How to model plates with a spacing similar to angles modeling with space

Hello .

Model details :

Its  a steel structure with the outer frame made of channels (ISMC 300) & the inter connection between channels is made by beams (ISMB 300 ) & angles. 

Finally the plates attached to the both the ends (front and back ) of frame. 

My doudt : how to create the plate at the ends (i.e two plates with spacing) . I am able to crate the plate only at the mid plane.

Pls. some one help to clear the modeling problem

model details and fig attached.

STAAD SPACE
START JOB INFORMATION
ENGINEER DATE 27-Mar-19
END JOB INFORMATION
INPUT WIDTH 79
UNIT MMS KN
JOINT COORDINATES
1 0 0 0; 2 1130 0 0; 3 2260 0 0; 4 3390 0 0; 5 4520 0 0; 6 5650 0 0;
7 0 14950 0; 8 1130 14950 0; 9 2260 14950 0; 10 3390 14950 0; 11 4520 14950 0;
12 5650 14950 0; 13 0 1475 0; 14 0 2975 0; 15 1130 1475 0; 16 2260 1475 0;
17 3390 1475 0; 18 4520 1475 0; 19 5650 1475 0; 20 1130 2975 0; 21 2260 2975 0;
22 3390 2975 0; 23 4520 2975 0; 24 5650 2975 0; 25 0 4475 0; 26 1130 4475 0;
27 2260 4475 0; 28 3390 4475 0; 29 4520 4475 0; 30 5650 4475 0; 31 0 5975 0;
32 1130 5975 0; 33 2260 5975 0; 34 3390 5975 0; 35 4520 5975 0; 36 5650 5975 0;
37 0 7475 0; 38 1130 7475 0; 39 2260 7475 0; 40 3390 7475 0; 41 4520 7475 0;
42 5650 7475 0; 43 0 8975 0; 44 1130 8975 0; 45 2260 8975 0; 46 3390 8975 0;
47 4520 8975 0; 48 5650 8975 0; 49 0 10475 0; 50 1130 10475 0; 51 2260 10475 0;
52 3390 10475 0; 53 4520 10475 0; 54 5650 10475 0; 55 0 11975 0;
56 1130 11975 0; 57 2260 11975 0; 58 3390 11975 0; 59 4520 11975 0;
60 5650 11975 0; 61 0 13475 0; 62 1130 13475 0; 63 2260 13475 0;
64 3390 13475 0; 65 4520 13475 0; 66 5650 13475 0; 67 0 3725 0; 68 1130 3725 0;
69 2260 3725 0; 70 3390 3725 0; 71 4520 3725 0; 72 5650 3725 0; 73 0 5225 0;
74 1130 5225 0; 75 2260 5225 0; 76 3390 5225 0; 77 4520 5225 0; 78 5650 5225 0;
MEMBER INCIDENCES
1 1 13; 2 1 2; 3 2 3; 4 3 4; 5 4 5; 6 5 6; 7 6 19; 8 12 11; 9 11 10; 10 10 9;
11 9 8; 12 8 7; 13 2 15; 14 3 16; 15 4 17; 16 5 18; 17 13 14; 18 14 67;
19 15 20; 20 16 21; 21 17 22; 22 18 23; 23 19 24; 24 20 68; 25 21 69; 26 22 70;
27 23 71; 28 24 72; 29 25 73; 30 26 74; 31 27 75; 32 28 76; 33 29 77; 34 30 78;
35 31 37; 36 32 38; 37 33 39; 38 34 40; 39 35 41; 40 36 42; 41 37 43; 42 38 44;
43 39 45; 44 40 46; 45 41 47; 46 42 48; 47 43 49; 48 44 50; 49 45 51; 50 46 52;
51 47 53; 52 48 54; 53 49 55; 54 50 56; 55 51 57; 56 52 58; 57 53 59; 58 54 60;
59 55 61; 60 56 62; 61 57 63; 62 58 64; 63 59 65; 64 60 66; 65 61 7; 66 62 8;
67 63 9; 68 64 10; 69 65 11; 70 66 12; 71 13 15; 72 15 16; 73 16 17; 74 17 18;
75 18 19; 76 14 20; 77 20 21; 78 21 22; 79 22 23; 80 23 24; 81 25 26; 82 26 27;
83 27 28; 84 28 29; 85 29 30; 86 31 32; 87 32 33; 88 33 34; 89 34 35; 90 35 36;
91 37 38; 92 38 39; 93 39 40; 94 40 41; 95 41 42; 96 43 44; 97 44 45; 98 45 46;
99 46 47; 100 47 48; 101 49 50; 102 50 51; 103 51 52; 104 52 53; 105 53 54;
106 55 56; 107 56 57; 108 57 58; 109 58 59; 110 59 60; 111 61 62; 112 62 63;
113 63 64; 114 64 65; 115 65 66; 116 67 25; 117 68 26; 118 69 27; 119 70 28;
120 71 29; 121 72 30; 122 67 68; 123 68 69; 124 69 70; 125 70 71; 126 71 72;
127 73 31; 128 74 32; 129 75 33; 130 76 34; 131 77 35; 132 78 36; 133 73 74;
134 74 75; 135 75 76; 136 76 77; 137 77 78;
ELEMENT INCIDENCES SHELL
138 2 3 16 15; 139 3 4 17 16;
ELEMENT PROPERTY
138 139 THICKNESS 6
DEFINE MATERIAL START
ISOTROPIC STEEL
E 205
POISSON 0.3
DENSITY 7.68195e-008
ALPHA 1.2e-005
DAMP 0.03
TYPE STEEL
STRENGTH FY 0.2532 FU 0.4078 RY 1.5 RT 1.2
END DEFINE MATERIAL
MEMBER PROPERTY INDIAN
1 TO 12 17 18 23 28 29 34 35 40 41 46 47 52 53 58 59 64 65 70 116 121 127 -
132 TABLE ST ISMC300
13 TO 16 19 TO 22 24 TO 27 30 TO 33 36 TO 39 42 TO 45 48 TO 51 54 TO 57 60 -
61 TO 63 66 TO 69 76 TO 80 91 TO 95 106 TO 110 117 TO 120 128 TO 130 -
131 TABLE ST ISMB300
72 TO 75 TABLE SD ISA75X75X8 SP 150
CONSTANTS
BETA 90 MEMB 1 8 TO 22 24 TO 27 29 TO 33 35 TO 39 41 TO 45 47 TO 51 -
53 TO 57 59 TO 63 65 TO 69 76 TO 80 91 TO 95 106 TO 110 116 TO 120 -
127 TO 131
BETA 270 MEMB 2 TO 7 23 28 34 40 46 52 58 64 70 121 132
MATERIAL STEEL MEMB 1 TO 70 72 TO 80 91 TO 95 106 TO 110 116 TO 121 -
127 TO 132 138 139
SURFACE CONSTANTS
FINISH