Hello Hanfg
Firstly , send us the model.
Load combination are used for linear combination types, the use of Compression Spring makes the structure non-linear and hence REPEAT Load should be used instead of LOAD COMBINATIONS.
A Repeat Load type is a Primary load case. That is because, when the program runs into this command, it physically creates the load data for this case by assembling together the load information from all the component load cases (after factoring them by the respective load factors) which the user wants to "REPEAT". Thus, when you specify
LOAD 10
REPEAT LOAD
4 1.4 5 1.7
STAAD creates a physical load case called 10 whose contents will include all of the data of load case 4 factored by 1.4, and all of the data of load case 5 factored by 1.7.
Regards,