Allowable bending stress for rolled I-sections and channels (extreme fibres on the effective section) What is the permissible tensile/compressive bending stress at extreme fibres for rolled I- and channel sections when using the effective section concept?

Introduction / Context:Traditional steel design using the working-stress method specifies permissible bending stresses for standard rolled sections. For mild steel (yield ≈ 2500 kg/cm²), the allowable bending stress is typically a fraction of the yield strength to ensure adequate margin.

Given Data / Assumptions:

• Rolled I-sections and channels.
• Effective section considered for bending.
• Working-stress philosophy with legacy units.

Concept / Approach:A common allowable bending stress for MS sections is about 0.66 of the yield strength. With fy ≈ 2500 kg/cm² (≈ 250 MPa), the permissible stress is about 0.66 * 2500 ≈ 1650 kg/cm².

Step-by-Step Solution:Assume fy ≈ 2500 kg/cm².Compute 0.66 * fy ≈ 0.66 * 2500 = 1650 kg/cm².Select 1650 kg/cm² as the allowable bending stress.

Verification / Alternative check:Design handbooks from the working-stress era list 1650 kg/cm² (tension/compression at extreme fibres) for standard sections in bending.

Why Other Options Are Wrong:

• 1500 or 1420 kg/cm²: conservative but not the standard tabulated limit.
• 2285 kg/cm²: too high, approaching yield without adequate safety margin.

Common Pitfalls:Mixing limit-state design (factored) values with working-stress allowable stresses; the question clearly uses allowable-stress numbers in kg/cm².

Final Answer:1650 kg/cm²