Torsional Stiffness

Please upload the model. Also what is the TUBE section you are using ?

Hi Sye,

Please find below staad input file. This is based on hollow section 450x280x10mm .

STAAD SPACE
START JOB INFORMATION
ENGINEER DATE 12-Oct-15
END JOB INFORMATION
INPUT WIDTH 79
UNIT METER KN
JOINT COORDINATES
1 0 0 0; 2 4.5 0 0; 3 0 0 -0.3; 4 0 0 1.5; 5 0 0 1.3; 6 4.5 0 1.3;
7 4.5 0 -0.3; 8 4.5 0 1.5; 9 1.35 0 0; 10 2.15 0 0; 11 0.748 0 0; 12 2.752 0 0;
13 1.35 -0.7 0; 14 2.15 -0.7 0; 15 1.35 -0.15 0; 16 2.15 -0.15 0;
19 1.35 -0.7 -2.1; 20 2.15 -0.7 -2.1; 21 1.35 -0.7 -1.35; 22 2.15 -0.7 -1.35;
23 2.15 -0.515873 -0.994898; 24 1.35 -0.515873 -0.994898;
25 2.15 -0.147619 -0.284695; 26 1.35 -0.147619 -0.284695;
27 1.75 -0.331746 -0.639796;
MEMBER INCIDENCES
1 3 1; 2 1 5; 3 5 4; 4 7 2; 5 2 6; 6 6 8; 7 1 11; 8 5 6; 9 9 10; 10 10 12;
11 11 9; 12 12 2; 13 13 15; 14 14 16; 15 14 12; 16 13 11; 17 15 9; 18 16 10;
19 15 16; 20 13 14; 25 21 13; 26 22 14; 27 22 23; 28 21 24; 29 21 22; 30 20 22;
31 19 21; 32 23 25; 33 24 26; 34 25 10; 35 26 9; 36 23 27; 37 24 27; 38 27 26;
39 27 25; 40 20 19;
START GROUP DEFINITION
MEMBER
_SHS100X100X5MM 15 16 19 20 27 TO 29 32 TO 35 40
_EA50X50X5MM 36 TO 39
_8"X8"X46.1KG.M. 13 14 17 18 25 26 30 31
_NEW_STRUCTURE 13 TO 20 25 TO 40
END GROUP DEFINITION
START USER TABLE
TABLE 1
UNIT METER KN
WIDE FLANGE
350X250X69.2
0.00884459 0.341045 0.00891324 0.247812 0.012149 0.000186514 3.08333e-005 -
3.48684e-007 0.00282324 0.00602135
TABLE 2
UNIT METER KN
WIDE FLANGE
300X300X87
0.0111137 0.300237 0.00985337 0.297736 0.0141643 0.000189212 6.23289e-005 -
6.11537e-007 0.00267922 0.00843444
TABLE 3
UNIT METER KN
WIDE FLANGE
W300X00X93KGM
0.0117 0.3 0.01 0.3 0.015 0.000199328 6.75225e-005 7.65e-007 0.003 0.009
END
DEFINE MATERIAL START
ISOTROPIC STEEL
E 2.05e+008
POISSON 0.3
DENSITY 76.8195
ALPHA 1.2e-005
DAMP 0.03
END DEFINE MATERIAL
MEMBER PROPERTY AMERICAN
8 UPTABLE 1 350X250X69.2
MEMBER PROPERTY BRITISH
15 16 19 20 27 TO 29 32 TO 35 40 TABLE ST TUBE TH 0.005 WT 0.1 DT 0.1
36 TO 39 TABLE ST UA50X50X5
13 14 17 18 25 26 30 31 TABLE ST UC203X203X46
MEMBER PROPERTY BRITISH
1 TO 6 UPTABLE 3 W300X00X93KGM
7 9 TO 12 TABLE ST TUBE TH 0.01 WT 0.28 DT 0.45
CONSTANTS
BETA 45 MEMB 36 TO 39
BETA 90 MEMB 13 14 17 18
MATERIAL STEEL ALL
SUPPORTS
3 4 7 8 PINNED
MEMBER OFFSET
13 TO 20 25 TO 40 START 0 -0.23 0
13 TO 20 25 TO 40 END 0 -0.23 0
************************************************UNFACTORED LOADS
LOAD 1 LOADTYPE Dead TITLE SELFWEIGHT
SELFWEIGHT Y -1
LOAD 2 LOADTYPE None TITLE UNFACTORED LOAD 1
MEMBER LOAD
9 CON GY -110.61 0.6455
9 CON GZ -11.061 0.6455
10 CON GY -110.61 0.4865
10 CON GZ -11.061 0.4865
LOAD 3 LOADTYPE None TITLE UNFACTORED LOAD 2
JOINT LOAD
1 FY -67.1
LOAD 4 LOADTYPE None TITLE UNFACTORED LOAD 3
JOINT LOAD
5 FY -62.195
LOAD 5 LOADTYPE None TITLE UNFACTORED LOAD 4
MEMBER LOAD
5 CON GY -117.03 0.8
LOAD 6 LOADTYPE Dead TITLE UNFACTORED CABIN WEIGHT
MEMBER LOAD
30 31 UNI GY -3.4
25 26 UNI GY -3.4 0 0.705
**************************************************FACTORED LOADS
LOAD 7 LOADTYPE None TITLE FACTORED LOAD 1
REPEAT LOAD
2 1.5
LOAD 8 LOADTYPE None TITLE FACTORED LOAD 2
REPEAT LOAD
3 1.5
LOAD 9 LOADTYPE None TITLE FACTORED LOAD 3
REPEAT LOAD
4 1.5
LOAD 10 LOADTYPE None TITLE FACTORED LOAD 4
REPEAT LOAD
5 1.5
LOAD 11 LOADTYPE Dead TITLE FACTORED LOAD 5
REPEAT LOAD
6 1.5
LOAD COMB 12 SLS
2 1.0 3 1.0 4 1.0 5 1.0 6 1.0
LOAD COMB 13 ULS
7 1.0 8 1.0 9 1.0 10 1.0 11 1.0
PERFORM ANALYSIS



LOAD LIST 12
PARAMETER 1
CODE EN 1993-1-1:2005
SGR 1 ALL
TRACK 2 ALL
NA 7 ALL
CHECK CODE ALL
FINISH

Hi Geeky biswa,

Yes that may be a big contributing factor for the sudden change in displacement but my follow up question is how does STAAD accounts for torsional constant in the calculation of deflection? Any reference? I'm currently going thru all the technical references as i need to provide a straight forward answer relating torsional stiffness in calculation of member displacement.

Cheers,
m.mac

The torsional constant is computed by the St. Venant principle.
See the following discussion below that will explain your query.

communities.bentley.com/.../274896

Hi,

Thanks for pointing me on this article but the discussion pertains to "calculation of the torsional constant" and not "how torsional constant is accounted for the calculation of member displacement subject to torsion".

Or should i say, how does torsional constant affects the calculation of member displacement subject to torsion? 

Cheers,
M.Mac

The calculation of rotation based on the Torsional stiffness is similar to the calculation of the displacent due to flexure and axial forces. Staad first solves the stiffness matrix to determine the 6 displacement like translation against X, Y and z, rotation about local X, Y and Z . Now, the corresponding forces like Fx, Fy ,Fz ,Mx ,My and MZ is determined based on the stiffness offered along those respective degree of freedom. Now in your model as the Torsional Constant for HSS is too high compared to the I shaped section, hence the corresponding rotation is also too low in case in HSS section. I don't see any problem in the displacement.

Hi Geeky,

Thank you.

When does STAAD calculates stiffness matrix based from torsional constant? Is it when we set TORSION parameter to other values than 0 or the usual/default assumption (TORSION 0) when a member is subjected to torsion only?

I will be grateful if you can give me any reference or sample calculation that accounts for torsional rigidity for the calculation of rotation and displacement.

Cheers,
M.Mac

Hi Geeky,

Thank you.

When does STAAD calculates stiffness matrix based from torsional constant? Is it when we set TORSION parameter to other values than 0 or the usual/default assumption (TORSION 0) when a member is subjected to torsion only?

I will be grateful if you can give me any reference or sample calculation that accounts for torsional rigidity for the calculation of rotation and displacement.

Cheers,
M.Mac