Surface Springs

Springs are flexible restraints.  Springs applied to shells are referred to as surface springs. You may assign a surface spring to a shell with one or more global DOFs (Dx, Dy and Dz) restrained. To qualify as a valid flexible restraint, the corresponding spring constant must be specified.  A restraint may be designated as linear, compression-only or tension-only.  A compression-only restraint is active only when the nodal displacement in the restrained direction is negative.  A tension-only restraint is active only when the nodal displacement in the restrained direction is positive.  If a model contains one or more compression-only or tension-only springs, the whole problem becomes nonlinear and the solution becomes iterative for each load combination.

The forces or moments required to enforce flexible restraints are called spring reactions.  They are computed by the program.  Surface springs may be used to model Winkler mat foundations.  It may be worthwhile to note that in modeling a mat foundation, surface spring constants are the soil subgrade moduli while surface spring reactions are the soil pressures.