Available for Arbitrary systems only.

This modelling feature is used to specify geotechnical springs to model horizontal soil contact with cyclic stiffness degradation. The geotechnical spring will always have the same properties in all horizontal directions.

Geotechnical springs are activated together with global springs, either in the static load group with Activate global springs (load type SPRI in standalone) or at the start of dynamic analysis.

The spring will initially follow the input displacement-force curve as long as the horizontal displacement increases. When the displacement is reversed, the secant stiffness from the origin to the spring force value corresponding to the maximum horizontal displacement is used as the new spring stiffness. This constant "degraded" stiffness is then used as long as the displacement in any horizontal direction does not exceed the previous maximum. If the displacement exceeds the previous maximum, the spring will follow the initial curve until the displacement reverses.

The figure shows the initial displacement-force curve input by the user (black line) and how the stiffness is modified depending on the maximum displacement encountered so far in the analysis. The displacement is the absolute value of the displacement in the horizontal plane. The resulting spring force is applied in the opposite direction.

The spring first follows the initial curve (the green arrows over the black line) as the displacement increases. When the displacement reverses (at the violet X), the spring follows the secant stiffness back through the origin (the violet arrow). The spring follows this constant secant stiffness in all directions as long as the displacement is less than the previous maximum. If this value is exceeded, the spring will again follow the initial curve (the violet arrow over the black line). After the next reversal (at the red X), the spring follows a new constant secant stiffness (red arrow).

Damping will be included based on the following model

\(\mathrm {F=-C(d)\times |\dot {d}|^P\times sign(\dot {d})}\)

where

Energy will also be dissipated in the cycles where the spring stiffness is updated.

Dynamic results for the geotechnical springs will be stored in the ASCII file <prefix>_spring_results.asc. The key file key_<prefix>_spring_results.txt will describe how data is stored in the file.

For details of the input file please see Geotechnical spring - input reference.