According to traditional working-stress design values, the permissible bending stress for rolled steel I-beams and channels is closest to which value (units: kg/cm^2)?

Introduction / Context:Before modern limit-state design became prevalent, steel members were sized using working-stress design (WSD) with permissible (allowable) stresses. For structural steel of typical yield strength, a standard allowable bending stress was adopted to ensure a safety margin against yielding and serviceability concerns.

Given Data / Assumptions:

• Rolled I-beams and channels in bending.
• Permissible stress values expressed in kg/cm^2.
• Traditional WSD context (not LRFD/limit-state).

Concept / Approach:For mild steel with yield stress around 2500 kg/cm^2 (≈ 250 MPa), a typical WSD allowable bending stress is about 0.66 * Fy, giving ≈ 0.66 * 2500 ≈ 1650 kg/cm^2. This value was widely used for beams subject to bending about the major axis under service loads.

Step-by-Step Solution:Take Fy ≈ 2500 kg/cm^2 as a representative yield stress for mild steel.Compute 0.66 * Fy ≈ 1650 kg/cm^2.Select the option that matches this conventional permissible bending stress.

Verification / Alternative check:Older steel design handbooks list allowable bending stresses near 1650 kg/cm^2 for bending, with different values for tension, compression, and shear reflecting safety factors and instability checks.

Why Other Options Are Wrong:

• 1500 or 1575 kg/cm^2: Conservative relative to the classic 0.66Fy benchmark.
• 945 kg/cm^2: In the range of typical allowable shear, not bending.
• 1875 kg/cm^2: Too high for the classic WSD bending allowance for Fy ≈ 2500 kg/cm^2.

Common Pitfalls:

• Mixing units (MPa vs kg/cm^2) and misapplying conversion (1 MPa ≈ 10.197 kg/cm^2).

Final Answer:1650 kg/cm2.