Beam Torsional Stiffness by STAAD

Chris,

Can you help me this out?

There is a simple beam resisting a torque as shown in the file attached. The beam is fixed at both ends. Calculated by STAAD, the torsional angle at the loading point is 0.121 rad. But when I do the hand calculation (using formulas in AISC Steel Design Guideline Series 9), the angle is 0.067 rad (about 50% for this calculation case).

I checked the sectional properties of the beam given in STAAD, everyone looks good except the warping constant Cw is not given.  Coicidently, when I ignore Cw in the calculation the rotational angle is close to the value from STAAD. Does STAAD consider Cw?

Regards,

j1d

 

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  • By default, only St. Venant's torsional deformation is considered in the STAAD analysis. The torsional rigidity is calculated using the assumption that both ends are free to warp for non-circular cross sections.

    If the ends of the member are prevented from warping, you can convey that information to STAAD using a command called

    SET WARP f

    where f can take on a value in the range 0 to 1. 0.0 means no warping restraint which is the default option, 1.0 means full warping restraint. Cw, the warping constant, will be computed and used in the torsional rigidity calculation if f is assigned a non-zero value. Values between 0.0 and 1.0 will result in a partial warping restraint. You can find this described in Section "5.5 Set Command Specification" of the STAAD Technical Reference manual.

    SET WARP is a global command, which means it affects every member of the model which has an untapered I-shape. STAAD does not consider this condition for other shapes.

    The complete procedure is available in the book

    Roark's Formulas for Stress & Strain

    Author : Warren C.Young

    6th edition, Published by McGraw Hill

    Section 9.3 Effect of End Constraint

    Look at example 1 in the above section. It shows the calculation of the equivalent stiffness constant K'.



  • Kris,

    Thanks for the information of SET WARP for end warping restraint.

    But there is no warping restraint required in this calculation example (it is Case 3 in AISC Design Guideline Series 9). What I found is that the I-beam rotation under a concentrated torque from STAAD is about 2 times of that from the analytical equation. And it seems STAAD simply ignored the cross-sectional warping constant Cw. I just want to confirm this.

    Regards,

    j1d

Reply
  • Kris,

    Thanks for the information of SET WARP for end warping restraint.

    But there is no warping restraint required in this calculation example (it is Case 3 in AISC Design Guideline Series 9). What I found is that the I-beam rotation under a concentrated torque from STAAD is about 2 times of that from the analytical equation. And it seems STAAD simply ignored the cross-sectional warping constant Cw. I just want to confirm this.

    Regards,

    j1d

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