1、4.1 INVESTIGATION OF STRUCTURAL BEHAVIOR Investigating 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 materi
2、al 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: Functiona
3、lity. or the general physical relationships of the structures form. detail. durability. fire resistance. deformation resistance. and so on. Feasibility. including cost. availability of materials and products. and practicality of construction. Safety. or capacity 10 resist anticipated loads. Means An
4、 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 struc
5、tures 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 Visualize graphically the str
6、uctures deformation under load. Manipulate mathematical models. 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. mathemati
7、cal modeling precedes any actual construction-even of a test model. Limit direct mea-surementto 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. Three ty
8、pes 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 structure or som
9、e 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 static equil
10、ibrium. 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 bent. so o
11、ne 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 beams internal
12、 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 material is isola
13、ted, 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 load-deform
14、ed 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 deformation of t
15、he 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 deformation The scaled plot is a graph of some mathematical relationship or real data. For
