A structure that responds linearly possesses a constant stiffness regardless of the level of force it is subjected to. Such a structure can be thought of as infinitely strong. For linear systems, magnifying the excitation results in a response that is magnified by the same amount i.e. that scales linearly with the excitation. Despite the fact that many structures are expected to respond outside the linear range during severe earthquakes, the linear response can provides valuable information about the actual response.

Linear elastic behavior is illustrated in Figure 22.

Figure 22: Hysteresis Behavior for an Elastic Linear Model