Effect of Angle Bar Offset

Hi to all,

Please find attached STAAD input file below.

I am a bit confused with the design ratio (strength) of angle bar members which has offset property. My structure is a mechanical car lift with angle bars and channels as frame and steel plate as the platform. I applied offsets and beta angle property to account for the effect of actual positioning of each members but somehow found some hard time  to understand the design consideration used by STAAD. The two small piece of angle bar (350mm) is acting inconsistently, using section of 75x75x6.0mm angle bar, i get max design ratio of 1.126, but if i used a section of 50x50x5.0mm all members passed the analysis and safe. I have gone all thru the previous discussions regarding member offsets and i just want to know the real story behind this offset function of STAAD. Is this applicable for offsetting members perpendicular to its cross section? How does STAAD recalculates the stiffness of the structure based on the offset value of the members?

Any advice and clarifications will be highly appreciated! Thank you in advance!

Master Mac

STAAD INPUT FILE:

STAAD SPACE
START JOB INFORMATION
ENGINEER DATE 28-Oct-15
END JOB INFORMATION
INPUT WIDTH 79
UNIT METER KN
JOINT COORDINATES
1 0 0 0; 2 5 0 0; 3 0 0 2.3; 4 5 0 2.3; 5 1.3 0 0; 6 1.3 0 2.3; 7 2.5 0 0;
8 2.5 0 2.3; 9 3.7 0 0; 10 3.7 0 2.3; 11 0 0 1.95; 12 5 0 1.95; 13 1.3 0 1.95;
14 2.5 0 1.95; 15 3.7 0 1.95; 16 0 0 0.35; 17 5 0 0.35; 18 1.3 0 0.35;
19 2.5 0 0.35; 20 3.7 0 0.35; 22 0.315 0 2.3; 24 4.685 0 2.3; 25 0.315 0 0;
26 4.685 0 0; 27 0 0 1.51; 28 5 0 1.51; 29 0 0 0.79; 30 5 0 0.79;
31 1.3 0 0.79; 32 3.7 0 0.79; 33 1.3 0 1.51; 34 3.7 0 1.51; 35 0.315 0 0.79;
36 0.315 0 1.51; 37 4.685 0 0.79; 38 4.685 0 1.51; 39 0.52 0 2.3;
40 0.75 0 2.3; 41 4.25 0 2.3; 42 4.48 0 2.3; 43 4.25 0 1.95; 44 4.48 0 1.95;
45 0.52 0 1.95; 46 0.75 0 1.95;
MEMBER INCIDENCES
2 3 22; 3 1 16; 4 2 17; 6 6 8; 7 5 18; 9 8 10; 10 7 19; 12 10 41; 13 9 20;
14 11 3; 15 12 4; 16 13 6; 17 14 8; 18 15 10; 19 16 29; 20 17 30; 21 18 31;
22 19 14; 23 20 32; 24 16 18; 25 11 45; 31 22 39; 33 24 4; 34 13 14; 35 14 15;
36 15 43; 37 18 19; 38 19 20; 39 20 17; 40 2 26; 41 5 25; 42 7 5; 43 9 7;
44 25 1; 45 26 9; 47 18 14; 48 19 15; 62 27 11; 63 28 12; 64 29 27; 65 30 28;
66 31 33; 67 32 34; 68 33 13; 69 34 15; 70 29 35; 71 32 37; 72 27 36; 73 34 38;
80 35 31; 81 36 33; 82 37 30; 83 38 28; 84 16 13; 85 20 12; 86 39 40; 87 40 6;
88 41 42; 89 42 24; 90 43 44; 91 44 12; 92 45 46; 93 46 13; 94 45 39; 95 46 40;
96 43 41; 97 44 42;
ELEMENT INCIDENCES SHELL
50 16 1 5 18; 51 18 5 7 19; 52 19 7 9 20; 53 20 9 2 17; 54 3 11 13 6;
55 6 13 14 8; 56 8 14 15 10; 57 10 15 12 4; 59 13 18 19 14; 60 14 19 20 15;
74 16 18 31 29; 75 27 29 31 33; 76 11 27 33 13; 77 32 20 17 30; 78 34 32 30 28;
79 15 34 28 12;
START GROUP DEFINITION
MEMBER
_CH200X75X3.0MM 2 6 9 12 31 33 40 TO 45 86 TO 89
_EA75X75X6.0MM 3 4 7 10 13 TO 25 34 TO 39 62 TO 73 80 TO 83 90 TO 97
_EA50X50X5.0MM 47 48 84 85
_FRAMING 2 TO 4 6 7 9 10 12 TO 25 31 33 TO 45 47 48 62 TO 73 80 TO 97
ELEMENT
_PLATE_4.0MM 50 TO 57 59 60 74 TO 79
END GROUP DEFINITION
START USER TABLE
TABLE 1
UNIT METER KN
ANGLE
75X75X5MM
0.075 0.075 0.005 0.0148867 0.00025 0.00025
TABLE 2
UNIT METER KN
CHANNEL
LIP_C_200X75X20X3.0MM
0.001032 0.2 0.003 0.075 0.003 6.1917e-006 5.40272e-007 3.096e-009 -
0.0171977 0.0006 0.0003
TABLE 3
UNIT METER KN
ANGLE
130X130X8MM
0.13 0.13 0.008 0.0258489 0.000693333 0.000693333
TABLE 4
UNIT METER KN
ANGLE
75X75X6.0MM
0.075 0.075 0.006 0.0148239 0.0003 0.0003
END
ELEMENT PROPERTY
50 TO 57 59 60 74 TO 79 THICKNESS 0.004
DEFINE MATERIAL START
ISOTROPIC STEEL
E 2.05e+008
POISSON 0.3
DENSITY 76.8195
ALPHA 1.2e-005
DAMP 0.03
TYPE STEEL
STRENGTH FY 253200 FU 407800 RY 1.5 RT 1.2
END DEFINE MATERIAL
MEMBER PROPERTY BRITISH
2 6 9 12 31 33 40 TO 45 86 TO 89 UPTABLE 2 LIP_C_200X75X20X3.0MM
MEMBER PROPERTY BRITISH
47 48 84 85 TABLE ST UA50X50X5
MEMBER PROPERTY BRITISH
3 4 7 10 13 TO 25 34 TO 39 62 TO 73 80 TO 83 90 TO 97 UPTABLE 4 75X75X6.0MM
CONSTANTS
BETA 135 MEMB 94 96
BETA 225 MEMB 95 97
BETA 45 MEMB 3 7 10 14 16 17 19 21 22 25 34 TO 36 47 62 64 66 68 72 73 81 -
83 84 90 TO 93
BETA 315 MEMB 4 13 15 18 20 23 24 37 TO 39 48 63 65 67 69 TO 71 80 82 85
MATERIAL STEEL ALL
SUPPORTS
22 24 TO 26 35 TO 38 PINNED
MEMBER OFFSET
3 4 7 10 13 TO 25 34 TO 39 47 48 62 TO 69 84 85 90 TO 93 START 0 -0.0454 0
3 4 7 10 13 TO 25 34 TO 39 47 48 62 TO 69 84 85 90 TO 93 END 0 -0.0454 0
70 TO 73 80 TO 83 START 0 -0.1204 0
70 TO 73 80 TO 83 END 0 -0.1204 0
94 TO 97 START 0 -0.0796 0
94 TO 97 END 0 -0.0796 0
LOAD 1 LOADTYPE Dead TITLE SELFWEIGHT
SELFWEIGHT Y -1 LIST 2 TO 4 6 7 9 10 12 TO 25 31 33 TO 45 47 48 50 TO 57 59 -
60 62 TO 85
LOAD 2 LOADTYPE Live TITLE WHEEL LOAD, UNFACTORED
MEMBER LOAD
24 CON GY -4.91 1
93 CON GY -4.91 0.25
36 39 CON GY -4.91 0.3
LOAD COMB 3 1.35DL + 1.5P
1 1.35 2 1.5
LOAD COMB 4 1.0DL + 1.0P
1 1.0 2 1.0
PERFORM ANALYSIS
LOAD LIST 3
PARAMETER 1
CODE EN 1993-1-1:2005
RATIO 1 mem _FRAMING
SGR 1 mem _FRAMING
TORSION 0 mem _FRAMING
TRACK 2 mem _FRAMING
CHECK CODE mem _FRAMING
FINISH