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The bolts used for the connections of this steel framework are subjected to stress. In this chapter we will discuss how engineers design these connections and their fasteners.

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CHAPTER

1
CHAPTER OBJECTIVES

Stress

In this chapter we will review some of theimportant principles of statics and show how they are used to determine the internal resultant loadings in a body. Afterwards the concepts of normal and shear stress will be introduced, and specific applications of the analysis and design of members subjected to an axial load or direct shear will be discussed.

1.1 Introduction
Mechanics of materials is a branch of mechanics that studies therelationships between the external loads applied to a deformable body and the intensity of internal forces acting within the body. This subject also involves computing the deformations of the body, and it provides a study of the body’s stability when the body is subjected to external forces. In the design of any structure or machine, it is first necessary to use the principles of statics to determine theforces acting both on and within its various members. The size of the members, their deflection, and their stability depend not only on the internal loadings, but also on the type of material from which the members are made. Consequently, an accurate determination and fundamental understanding of material behavior will be of vital importance for developing the necessary equations used in mechanicsof materials. Realize that many formulas and rules for design, as defined in engineering codes and used in practice, are based on the fundamentals of mechanics of materials, and for this reason an understanding of the principles of this subject is very important.
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CHAPTER 1 Stress

Historical Development. The origin ofmechanics of materials dates back to the beginning of the seventeenth century, at which time, Galileo performed experiments to study the effects of loads on rods and beams made of various materials. For a proper understanding, however, it was necessary to establish accurate experimental descriptions of a material’s mechanical properties. Methods for doing this were remarkably improved at thebeginning of the eighteenth century.At that time both experimental and theoretical studies in this subject were undertaken primarily in France by such notables as SaintVenant, Poisson, Lamé, and Navier. Because their efforts were based on material-body applications of mechanics, they called this study “strength of materials.” Currently, however, it is usually referred to as “mechanics of deformablebodies” or simply “mechanics of materials.” Over the years, after many of the fundamental problems of mechanics of materials had been solved, it became necessary to use advanced mathematical and computer techniques to solve more complex problems. As a result, this subject expanded into other subjects of advanced mechanics such as the theory of elasticity and the theory of plasticity. Research in thesefields is ongoing, not only to meet the demands for solving advanced engineering problems, but to justify further use and the limitations upon which the fundamental theory of mechanics of materials is based.

1.2

Equilibrium of a Deformable Body
Since statics plays an important role in both the development and application of mechanics of materials, it is very important to have a good graspof its fundamentals. For this reason we will review some of the main principles of statics that will be used throughout the text.
G C

Concentrated force idealization

s
Surface force

External Loads. A body can be subjected to several different types of external loads; however, any one of these can be classified as either a surface force or a body force, Fig. 1–1.
Surface Forces. As...