Typically plate mat foundation supports are set up to generate vertical springs with the direction defined as either "Y" or "Y Only" as described in the following article:
However, there may be instances in which springs are required in all three global directions. In such scenarios one can specify the direction as "All" and enter three different subgrade modulus values for the three global directions X, Y, and Z. This can only be done using the STAAD Editor to modify the input command file (it is not possible from the GUI). The following excerpt from section TR.27.3 contains the proper syntax:
Consider an example in which springs are to be assigned to the corner nodes of plates 1 to 100. The command would appear as follows:
1 TO 100 PLATE MAT DIRECT ALL SUBGRADE 10 5 5
The subgrade modulus values are 10, 5, and 5. As indicated in the Help documentation, the first value (10) corresponds to the Y direction. The subsequent values correspond to the X and Z directions.