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D1.B.1.1.4 Bending Stress

Allowable bending stress for tension and compression for a symmetrical member loaded in the plane of its minor axis, as given in Section is:

Fb = 0.66Fy

If meeting the requirements of this section of:

  1. bf/2tf ≤ 65/√(Fy)
  2. bf/tf ≤ 190/√(Fy)
  3. d/t ≤ 640(1 - 3.74 (fa/Fy))/√(Fy) when (fa/Fy) < 0.16, or than 257/√(Fy) if (fa/Fy) > 0.16
  4. The laterally unsupported length shall not exceed 76.0 bf/Fy (except for pipes or tubes), nor 20,000/(d Fy/Af)
  5. The diameter:thickness ratio of pipes shall not exceed 3,300/√(Fy)

If for these symmetrical members, bf/2tf exceeds 65/√(Fy), but is less than 95/√(Fy), Fb = Fv(0.79 - 0.002(bf/2tf)√(Fy)

For other symmetrical members which do not meet the above, Fb is calculated as the larger value computed as per AISC formulas F1-6 or F1-7 and F1-8 as applicable, but not more than 0.60Fy. An unstiffened member subject to axial compression or compression due to bending is considered fully effective when the width-thickness ratio is not greater than the following:

  • 76.0/√(Fy), for single angles or double angles with separators
  • 95.0/√(Fy), for double angles in contact
  • 127.0/√(Fy), for stems of tees

When the actual width-thickness ratio exceeds these values, the allowable stress is governed by B5 of the AISC code.

Tension and compression for the double symmetric (I & H) sections with bf/2tf less than 65/√(Fy) and bent about their minor axis, Fb = 0.75 Fy . If bf/2tf exceeds 65/√(Fy), but is less than 95/√(Fy), Fb = Fy(1.075 - 0.005(bf/2tf)√(Fy).

For tubes, meeting the subparagraphs b and c of this Section, bent about the minor axis, Fb = 0.66Fy ; failing the subparagraphs B and C but with a width:thickness ration less than 238/√(Fy), Fb = 0.6Fy .