# Spline Stiffness Model

The spline stiffness formulation interpolates a Force versus Deflection table using Akima’s cubic interpolation method. For values of deflection outside the supplied range, the force is linearly extrapolated using the slope at the corresponding end-point. For each direction employing the spline-stiffness formulation, you must supply a Force versus Deflection table in the property file. The force is calculated as follows:

${F}_{i}=Akispl\left({q}_{i},\text{\hspace{0.17em}}table\right)$

Where:

${q}_{i}$ is the displacement of Body-I relative to Body-J in the ${i}^{th}$ direction after offset, scaling and coupling effects have been considered.

$table$ is the force-deflection table specified in the [SPLINE_STIFFNESS_xx] block in the .gbs file.