The design bending moment capacity of a section is primarily dependent on whether the member is laterally supported or unsupported.
You can control the lateral support condition of the member by the use of LAT parameter.
If the member is laterally supported, then the design strength is calculated as per the provisions of the section 8.2.1 of IS 800:2007, based on the following factors:
- Whether section with webs susceptible to shear buckling before yielding
- Ratio of shear force to design shear strength
- Section classification
If the member is laterally unsupported, then the design strength is calculated as per the provisions of the section 8.2.2 of IS 800:2007, based on the following factors:
- Lateral Torsional Buckling
- Section Classification
Working Stress Design
Actual bending stress values are given by, about major (Z) and minor (Y) axes, respectively:
|fbcz = Mz/Zecz|
|fbtz = Mz/Zetz|
|fbcy = My/Zecy|
|fbty = My/Zety|
The permissible bending stress is given as follows:
For laterally supported beams:
- Fabc = Fabt = 0.66·Fy for Plastic or Compact sections
- Fabc = Fabt = 0.60·Fy for Semi-compact sections
- Yield strength of steel, indicated by the FYLD parameter.
For laterally unsupported beams:
About the major axis:
fabcz = 0.60·Md/Zecz
fabtz = 0.60·Md/Zetzwhere
- Design Bending Strength as per Clause 8.2.2
= βb · Zpz · fbd
- χLT · Fy / γmo
- Elastic Section Modulus of the Section
- Plastic Section Modulus of the Section
- 0.21 for Rolled Steel Section and 0.49 for Welded Steel Section
- 1.0 for Plastic and Compact Section or Zez/Zpz for Semi-Compact Section
- Non-dimensional slenderness ratio
- (βb · Zpz · Fy / Mcr)1/2 ≤ (1.2 · Zez · Fy / Mcr )1/2
- 0.5 · ( 1 + αLT · ( λLT – 0.2 ) + λLT 2)
- The Bending Stress Reduction Factor to account for Lateral Torsional Buckling
- Elastic Section Modulus of the section about Major Axis for the compression side
- Elastic Section Modulus of the section about Major Axis for the tension side
- Moment of inertia about the minor axis
- Effective length for lateral torsional buckling as determined using either the KX or LX parameters
- Torsional constant of the section
- Warping constant of the section
- Shear modulus of the material
About the minor axis, the permissible bending stress is calculated as for a laterally supported section.
For member with slender section subjected to bending, moment is taken by flanges alone. Design bending strength should be calculated with effective elastic modulus disregarding the contribution of web of the section.
|Zez = 2·[Bf · tf 3/12 + (Bf · tf) · (D/2 - tf/2)2 )] ⁄ (0.5 · D)|
|Zey = 2·(Bf · tf 3/12) ⁄ (0.5 · Bf)|
The Moment Capacity will be Md = Ze· fy/γm0 for "Laterally Supported" condition.
The Moment Capacity will be Md = Ze· fbd/γm0 for "Laterally Un-Supported" condition.
Where, fbd is defined in clause 8.2.2 of IS:800-2007 (described in previous Working Stress Design section).