I tried the benchmark problem, case 1, shown in AISC 13th edition, page 16.1-435 in STAAD(X). I was unable to get the correct answer. I used P-Delta analysis with P-large delta and P-small delta. I've done the same problem in STAAD.Pro and got the correct answer. Has anybody been successful at running the benchmark problem? Don't forget for your test case Mmax/Mo and ymax/yo should be greater than 2.5.