面积比相连的构件的总的表面面积小很多。将一个实际的结构简化成一种理想的形式的能力只有通过经验才能获得。在工程实践中，如果就怎样模拟一个结构或将荷载传递给构件变得难以确定时，最好考虑几个理想的结构和荷载，然后设计实际的结构，使它在所有理想的模型中都能抵抗荷载。
It may be recalled（回想）from statics that a structure or one of its members is in equilibrium（处于平衡） when it maintains a balance of force and moment. When all the forces in a structure can be determined strictly from these equations, the structure is referred to as statically determinate（静定的）. Structures having more unknown forces than available equilibrium equations（平衡方程）are called statically indeterminate. As a general rule, a structure can be identified as（确定）being either statically determinate or statically indeterminate by drawing free-body diagrams（隔离体图）of all its members, or selective parts of its members, and then comparing the total number of unknown reactive force and moment components（分量）with the total number of available equilibrium equations.
从静力学可以回想起当一个结构或它的一个构件维持力和弯矩的平衡时即处于平衡状态。当一个结构中所有的力能严格地根据这些方程式来确定，该结构称为静定的。如果结构上未知的力比能得到的平衡方程多时称为超静定结构。作为一般的规律，一个结构可以通过画出所有构件或经选择的部分构件的隔离体图，然后比较未知的反力和弯矩的分量总数目与可用的平衡方程总数目是否相等来确定其是静定结构还是超静定结构。 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。