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 mat

2、erial 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: Functi

3、onality. 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

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

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

6、rmation 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. mathematical modeling

7、 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. Three types of graphi

8、cs 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 some part of it.

9、 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 equilibrium. Figur

10、e 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 one purpose of

11、 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 force action

12、s. 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 isolated, illustra

13、ting 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-deformed structure

14、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 the bent in Fig

15、ure 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 is a graph of some mat

16、hematical 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 a damped cyclic motion

17、.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 METHODS OF INVESTIGATI

18、ON 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: an acceptable maximum st

19、ress (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 adequacy against its absolut

20、e 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 studied both for researc

21、h 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 should experience. Clearl

22、y the stress and strength methods arc different. but the difference is mostly procedural.The Stress Method (ASD) The stress method is as follows: Visualize and quantify the service (working) load conditions as intelligently as possible. You can make adjustments by determining statistically likely lo

23、ad combinations (i.e , dead load plus live load plus wind load). considering load duration. and so on. Establish standard stress. stability, and deformation limits for the various structural responses-in tension. bending, shear, buckling. deflection, and so on. Evaluate the structures response. An a

24、dvantage 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 they approach their failure limits

25、. The Strength Method (LRFD) The strength method is as follows: Quantify the service loads. Then multiply them by an adjustment factor( essentially a safety factor) to produce thejaclOred load. Visualize the various structural responses and quantify the structures ultimate (maximum, failure) resista

26、nce 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 and resistancefaaor design (LRFD) (see Section 5.9). Compare the usable resistance ofthe structu re to the u ltirnatc resistance required