How to model a base plate with anchors in STAAD.Pro
In STAAD.Pro you can model the base plate as a mesh of plates. Model the soil support as springs using Elastic Mat or Plate Mat. Model the anchors as FIXED BUT supports. Assign the compression only attribute to the springs representing the soil and tension only attribute to the anchors.
A sample model of base plate modeled in STAAD.Pro is attached
Note that in the attached model, the load cases 1 and 2 are primary loads and hence these not analyzed with compression/tension springs just in case the iterative analysis fails to converge. The actual combination load case where DL and WL are acting together is analyzed with the tension/compression springs.