Torsion design general checks (Beams: AISC 360)

Auto design is not carried out in the current release, only check design (and check design is only carried out if the “check for torsion” flag is set to on in the Edit dialog or Properties Window)
Lateral restraint amplification factor (in accordance with section 4.7.3 of DG9):
1. It is assumed lateral displacement and twist are not restrained at any load point. Therefore, in accordance with section 4.7.3 of DG9, σ_by and σ_w will always be amplified in the presence of torsion.
2. To avoid a negative value, Tekla Structural Designer applies a lower limit of 0.001ksi OR N/mm² to the denominator of the amplification factor,
  (ɸF_cr^e - σ_bx)
3. F_cr^e = F_cr,bx
4. Amplification factor = 1.0 when σ_bx = 0.0
Both major and minor axis shear buckling are checked if loaded in the relevant axis. A warning is issued if the buckling limit defined in AISC Sect G2 is exceeded. Torsion design will, however, be continued - the engineer is expected to deem if the shear buckling condition is safe.
Torsion shear stresses:
1. A cross-section check is carried out at points of interest taken from the load analysis diagram.
Combined forces and torsion:
1. HSS - sections
  1. A cross-section check is carried out at points of interest taken from the load analysis diagrams as well as 10th positions along the member. In cases where the final utilization ratio approaches 1.0 we strongly recommend the engineer considers other locations, where a more critical location than that chosen in Tekla Structural Designer may exist.
2. I symmetric
  1. We take the most critical axial stress value across all axial strut lengths to determine F_cr,a
  2. We take the most critical major bending stress value across all LTB lengths to determine F_cr,bx
  3. In ASD design checks, the value used for F_cr,a and F_cr,bx is F_cr / 1.67 since F_a and F_b used in DG9 relate to the 1989 ASD Specification where this factor was effectively already taken into account.