As per the AISC 360 specifications, I have to use DIRECT ANALYSIS when ratio of second order drift/first order drift exceeds 1.5. How can I find this ratio in STAAD.Pro ?

You have to carry out two separate analysis for the same model and compare the drifts to find out the ratio. Start with a first order analysis ( PEERFORM ANANLYSIS ) to find out the story drift corresponding to the first order analysis. Include the command PRINT STORY DRIFT in the file and the software would output the drift for each story and for each load case/combination. These drifts are reported as part of the analysis output file ( anl file ). Carry out a second analysis with the same file but this time using a PDELTA nn ANALYSIS ( where nn stands for the number of iterations, usually 15 is a good value to use ) instead of a PERFROM ANALYSIS. Divide the maximum drift obtained from case 2 by the corresponding drift obtained from case 1. If this ratio exceeds 1.5, carrying out a DIRECT ANALYSIS is mandatory.