Under working stress design for mild steel rolled I-sections and channels, the permissible (allowable) axial tensile stress is taken as approximately 1500 kg/cm^2.

Introduction / Context:
In traditional working stress design (WSD) as per older steel codes, permissible stresses are fractions of yield stress, providing a safety margin under service loads. For common rolled I-sections and channels in mild steel, a standard allowable axial tensile stress is used for sizing and checking tension members.

Given Data / Assumptions:

• Material: mild steel with nominal yield strength around 2500 kg/cm^2.
• Design philosophy: working stress method, not limit state.
• Members considered: rolled I-sections and channels in axial tension.

Concept / Approach:
Permissible stress sigma_perm in WSD is typically sigma_y multiplied by a factor less than 1 (for example, around 0.6 for tension in mild steel). For sigma_y ≈ 2500 kg/cm^2, sigma_perm ≈ 0.6 * 2500 ≈ 1500 kg/cm^2, which has been widely adopted historically for rolled shapes in axial tension.

Step-by-Step Solution:
Assume mild steel yield stress sigma_y ≈ 2500 kg/cm^2Use WSD factor for tension ≈ 0.6Compute permissible stress: sigma_perm ≈ 0.6 * 2500 = 1500 kg/cm^2

Verification / Alternative check:
Cross-check with classic WSD tables: values for allowable axial tension for rolled sections customarily lie near 1500 kg/cm^2, validating the selection.

Why Other Options Are Wrong:

• 1420 kg/cm^2: conservative but not the standard tabulated value for rolled I- and channel sections in axial tension.
• 2125 kg/cm^2: too high for WSD; leaves inadequate safety margin.
• 1810 kg/cm^2: also higher than typical WSD permissible tension values.

Common Pitfalls:

• Mixing limit state design values with working stress permissible values.
• Ignoring net area reductions due to holes when checking actual member stress.

Final Answer:
1500 kg/cm^2