Design of Members for Torsion

Design methods of torsion member in AISC 360-16 are introduced. Only closed sections are included for the torsion calculation in AISC 360-16. Pure torsion is considered mainly in torsion calculation with consideration for helpful factors of constrained torsion. Torsion of members is divided into pure torsion and constrained torsion. For open sections, normal stress and shear stress under pure torsion and constrained torsion are both high, while for closed sections, pure torsion plays a controlling role and the stress under constrained torsion is negligible. Hence, the torsion design of closed sections in AISC 360-16 assumes that the torque is all borne by pure torsion and is then modified in terms of the constraints. Pure torsional shear stress is uniformly distributed along the section and is equal to the torque divided by torsion constant C. Considering the buckling effect, torsional capacity is the product of torsion constant C and critical shear stress F_cr.
Design torsion strength of round and rectangular hollow structural sections is φ_TT_n and determined by the critical force of torsional yield and torsional buckling, namely and torsional resistance coefficient φ_T=0. 9. F_cr is different because of different members.
1) Local buckling critical stress of long tubes under torsion is not affected by the end constraints, in which certain initial imperfections are considered. For medium and short tubes, end constraints will increase the local buckling critical force.
2) The critical stress of rectangular tubes has the same shear buckling coefficient k_v=5. 0 as the bending shear stress of Chapter G. The distribution of torsional shear stress on the long sides of rectangular tubes is consistent with the shear stress in the webs of I-shaped beams.
The external torque consists of free torsional resistance and restrained torsional resistance. The two resistances of the open section are very small, so the AISC 360-10 only considers the torsion of the closed section. In practical application, structural measures shall be taken for the open section to prevent torsion.