Spring Constant

Axial Stiffness

C_{u, x} = \frac{E \cdot A}{l}

d	Diameter of the fastener
K_ser	Displacement modulus of a fastener
γ_M	Partial safety factor for connections according to [1] Table 2.3
I_p	Polar moment of inertia without component of fastener surfaces

Do you have any questions?