Lateral stiffness is a way to define the ability of a frame or beam to resist lateral loads, such as wind loads or seismic loads. Different systems (e.g. moment frames, braced frames, etc.) have different derivations of lateral stiffness.
What is Lateral Stiffness?
Lateral stiffness is not the same as axial or bending stiffness, which apply to individual members. Lateral stiffness defines the ability of a system of members to resist lateral loads. By knowing the lateral stiffness and applying some force, we can calculate the deflection of said frame.
We can refer to the example of a braced frame to see the derivation of lateral stiffness...