Modal and Rigid Diaphragm

Hi,

I several question regarding STAAD. My example model is attached below.
1. I expect that the Mode 1 and Mode 2 will be pure translation in X and Z since my building and the load was symmetric. But, I got Mode 1 and 2 is not pure translation. Is there some mistake in my model? 
2. I think my building is not rigid enough so that I apply rigid diaphragm to make the building more rigid and also increasing my column size. Still Mode 1 and 2 is not pure translation. Did I make a mistake while assigning the rigid diaphragm?
3. How to determine the center of mass if I choose "calculating" in the diaphragm definition?
4. In respond spectrum definition, what is the function of the "fill box" beside X Y X ?

Thank you in advance.

Regards,
Charismawelly

STAAD SPACE
START JOB INFORMATION
ENGINEER DATE 18-Sep-18
END JOB INFORMATION
INPUT WIDTH 79
UNIT METER KN
JOINT COORDINATES
1 0 0 0; 2 0 4 0; 3 4 4 0; 4 4 0 0; 5 0 0 4; 6 0 4 4; 7 4 4 4; 8 4 0 4;
9 0 0 8; 10 0 4 8; 11 4 4 8; 12 4 0 8; 13 8 4 0; 14 8 0 0; 15 8 4 4; 16 8 0 4;
17 8 4 8; 18 8 0 8; 19 0 8 0; 20 4 8 0; 21 0 8 4; 22 4 8 4; 23 0 8 8; 24 4 8 8;
25 8 8 0; 26 8 8 4; 27 8 8 8; 28 0 12 0; 29 4 12 0; 30 0 12 4; 31 4 12 4;
32 0 12 8; 33 4 12 8; 34 8 12 0; 35 8 12 4; 36 8 12 8;
MEMBER INCIDENCES
1 1 2; 2 4 3; 3 2 3; 4 2 6; 5 3 7; 6 5 6; 7 8 7; 8 6 7; 9 6 10; 10 7 11;
11 9 10; 12 12 11; 13 10 11; 14 14 13; 15 3 13; 16 13 15; 17 16 15; 18 7 15;
19 15 17; 20 18 17; 21 11 17; 26 2 19; 27 3 20; 28 19 20; 29 19 21; 30 20 22;
31 6 21; 32 7 22; 33 21 22; 34 21 23; 35 22 24; 36 10 23; 37 11 24; 38 23 24;
39 13 25; 40 20 25; 41 25 26; 42 15 26; 43 22 26; 44 26 27; 45 17 27; 46 24 27;
47 19 28; 48 20 29; 49 28 29; 50 28 30; 51 29 31; 52 21 30; 53 22 31; 54 30 31;
55 30 32; 56 31 33; 57 23 32; 58 24 33; 59 32 33; 60 25 34; 61 29 34; 62 34 35;
63 26 35; 64 31 35; 65 35 36; 66 27 36; 67 33 36;
ELEMENT INCIDENCES SHELL
22 2 6 7 3; 23 6 10 11 7; 24 3 7 15 13; 25 7 11 17 15; 68 19 21 22 20;
69 21 23 24 22; 70 20 22 26 25; 71 22 24 27 26; 72 28 30 31 29; 73 30 32 33 31;
74 29 31 35 34; 75 31 33 36 35;
ELEMENT PROPERTY
22 TO 25 68 TO 75 THICKNESS 0.15
DEFINE MATERIAL START
ISOTROPIC CONCRETE
E 2.17185e+007
POISSON 0.17
DENSITY 23.5616
ALPHA 1e-005
DAMP 0.05
TYPE CONCRETE
STRENGTH FCU 27579
END DEFINE MATERIAL
MEMBER PROPERTY
1 2 6 7 11 12 14 17 20 26 27 31 32 36 37 39 42 45 47 48 52 53 57 58 60 63 -
66 PRIS YD 0.4 ZD 0.4
3 TO 5 8 TO 10 13 15 16 18 19 21 28 TO 30 33 TO 35 38 40 41 43 44 46 -
49 TO 51 54 TO 56 59 61 62 64 65 67 PRIS YD 0.4 ZD 0.2
CONSTANTS
MATERIAL CONCRETE ALL
SUPPORTS
1 4 5 8 9 12 14 16 18 FIXED
FLOOR DIAPHRAGM
DIA 1 TYPE RIG HEI 4 MASTER 7
DIA 2 TYPE RIG HEI 8 MASTER 22
DIA 3 TYPE RIG HEI 12 MASTER 31
LOAD 1 LOADTYPE Dead TITLE DEAD
SELFWEIGHT Y -1
ELEMENT LOAD
22 TO 25 68 TO 75 PR GY -1.5
LOAD 2 LOADTYPE Live TITLE LIVE
ELEMENT LOAD
22 TO 25 68 TO 75 PR GY -2.5
LOAD 3 LOADTYPE None TITLE MODAL
SELFWEIGHT X 1
SELFWEIGHT Z 1
SELFWEIGHT Y 1
ELEMENT LOAD
22 TO 25 68 TO 75 PR GX 1.5
22 TO 25 68 TO 75 PR GZ 1.5
22 TO 25 68 TO 75 PR GY 1.5
22 TO 25 68 TO 75 PR GX 2.5
22 TO 25 68 TO 75 PR GZ 2.5
22 TO 25 68 TO 75 PR GY 2.5
MODAL CALCULATION REQUESTED
LOAD 4 LOADTYPE None TITLE SEISMICX
SELFWEIGHT X 1
SELFWEIGHT Z 1
SELFWEIGHT Y 1
ELEMENT LOAD
22 TO 25 68 TO 75 PR GX 1.5
22 TO 25 68 TO 75 PR GZ 1.5
22 TO 25 68 TO 75 PR GY 1.5
22 TO 25 68 TO 75 PR GX 2.5
22 TO 25 68 TO 75 PR GZ 2.5
22 TO 25 68 TO 75 PR GY 2.5
SPECTRUM CQC X 1 ACC SCALE 9.806 DAMP 0.05 LIN
0 0.211; 0.183 0.528; 0.915 0.528; 0.965 0.476; 1.015 0.433; 1.115 0.397;
1.215 0.367; 1.315 0.341; 1.415 0.319; 1.515 0.299; 1.615 0.281; 1.715 0.266;
1.815 0.252; 1.915 0.24; 2.015 0.228; 2.115 0.218; 2.215 0.208; 2.315 0.2;
2.415 0.192; 2.515 0.185; 2.615 0.178; 2.715 0.171; 2.815 0.166; 2.915 0.16;
3.015 0.155; 3.115 0.15; 3.215 0.146; 3.315 0.141; 3.415 0.137; 3.515 0.134;
3.615 0.13; 3.715 0.127; 3.815 0.123; 4 0.121;
LOAD 5 LOADTYPE None TITLE SEISMICZ
SPECTRUM CQC Z 1 ACC SCALE 9.806 DAMP 0.05 LIN
0 0.211; 0.183 0.528; 0.915 0.528; 0.965 0.476; 1.015 0.433; 1.115 0.397;
1.215 0.367; 1.315 0.341; 1.415 0.319; 1.515 0.299; 1.615 0.281; 1.715 0.266;
1.815 0.252; 1.915 0.24; 2.015 0.228; 2.115 0.218; 2.215 0.208; 2.315 0.2;
2.415 0.192; 2.515 0.185; 2.615 0.178; 2.715 0.171; 2.815 0.166; 2.915 0.16;
3.015 0.155; 3.115 0.15; 3.215 0.146; 3.315 0.141; 3.415 0.137; 3.515 0.134;
3.615 0.13; 3.715 0.127; 3.815 0.123; 4 0.121;
LOAD 6 LOADTYPE Mass TITLE MASS
SELFWEIGHT Y -1 LIST ALL
PERFORM ANALYSIS
FINISH