Why results for following load-cases are always +ve :
Response Spectrum Analysis
Force Spectrum analysis
Time History Analysis
how to get -ve (Negative) results?
In short, in order to run a Dynamic Analysis first must obtain the result from a Model Analysis. Note that calculated values from modal analysis are all +ve. Therefore the results for all dynamic analysis are +ve.
See AutoPIPE help
Help > Contents> Contents Tab> Reference Information> Results Interpretation> Dynamic Support Forces Results
After reading the help reference above, to include a -ve (negative) dynamic load case (ex. SAM), create a user combination and set the scale factor to -1.00, as shown below.
Create additional User Code / Non Code combinations as needed to consider the -ve direction of dynamic load cases.