土木工程(外文翻译)--investigation-of-structural-behavior—-毕业论文设计.doc

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1、4.1 INVESTIGATION OF STRUCTURAL BEHAVIORInvestigating how structures behave is an important part of structural design: it provides a basis for ensuring the adequacy and safety of a design, In this section I discuss structural investigation in general. As I do throughout this book. I focus on materia

2、l relevant to structural design tasks. Purpose of Investigation Most structures exist because they are needed. Any evaluation of a structure thus must begin with an analysis of how effectively the structure meets the usage requirements. Designers must consider the following three factors: l Function

3、ality. or the general physical relationships of the structures form. detail. durability. fire resistance. deformation resistance. and so on. l Feasibility. including cost. availability of materials and products. and practicality of construction. l Safety. or capacity 10 resist anticipated loads. Mea

4、ns An investigation of a fully defined structure involves the following: 1. Determine the structures physical being-materials, form, scale. orientation. location. support conditions, and internal character and detail. 2. Determine the demands placed on the structure-that is. loads. 3. Determine the

5、structures deformation limits. 4. Determine the structures load response-how it handles internal forces and stresses and significant deformations. 5. Evaluate whether the structure can safely handle the required structural tasks. Investigation may take several forms. You can l Visualize graphically

6、the structures deformation under load. l Manipulate mathematical models. l Test the structure or a scaled model, measuring its responses to loads. When precise quantitative evaluations are required. use mathematical models based on reliable theories or directly measure physical responses. Ordinarily

7、. mathematical modeling precedes any actual construction-even of a test model. Limit direct measurementto experimental studies or to verifying untested theories or design methods. Visual Aids In this book, I emphasize graphical visualization; sketches arc invaluable learning and problem-solving aids

8、. Three types of graphics are most useful: the free-body diagram. the exaggerated profile of a load-deformed structure. and the scaled pial. A free-body diagram combines a picture of an isolated physical clemen I with representations of all external forces. The isolated clement may be a whole struct

9、ure or some part of it. For example. Figure 4.1a shows an entire structure-a beamand-eolumn rigid bent-and the external forces (represented by arrows). which include gravity. wind. and the reactive resistance of the supports (called the reactions). Note: Such a force system holds the structure in st

10、atic equilibrium. Figure 4.lb is a free-body diagram of a single beam from the bent. Operating on the beam are two forces: its own weight and the interaction between the beam ends and the columns 10 which the beam is all ached. These interactions are not visible in the Ireebody diagram of the whole

11、bent. so one purpose of the diagram for the beam is to illustrate these interactions. For example. note that the columns transmit to theendsofthe beams horizontal and vertical forces as well as rotational bending actions. Figure 4.lc shows an isolated portion ofthe beam length. illustrating the beam

12、s internal force actions. Operating on this free body arc its own weight and the actions of the beam segments on the opposite sides of the slicing planes. since it is these actions that hold the removed portion in place in the whole beam. Figure 4.ld. a tiny segment. or particle. of the beam materia

13、l is isolated, illustrating the interactions between this particle and those adjacent to it. This device helps designers visualize stress: in this case. due to its location in the beam. the particle is subjected to a combination of shear and linear compression stresses. An exaggerated profile of a l

14、oad-deformed structure helps establish the qualitative nature of the relationships between force actions and shape changes. Indeed. you can infer the form deformation from the type of force or stress. and vice versa. FIGURE 4.1 Free-body diagrams.For example. Figure 4.la shows he exaggerated deforma

15、tion of the bent in Figure 4.1 under wind loading. Note how you can determine the nature of bending action in each member of the frame from this figure. Figure 4.2b shows the nature of deformation of individual particles under various types of stress. FIGURE 4.2 Structural deformationThe scaled plot

16、 is a graph of some mathematical relationship or real data. For example, the graph in Figure 4.3 represents the form of a damped ibration of an elastic spring. It consists of the plot of the displacements against elapsed time t. and represents the graph of the expression.FIGURE 4.3 Graphical plot of

17、 a damped cyclic motion.Although the equation is technically sufficient to describe the phenomenon, the graph illustrates many aspects of the relationship. such as the rate of decay of the displacement. the interval of the vibration. the specific position at some specific elapsed time. and so on.4.2

18、 METHODS OF INVESTIGATION AND DESIGN Traditional structural design centered on the working stress method. a method now referred to as stress design or allowable stress design (ASD). This method. which relies on the classic theories of elastic behavior, measures a designs safety against two limits: a

19、n acceptable maximum stress (called allowable working stress) and a tolerable extent of deformation (deflection. stretch. erc.). These limits refer to a structures response to service loads-that is. the loads caused by normal usage conditions. The strength me/hod, meanwhile, measures a designs adequ

20、acy against its absolute load limit-that is. when the structure must fail. To convincingly establish stress. strain. and failure limits, tests were performed extensively in the field (on real structures) and laboratories (on specimen prototypes. or models). Note: Real-world structural failures are s

21、tudied both for research sake and to establish liability. In essence. the working stress method consists of designing a structure to work at some established percentage of its total capacity. The strength method consists of designing a structure tofail. but at a load condition well beyond what it sh

22、ould experience. Clearly the stress and strength methods arc different. but the difference is mostly procedural.The Stress Method (ASD) The stress method is as follows: 1. Visualize and quantify the service (working) load conditions as intelligently as possible. You can make adjustments by determini

23、ng statistically likely load combinations (i.e , dead load plus live load plus wind load). considering load duration. and so on. 2. Establish standard stress. stability, and deformation limits for the various structural responses-in tension. bending, shear, buckling. deflection, and so on. 3. Evalua

24、te the structures response. An advantage of working with the stress method is that you focus on the usage condition (real or anticipated). The principal disadvantage comes from your forced detachment from real failure conditions-most structures develop much different forms of stress and strain as th

25、ey approach their failure limits. The Strength Method (LRFD) The strength method is as follows: 1. Quantify the service loads. Then multiply them by an adjustment factor( essentially a safety factor) to produce thejaclOred load. 2. Visualize the various structural responses and quantify the structur

26、es ultimate (maximum, failure) resistance in appropriate terms (resistance to compression, buckling. bending. etc.). Sometimes this resistance is subject to an adjustment factor, called theresistancefacror. When you employ load and resistance factors. the strength method is now sometimes called foad

27、 and resistancefaaor design (LRFD) (see Section 5.9). 3. Compare the usable resistance ofthe structu re to the u ltirnatc resistance required (an investigation procedure), or a structure with an appropriate resistance is proposed (a design procedure). A major reason designers favor the strength meth

28、od is that structural failure is relatively easy to test. What is an appropriate working condition is speculation. In any event, the strength method which was first developed for the design of reinforced concrete structures, is now largely preferred in all professional design work. Nevertheless, the

29、 classic theories of clastic behavior still serve as a basis for visualizing how structures work. But ultimate responses usually vary from the classic responses, because of inelastic materials, secondary effects, multi mode responses, and so on. In other words, the usual procedure is to first consid

30、er a classic, elastic response, and then to observe (or speculate about) what happens as failure limits are approached. 中文翻译4. 1结构特性分析 研究结构的特性在结构设计中是一个很重要的部分，它是保证设计安全性和适用性的 基础。本节讨论常用的结构分析方法。正如贯穿本书所讨论的一样，本章集中讲述与钢结 构设计相关的材料问题。1.分析的目的 绝大数的结构是因需而生的。因此，任何一个结构的评价都是从分析结构如何有效地满足使用要求开始的。 设计人员必须考虑以下三个因素：（1）实用

31、性指结构的形式、构造、耐久性、抗火性以及抗变形能力等的一般物理关系。（2）可行性包括造价、材料及产品的实用性和结构的实用性。 （3）安全性指抵抗设计荷载的能力。 2.方法 一个完整的结构分析包括以下几点：（1）确定结构的物理特性一一材料、形式、尺寸、方向、位置、支承条件以及内部特征和构造。（2）确定施加在结构上的负荷.即荷载. （3）确定结构的变形极限。 （4）确定结构的荷载效应，即荷载作用对结构的内力、应力和主要变形的影响。 （5）评定结构是否能够安全地承担所需的结构要求。 结构研究可以采用以下三种方法: （1）图解表示街载下结构的变形。 （2）使用数学模型。 （3）对结构或比例模型进行试验

32、，测量其在荷载下的效应. 当需要精确的定量评定时，可以采用基于可靠度理论的数学模型或直接测量物理效 应。-般地.建立数学模型先于实际结构.甚至是先于试验模型。括号直接测定限制在试验 研究上或是限定在验证未被检验过的理论上或是限定在设计方法土。 3.直观法 本书强调图解法，草图是一种非常有价值的学习及解决问题的建助工具。最有用的三 种图解法是:隔离体图解法、荷载变形结构放大示意图和比例图. 隔离体图解法是用图解的方法表示一个隔离单元所受的所有外力.这个隔离单元可以 是整体结构或是结构的一部分。 例如，图4. 1（a）为一整体结构梁柱刚性框架和框架研受外力(由箭头表示)。结构所受的外力包括自重、风

33、荷载和支座反力(即反力)。注意：结构所受的力系使结构处于静力平衡状态。 图4. 1 （b）为从框架上隔离出来的单个梁的隔离体图。该梁承受两种力：自重以及梁端部和与梁相连码在之间的相互作用力。梁和柱之间的相互作用力在框架隔离体图中是看不到的。因此梁的隔离体图目的之一是阐明此相互作用力。注意：柱子传递给梁端的不仅有弯距，还有水平力和竖向力。 图4.1（c）为沿梁长度方向上部分梁的隔离体，给出了梁的内力作用。在该隔离体上作用有自重和剖面相反一侧对该梁段所施加的作用力，正是由于此内力使得整个梁的剩余部分保持平衡。图4.1（d）为梁截面隔离体中的一小段或一部分，该图显示了这部分与相邻部分间的作用力。此图

34、有助于设计人员了解结构所受的应力。既然这样，由于它是梁的一部分，因此受到剪应力和线性压应力 的作用。荷载变形结构放大示意图有助于定性确定作用力和形状改变之间的关系。实际上，可以从力或应力的类型来推断变形的形式，反之亦然。 例如，图4. 2（a）表示的是图4. 1所示框架在风荷载作用下的变形放大示意图。应注意从图中如何确定框架的每个构件的弯曲作用特性。图4.2 （b）给出了在不同类型应力下，单个隔离体的变形特性。 比例图为一些数学关系或实测数据的图形。例如，图4. 3代表一弹性弹簧阻尼振动的形式。该图是位移-时间（s-t）关系图，其关系式如下: 虽然方程已经足够描述位移-时间的关系，但是图示还可

35、以表示位移-时间关系的很多方面，比如位移衰减的比率，振动周期以及在某一特定时间里振动的具体位置等。 4.2分析与设计方法 传统的结构设计方法是围绕着工作应力法展开的，此方法现在称为应力设计或容许应力设计（allowable stress design， ASD)。此方法依赖于经典的弹性特性理论，用两个极限值来衡量设计的安全性:可接受最大应力(称为容许工作应力)和容许的变形极限值(挠度、伸长等)。这两个极限值是结构在使用荷载下的效应，即正常使用条件下的荷载效应。 同时，承载力法是用来衡量设计是否足以抵抗其绝对荷载极限，即当结构必须破坏时，结构抗力是否大于结构效应。 为了得到令人信服的应力极限值、

36、应变极限值以及破坏极限，大量进行现场(在实际结构上)和试验室(在结构样本原型或模型上)试验。 提示 研究实际结构的破坏是为了研究和确定结构的可靠性。 实际上，工作应力法是指设计一个结构，使其在工作状态下只发挥部分承载力。承载力法是设计一个结构使其发生破坏，但是当实际荷载没有超过破坏荷载时，结构不会发生破坏。显而易见，应力法和承载力法是不同的，但是这种不同主要是设计程序上的不同。应力法（ASD法） 应力法应遵循以下规则： （1） 尽可能合理地假设和确定使用（工作）荷载的状况。可以通过确定可能的统计荷载组合（如恒载+活载+风载）来调整荷载状况，同时考虑荷载的持久性等。（2） 确定不同结构效应下受拉

37、、受弯、受剪、屈曲及变形等的标准应力、应变和 变形极限值结构效应。 （3） 评定结构效应。 使用应力法的优点是集中于结构的使用状态（真实的或期望的）。主要的不足之处在于人为的把破坏状态分离出来大多数结构接近破坏极限时，应力和应变很不相同。 承载力法（LRFD法） 承载力法应遵循以下规则: （1） 确定使用荷载值。然后乘以一修正系数（本质上是一安全系数），即得到设计荷载。 （2） 假设结构的各种效应，并确定结构在适当效应下的极限（最大或破坏）抗力（如受压、屈曲及受弯等的抗力）。有时该抗力受到某一修正系数的影响，即抗力系数。在设计中使用了荷载和抗力系数，则承载力法有时又被称为荷载抗力系数设计法(l

38、oad and resistance factor design， LRFD) （见第4. 9节）。 （3） 对照结构的使用抗力与极限设计抗力（分析过程），或建议采用适当抗力的结构 （设计过程）。 设计人员比较愿意使用承载力法的主要原因是结构的破坏比较容易检验。什么样的工 作状况才合适是一个值得思考的问题。无论如何，最初被用于设计钢筋混凝土结构的承载力法，现在己用于各专业设计。 不过，经典弹性理论作为基本方法仍然用于结构工作状况的假设上。但是，由于非线性材料、二次效应和多重模式效应等的影响，使得极限效应常常不同于传统的效应。换言之，通常的步骤是先考虑传统的弹性效应，然后观察（或推测）接近破坏极限时结构的反应。附：本翻译英文来自Simplified Design of Steel Structures（钢结构简化设计丛书 第七版）,作者James Ambrose（詹姆斯安布罗斯）。