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Design of Members for Torsion
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摘要: 介绍了AISC 360-16《建筑钢结构标准》（简称《美国钢标》）受扭杆件设计方法。《美国钢标》的受扭杆件计算只考虑闭口截面，计算公式采用自由扭转形式并考虑约束扭转的有利因素。杆件的扭转分自由扭转和约束扭转。对于开口截面，自由扭转和约束扭转的正应力和剪应力都比较大，而对于闭口截面，自由扭转起控制作用，约束扭转不大。因此，《美国钢标》的闭口截面扭转设计，首先假定扭矩全部由自由扭转即纯扭转剪应力承受（也即通常所说的圣维南扭转应力），之后根据约束情况加以修正。纯扭转剪应力沿截面均匀分布，其值等于扭矩除以扭转常数C。考虑屈曲影响，扭矩能力为扭转常数乘以临界剪应力Fcr。
圆管和矩形管的扭转设计强度φTTn由扭转屈服和扭转屈曲临界力确定，即：Tn=FcrC（Fcr为临界剪应力；C为扭转常数）；受扭抗力系数φT=0.9。不同构件Fcr取值不同。
1）长圆管的扭转局部屈曲临界应力不受端部约束条件的影响，并考虑了一定的初始缺陷。中长管和短管的端部约束会提高局部屈曲临界力。
2）矩形管临界应力具有与G章弯曲剪切应力相同的剪切屈曲系数kv=5.0。矩形管长边扭转剪应力分布与I形梁腹板中的剪应力是一致的。
外扭矩由自由扭转抗力和约束扭转抗力组成抵抗矩。开口截面的这两个抗力都很小，因此《美国钢标》只考虑闭口截面受扭，在实际应用时，对于开口截面应采取构造措施防止受扭。Abstract: 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 Fcr.
Design torsion strength of round and rectangular hollow structural sections is φTTn and determined by the critical force of torsional yield and torsional buckling, namely and torsional resistance coefficient φT=0. 9. Fcr 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 kv=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.-
Key words:
- torsion member /
- pure torsion /
- constrained torsion
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