Structural stiffness is the ability of a structure to resist deformation or deflection when subjected to an external load. The most common types of structural stiffness are bending stiffness (flexural stiffness) and torsional stiffness. Bending stiffness is the ability of a structure to resist bending or flexure, while torsional stiffness is the ability to resist twisting. However, there are other types of stiffness that may also be important for specific structures, such as axial, lateral, shear stiffness and rotational stiffness.
The amount of deflection and load that causes a particular type of deflection depends on the type of stiffness involved.
Axial stiffness, the elongation is related to the magnitude and direction of the axial load applied.
Lateral stiffness, the deflection is related to the magnitude and direction of the lateral load applied.
Shear stiffness, the deflection is related to the magnitude and direction of the shear force applied.
Bending stiffness, the flexure is related to the magnitude and distribution of the load applied, as well as to the span and cross-sectional properties of the structure.
Torsional stiffness, the twist is related to the magnitude and direction of the torque applied, as well as to the shape and size of the structure's cross-section.
Rotational stiffness, the rotation is related to the magnitude and direction of the rotational moment applied.
These factors are all considered in engineering design to ensure that a structure is designed to resist the specific types of loads and deflections that it is likely to experience during its intended use.