Structural nonlinearities occur routinely. For example, when you staple two pieces of paper together, the metal staples are permanently bent into a different shape, as shown in Figure 8.1: Common Examples of Nonlinear Structural Behavior (a).

If you load a wooden shelf heavily, it sags more and more over time, as shown in figure (b). As weight is added to a car or truck, the contact surfaces between its pneumatic tires and the underlying pavement change in response to the added load, as shown in figure (c).

If you were to plot the load-deflection curve for each example, you would discover that they exhibit the fundamental characteristic of nonlinear structural behavior: a changing structural stiffness.

Figure 8.1: Common Examples of Nonlinear Structural Behavior