I am going to incorporate into my model, rock anchors which will be installed down the center of the pipe piles. In case you are not familiar with these, they are a thread rod attached to the top of the pile, go down the center and are anchored into the rock and are used to take the uplift forces in the pier. I was planning on using the post tensioning command. Is this correct?
If the prestressing force is applied after the piles are driven into the soil and the pile cap is cast, the load type you may want to apply in analysing the model is the one involving the MEMBER PRESTRESS command. That is because, there is a small anomaly in STAAD in the definition of the terms PRESTRESS and POSTSTRESS. The way STAAD defines these terms and the way they are conventionally defined are opposite to each other. You will find an explanation of this anomaly in Section General Engineering Theory > G.15 Loads > G.15.5 Prestress and Poststress Member Load of the STAAD.Pro Help under items 4 and 5.
If you want an example which demonstrates the usage of these commands, please refer to example problem 6 in the STAAD.Pro Examples manual.