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。
             In particular, if a structure is statically indeterminate, the additional equations（附加方程）needed to solve for（求解）the unknown reactions（反力）are obtained by relating the applied loads and reactions to the displacement or slope（转角）at different points on the structure. These equations, which are referred to as compatibility equations（相容性方程或协调方程）, must be equal in number to the degree of indeterminacy（不确定次数）of the structure. Compatibility equations involve（涉及）the geometric and physical properties of the structure.
           特别地，如果一个结构是超静定的，可以通过建立作用力和反力与结构不同点上的位移或转角的关系来得到用以求解未知反力所需的附加方程。这些称为相容性方程的方程式在数量上必须等于结构的不确定次数。相容性方程涉及结构的几何和物理性能。
            There are two fundamental methods of analysis for trusses: the method of joints and the method of sections. Both start with（从..着手）a free-body diagram of the truss as a whole（基本上）, from which the equilibrium equations are written and solved for the support reactions（支座反力）.
           有两种分析桁架的基本方法：节点法和截面法。两种方法基本上都从桁架的隔离体图着手，根据它可以写出平衡方程并求解支座反力。
           The method of joints: After the support reactions have been found, a joint is selected that has no more than（不超过）two members connecting for which the axial forces are unknown. The free-body diagram of that joint is drawn, the forces are summed（合计）in two directions, and each sum is equated to（等于）zero. When drawing the free-body diagram, it is a good idea to assume that the unknown forces are tensions and to show（表示）them so on the free-body diagram by their exerting a pull on（对..施加拉力）the joint. When this is assumed, the resulting sign（符号）of the unknowns when evaluated（计算）will match（符合）the conventional（习惯的）+ for tension and – for compression. Once a joint has been analyzed, its members become knowns, and adjacent joints（相邻节点）, which might have had three or more unknowns, can then be solved since some of these unknowns have become knowns. This process（过程）continues from joint to joint, each time selecting a joint whose number of unknown members does not exceed 2.
    节点法：求出支座反力后，选择一个节点其上连接着轴向力未知的构件不超过两根。画出节点的隔离体图，将力在两个方向上进行合计，每个方向（力）的合计等于零。当画出隔离体图时，有个好主意是假定未知力是拉力，并在隔离体图上通过对该节点施加一个拉力来表示。这样假定后，未知力计算结果的符号将与习惯的正为拉力负为压力相符。一旦一个节点已经被分析，其上的构件成为已知构件，相邻的节点可能曾经有三个或更多的未知力，但因为其中的一些已经成为已知，因此也能求出。这个过程从一个节点到另一个节点连续进行，每次选择的节点其上未知构件（力）的数量不超过两根。
            Almost all truss systems are configured（装配）so that analysis using the method of joints must begin at one end and proceed（继续）joint by joint toward the other end. If it is necessary to evaluate the forces carried by a member located（位于）some distance from the ends, the method of joints requires the calculation of the forces in many members before the desired one is reached. The method of sections provides a means（方法）for a direct calculation in these cases. After the support reactions have been calculated the truss is cut through（切开）(analytically分析上) so that one part of the truss is completely severed from the rest. When this is done, no more than three unknown members should be cut. If possible（如果可能）the cut（切口）should pass through the member or members whose internal forces are to be found. A free-body diagram of the part of the truss on one side of（在..一边）this section is drawn, and the internal forces are found through the equilibrium equations. Since the system of forces（力系）on the free-body diagram is a plane non-concurrent（非共点）force system, three equilibrium equations may be written and solved for the three unknowns.