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Torsional Stiffness

Master Mac
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Hi,

I have this kind of model with  a cantilevered structure attached unto it as shown below in the image.

I'm a bit puzzled of the deflection result and i'm worried there is something wrong in my model. Can somebody explain this: whenever i use I section in the beam holding the cantilever structure the deflection is 184.069 mm whereas using hollow section for the same beam resulted to a minimal deflection of 4.523 mm. Is it because of the torsional stiffness greatly offered by hollow section than the open section? How does STAAD calculate deflection considering torsional constant of section? The deflection reduces to almost 95% and i want to be enlightened on this.

Thank you!

M.Mac

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



  • 0 Offline in reply to Sye
    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
  • 0 Offline in reply to Master Mac

    Sye,

    Please find below image of the properties for W18x76 and Tube 450x250x10mm (closest to 450x280x10mm). I am rooting for the Torsional Constant as the main caused for the sudden decreased in deflection.

    For I section, J = 33280cm^4

    For tube 450x250x10mm , J = 118cm^4 

    But still i need to know how does STAAD accounts for torsional constant for the calculation of deflection. 

    Cheers,

    Mac

    Sye,

  • 0 Offline in reply to Master Mac
    The screenshot that you illustrated shows that

    For I section, J = 118cm^4

    For tube 450x250x10mm , J = 33280cm^4 ,

    which implies that the tube section is far more high torsionally rigid than the I shaped section and hence the Tube section is providing higher degree of resistance against the cantilever system to swivel down about its axis.



  • 0 Offline in reply to Biswatosh
    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
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