Torsion for Dead Load?

Question

Hello, 
 I have a very simple model in STAAD right now. I don't understand why STAAD is showing Torsion in some members even for Load Case 1 which is a Dead Load case. Can someone please explain this. 
 Thanks 
 STAAD SPACE START JOB INFORMATION ENGINEER DATE 18-Jun-19 END JOB INFORMATION INPUT WIDTH 79 UNIT FEET KIP JOINT COORDINATES 1 0 0 0; 2 6.5 0 0; 3 6.5 0 6.5; 4 0 0 6.5; 5 0 12.5 0; 6 0 12.5 6.5; 7 6.5 9.5 0; 8 6.5 9.5 6.5; 9 0 8 0; 10 0 8 6.5; 11 6.5 8 0; 12 6.5 8 6.5; 13 -0.25 0 6.5; 14 -0.25 0 0; 15 6.75 0 0; 16 6.75 0 6.5; 17 0 3 0; 18 0 3 6.5; 19 6.5 3 6.5; 20 6.5 3 0; MEMBER INCIDENCES 1 1 2; 3 4 3; 5 5 9; 6 6 10; 7 2 20; 8 3 19; 9 5 7; 10 6 8; 11 7 8; 12 5 6; 13 10 18; 14 11 7; 15 12 8; 16 9 10; 17 10 12; 18 11 12; 19 9 11; 20 9 17; 21 14 1; 22 13 4; 23 2 15; 24 3 16; 25 17 1; 26 18 4; 27 17 18; 28 19 12; 29 18 19; 30 20 11; 31 20 19; 32 17 20; DEFINE MATERIAL START ISOTROPIC STEEL E 4.176e+006 POISSON 0.3 DENSITY 0.489024 ALPHA 6e-006 DAMP 0.03 TYPE STEEL STRENGTH FY 5184 FU 8352 RY 1.5 RT 1.2 END DEFINE MATERIAL MEMBER PROPERTY AMERICAN 1 3 21 TO 24 TABLE ST W10X22 5 TO 8 13 TO 20 25 TO 32 TABLE ST HSST3X3X0.375 9 TO 12 TABLE ST HSSP1.660X0.14 CONSTANTS MATERIAL STEEL ALL SUPPORTS 13 TO 16 PINNED LOAD 1 LOADTYPE Dead TITLE DEAD SELFWEIGHT Y -1 LIST 1 3 5 TO 8 13 TO 32 MEMBER LOAD 16 TO 19 27 29 31 32 UNI GY -0.01625 LOAD 2 LOADTYPE Snow TITLE SNOW SLOPE JOINT LOAD 5 TO 8 FY -0.2 LOAD 3 LOADTYPE Ice TITLE ICE JOINT LOAD 5 TO 8 FY -0.152 LOAD 4 LOADTYPE Wind TITLE WINDX+ MEMBER LOAD 5 6 13 16 20 25 TO 27 UNI GX 0.01 1 3 UNI GX 0.21 1 3 UMOM GZ -1.12 LOAD 5 LOADTYPE Wind TITLE WINDX- MEMBER LOAD 7 8 14 15 18 28 30 31 UNI GX -0.01 1 3 UNI GX -0.21 1 3 UMOM GZ 1.12 LOAD 6 LOADTYPE Wind TITLE WINDZ+ MEMBER LOAD 5 7 14 19 20 25 30 32 UNI GZ 0.01 1 3 UNI GZ 0.22 1 3 UMOM GX 1.21 LOAD 7 LOADTYPE Wind TITLE WINDZ- MEMBER LOAD 6 8 13 15 17 26 28 29 UNI GZ -0.01 1 3 UNI GZ -0.22 1 3 UMOM GX -1.21 LOAD COMB 11 1.4D 1 1.4 LOAD COMB 12 1.2D+0.5S 1 1.2 3 0.5 LOAD COMB 13 1.2D+1.6S+0.8WX+ 1 1.2 2 1.6 4 0.8 LOAD COMB 14 1.2D+1.6S+0.8WX- 1 1.2 2 1.6 5 0.8 LOAD COMB 15 1.2D+1.6S+0.8WZ+ 1 1.2 2 1.6 6 0.8 LOAD COMB 16 1.2D+1.6S+0.8WZ- 1 1.2 2 1.6 7 0.8 LOAD COMB 17 1.2D+1.6WX++0.5S 1 1.2 4 1.6 2 0.5 LOAD COMB 18 1.2D+1.6WX-+0.5S 1 1.2 5 1.6 2 0.5 LOAD COMB 19 1.2D+1.6WZ++0.5S 1 1.2 6 1.6 2 0.5 LOAD COMB 20 1.2D+1.6WZ-+0.5S 1 1.2 7 1.6 2 0.5 LOAD COMB 21 1.2D+0.2S 1 1.2 3 0.2 LOAD COMB 22 0.9D+1.6WX+ 1 0.9 4 1.6 LOAD COMB 23 0.9D+1.6WX- 1 0.9 5 1.6 LOAD COMB 24 0.9D+1.6WZ+ 1 0.9 6 1.6 LOAD COMB 25 0.9D+1.6WZ- 1 0.9 7 1.6 LOAD COMB 26 0.9D 1 0.9 LOAD COMB 31 1.2D+0.2I+0.5S 1 1.2 3 0.2 2 0.5 LOAD COMB 32 1.2D+I+0.5S 1 1.2 3 1.6 2 0.5 LOAD COMB 33 0.9D+I 1 0.9 3 1.0 LOAD COMB 41 D 1 1.0 LOAD COMB 42 D+S 1 1.0 2 1.0 LOAD COMB 43 D+0.75S 1 1.0 2 0.75 LOAD COMB 44 D+WX+ 1 1.0 4 1.0 LOAD COMB 45 D+WX- 1 1.0 5 1.0 LOAD COMB 46 D+WZ+ 1 1.0 6 1.0 LOAD COMB 47 D+WZ- 1 1.0 7 1.0 LOAD COMB 48 D+0.75WX++0.75S 1 1.0 4 0.75 2 0.75 LOAD COMB 49 D+0.75WX-+0.75S 1 1.0 5 0.75 2 0.75 LOAD COMB 50 D+0.75WZ++0.75S 1 1.0 6 0.75 2 0.75 LOAD COMB 51 D+0.75WZ-+0.75S 1 1.0 7 0.75 2 0.75 LOAD COMB 52 0.6D+WX+ 1 0.6 4 1.0 LOAD COMB 53 0.6D+WX- 1 0.6 5 1.0 LOAD COMB 54 0.6D+WZ+ 1 0.6 6 1.0 LOAD COMB 55 0.6D+WZ- 1 0.6 7 1.0 LOAD COMB 56 0.6D 1 0.6 LOAD COMB 61 D+0.7I 1 1.0 3 0.7 LOAD COMB 62 D+0.7I+S 1 1.0 3 0.7 2 1.0 LOAD COMB 63 0.6D+0.7I 1 0.6 3 0.7 PERFORM ANALYSIS DEFINE ENVELOPE 41 TO 56 61 TO 63 ENVELOPE 1 TYPE SERVICEABILITY 11 TO 26 31 TO 33 ENVELOPE 2 TYPE STRENGTH END DEFINE ENVELOPE LOAD LIST ENV 1 2 PARAMETER 1 CODE AISC UNIFIED 2010 METHOD LRFD FYLD 6624 MEMB 5 TO 8 13 15 TO 20 25 TO 32 FYLD 7200 MEMB 1 3 21 TO 24 FU 8352 MEMB 5 TO 8 13 15 TO 20 25 TO 32 FU 9360 MEMB 1 3 21 TO 24 RATIO 0.8 MEMB 1 3 5 TO 8 13 TO 32 DJ2 1 MEMB 5 20 25 DJ1 5 MEMB 5 20 25 DJ2 4 MEMB 6 13 26 DJ1 6 MEMB 6 13 26 DJ2 3 MEMB 8 15 28 DJ1 8 MEMB 8 15 28 DJ2 2 MEMB 7 14 30 DJ1 7 MEMB 7 14 30 DJ2 16 MEMB 3 22 24 DJ1 13 MEMB 3 22 24 DJ2 15 MEMB 1 21 23 DJ1 14 MEMB 1 21 23 DFF 360 MEMB 1 3 5 TO 8 13 TO 32 LZ 5 MEMB 5 TO 8 13 TO 15 20 25 26 28 30 LY 5 MEMB 5 TO 8 13 TO 15 20 25 26 28 30 UNT 0 MEMB 9 TO 12 CHECK CODE MEMB 1 3 5 TO 8 13 TO 32 FINISH

Tank · Answer

It's a matter of stiffness. Look at node 9, load case 1. It has four members framing into. The moments at this node must sum to zero. Member 16 has a major axis moment at node 9 of -1.003 k-in. Members 5 and 20 have a 0.615 k-in and 0.368 k-in minor axis moment which resist the end moment in member 16. That leaves 0.02 k-in unaccounted for. Guess where it went. Yep, torsion in member 19. Statics works once again! If you would have been doing this by hand, you would neglected the torsional stiffness in member 19 and divided that 0.02 k-in between the two column members. Computers think of everything though. (Well, more or less.) BTW, this tiny bit of torque is nothing to get your panties in a wad about. 
 Regarding load case 6 and 7. That is incorrect to apply a moment. The lateral forces being above the base is what applies the overturning moment. You don't have to add your own.