Stiffness is the
ability of a component or an assembly of components to resist deformations when subjected to actions, as shown in Figure 2.1 . It is expressed as the ratio
between action and deformation at a given level of either of the two quantities
and the corresponding value of the other. Therefore, stiffness is not a constant value. In Figure 2.1 , Ki is the stiffness at a required deformation δi and corresponds to force resistance Vi . If increments or first derivatives of actions and deformations are used, the ensuing stiffness is the tangent value. If total
actions and deformations are used, the ensuing stiffness is the secant value.
Strength is the capacity of a component or an assembly of components for
load resistance at a given response station. It is also not a constant value, as shown in Figure 2.1. In the figure, Vj and Vk are the force capacities corresponding to δj and δk , respectively. Vy , referred to as the yield strength, corresponds to the yield displacement δy , which is required for ductility calculations.
Ref. Fundamentals of earthquake engineering / Amr S. Elnashai and Luigi Di