What best describes the reciprocal relationship to stiffness in structural engineering?

In structural engineering, flexibility is often described as the reciprocal of stiffness. Stiffness refers to a material's ability to resist deformation under load. When a structure is stiff, it will deform less under a given load compared to a more flexible structure. Thus, the more flexible a structure is, the less stiff it is, establishing a direct relationship between the two concepts.

Flexibility allows a structure to bend or deflect under load, which can be a desirable trait in some applications to absorb energy or accommodate movement (like in seismic design). However, excessive flexibility can lead to issues with stability and serviceability, which is why understanding the balance between stiffness and flexibility is critical in design.

Elasticity, while related to stiffness, describes the property of a material to return to its original shape after deformation. Deflection concerns the amount of displacement a structural member undergoes when subjected to load, and rigidity is another term often used interchangeably with stiffness, emphasizing a structure's resistance to bending or twisting. Thus, flexibility stands out as the correct description of the reciprocal relationship to stiffness.