土木工程专业英语上册_翻译苏小卒_同济大学(考试手机专业版)(7)

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.

几乎所有的桁架体系是装配的，因此采用节点法进行的分析必须从一个端点开始，并一个节点连着一个节点地朝另一个端点继续进行。如果有必要计算位于端部一定距离的构件上的力，节点法需要在到达这根要求（计算）的构件之前计算很多构件中的力。在这些情况下截面法提供了一个直接计算的方法。当求出支座反力后，桁架（在分析上）被切开，从而一部分桁架同其余部分完全分离。当这样切开时，应该切出不超过三个构件的力是未知的。如果可能，切口应穿过将要求解内力的构件。画出在截面一边的桁架部分的隔离体图，并通过平衡方程式求解内力。由于隔离体图上的力系是平面非共点的，因而可以写出三个平衡方程式并求出三个未知力。

Influence lines（影响线）have important application for（应用）the design of structures that resist large live loads（活荷载）. An influence line represents（代表）the variation of either the reaction, shear, moment, or deflection at a specific （特定的）point in a member as concentrated force moves over the member. Once this line is constructed（作图）, one can tell at a glance（一眼便知）where a live load should be placed on the structure so that it creates（引起）the greatest influence at the specified point. Furthermore, the magnitude（大小）of the associated （相关的）reaction, shear, moment, or deflection at the point can then be calculated from the ordinates（纵坐标）of the influence-line diagram. For these reasons（因此）, influence lines play an important part in the design of bridges, industrial crane rails（吊车轨道）, conveyors, and other structures where loads move across their span（全长）. Although the procedure（步骤）

for constructing an influence line is rather basic（基本的）, one should clearly be aware of the difference between constructing an influence line and constructing a shear or moment diagram. Influence lines represent the effect of a moving load only at a specified point on a member, whereas shear and moment diagrams represent the effect of fixed loads at all points along the axis of the member.

影响线在设计抵抗大量活荷载的结构时有着重要的应用。一根影响线代表着当集中力在构件上移动时构件上一个特定点的反力、剪力、弯矩或挠度的变化。一旦画出这根线，任何人一眼便知活荷载应该置于结构的哪个位置才能对这个特定的点引起最大的影响。而且，这点上相关的反力、剪力、弯矩或挠度可从影响线图的纵坐标上计算出来。因此，影响线在桥梁、工业吊车轨道、输送机和其它有荷载在整个结构长度上移动的结构设计中扮演着重要的角色。虽然画出一条影响线的步骤是相当基本的，但任何人应该清楚地意识到画一条影响线与画一条剪力或弯矩图的区别。影响线只代表着移动荷载对构件上特定点的影响，而剪力和弯矩图代表固定荷载对沿着构件轴线的所有点的影响。

Deflections of structures can occur from various sources（原因）, such as loads, temperature, fabrication errors, or settlement. In design, deflections must be limited in order to prevent cracking of attached（附属的） brittle materials such as concrete or plaster (石膏) . Furthermore, a structure must not vibrate or deflect（变位）severely in order to “appear” safe for its occupants（居住者）. More important, though（然而）, deflections at specified points in a structure must be computed if one is to analyze statically indeterminate structures. We often …… 此处隐藏：5264字，全部文档内容请下载后查看。喜欢就下载吧 ……