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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  • J1d,

    You wrote that "But there is no warping restraint required in this calculation example (it is Case 3 in AISC Design Guideline Series 9)."

    Here is my opinion on that.

    Case 3 on page 58 of the AISC Design Guide stands for the following torsional end restraints :

    At left support : Theta = 0, Theta' (first derivative of Theta) = 0

    At right support : Theta' (first derivative of Theta) = 0

    In the table on page 108 of the AISC Design Guide 9,

    Theta = 0 stands for no rotation,

    Theta' (first derivative of Theta) = 0 stands for cross section cannot warp

    In other words, case 3 represents a condition where neither end of the member is allowed to warp, which means, warping is restrained at both ends. In this situation, as shown in equation 2.3 on page 3 of the Design Guide, the twist at any point along the longitudinal axis of the member comprises of a component which is associated with the St.Venant's Torsional Constant (J, or IX as it is known in STAAD) and another component associated with Cw, the warping constant.

    So, in my view, the AISC Guide does assume that warping restraint is available for case 3.

    As I wrote in my previous response, in STAAD, the default condition is that both ends of a member are allowed to warp freely. For this case, the twist in the member is solely a function of the St.venant's Torsional Constant J. That is why STAAD is not using Cw.

    To ensure that STAAD uses Cw, you need to specify SET WARP.



  • Kris,

    Case 3 with alpha=0.3 (Page 60) suits my calculation case better. Where you can find the Torsional End Restraint conditions are Theta=Theta''= 0 (Theta' not equal to 0). As the definition on page 108, the member can warp freely at the both end.

    Also on Page 60, it is indicated that "Concentrated torque at alpha=0.3 on member with pinned ends" You can compare this page with Page 70 for the difference of Torsional End Restraints.

    Regards,

    j1d

Reply
  • Kris,

    Case 3 with alpha=0.3 (Page 60) suits my calculation case better. Where you can find the Torsional End Restraint conditions are Theta=Theta''= 0 (Theta' not equal to 0). As the definition on page 108, the member can warp freely at the both end.

    Also on Page 60, it is indicated that "Concentrated torque at alpha=0.3 on member with pinned ends" You can compare this page with Page 70 for the difference of Torsional End Restraints.

    Regards,

    j1d

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