Of primary concern to a Structural engineer is ensuring that the structure will not collapse when subjected to its design loading. This requires firstly that the engineer properly identify the extreme loading that the structure may experience over

Table 1.2 Structures classified by makeup

Fig. 1.2 Examples of typical structures classified by makeup

its design life and secondly, ensure that the forces generated internally within the structure due to external loading combined satisfy the conditions for force equilibrium. In general, a structure will deform, i.e., change its shape, when loaded. It may also move as a rigid body if not properly restrained. Certain structures such as airplanes and automobiles are designed to move. However, civil structures are generally limited to small motion due to deformation, and rigid body motion is prohibited. Identifying the design loads is discussed later in this chapter. We focus here on the force equilibrium requirement for civil structures.

Reactions

Civil structures are connected to the ground at certain points called supports. When the external loading is applied to the structure, the supports develop forces which oppose the tendency of the structure to move. These forces are called reactions [2]. The nature and number of reactions depends on the type of support. Figure 1.3 shows the most common types of idealized structural supports for any planar structure. A roller support allows motion in the longitudinal direction but not in the transverse direction. A hinge prevents motion in both the longitudinal and transverse directions but allows rotation about the pin connection. Lastly, the clamped (fixed) support restrains rotation as well as translation with 2 reaction forces and one moment. Three-dimensional supports are similar in nature. There is an increase from 2 to 3 and from 3 to 6 in the number of reactions for the 3D hinge and a clamped supports.