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The tension flange acts like the string in an archer's bow and the maximum force that can be developed is limited only by the yield stress of the flange material.
The compression flange acts like a strut and is susceptible to buckling before yield stress can be developed. Unless the compression flange is fully restrained then the beam may fail by lateral torsional buckling.

Lateral torsional buckling.

Clause 9.6.1 requires all beams to be restrained at their supports. The strength and stiffness of the restraints are checked using clause 9.12.5.
The restraint is required to hold the compression flange in place and is usually provided at the support by use of the bearing stiffeners and a suitable bearing.

Restraint at support.

• From Clause 9.6
  Determine the effective length (l_e) based on the support condition of the compression flange. The Code requires the compression flange to be supported laterally at the beams' supports in accordance with Cl. 9.12.5. This ensures that the compression flange can be assumed to have at least a pinned end support so the maximum k₁ (Cl.9.6.2) that may be assumed is 1.0 for non-cantilever beams. If end diaphragms are provided to prevent the compression flange from rotating in plan then a smaller value of k₁can be used.

Rotational end restraint.

• From Clause 9.7
  Determine the slenderness (l_LT) using the geometrical parameters of the beam.
• From Clause 9.8
  Determine the limiting moment of resistance (M_R).
• From Clause 9.9
  Determine the bending capacity (M_D)
• From Clause 9.12.5
  Determine the strength of the end bearing stiffener to provide adequate support to the compression flange.