From the User's Guide:
Soil supports are modeled as bi-linear springs having an initial stiffness, an ultimate load, and a yield stiffness. The yield stiffness is typically set close to zero. After the ultimate load on the soil is reached, there is no further increase in load even though the displacement may continue.
A restraint starts off with the stiffness of
K1. Once the ultimate load is reached, the stiffness is switched from
K1 to
K2. (Numerically you can't have a zero stiffness, so we use 1.0 for
K2.)
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