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Basic assumptions of working stress method

Short answer

WSM assumes steel is homogeneous, isotropic, and linearly elastic; plane sections remain plane. Working loads are used, stresses must not exceed permissible values (e.g., 0.6f_y tension). A single factor of safety covers all uncertainties, and residual stresses are ignored.

What are the basic assumptions of Working Stress Design method?

The Working Stress Method (IS 800:1984) is based on the theory of elasticity. Basic assumptions:

  1. Steel is homogeneous and isotropic.
  2. Stress is proportional to strain up to the permissible stress (Hooke's law); EE is constant (2×1052\times10^5 N/mm²).
  3. Plane sections before bending remain plane after bending (linear strain distribution).
  4. Loads used are working (service) loads without load factors.
  5. Stresses in members and connections under working loads must not exceed the permissible stresses, which are the yield stress divided by a factor of safety (about 0.6fy0.6f_y for tension, 0.66fy0.66f_y for bending).
  6. A single overall factor of safety accounts for variations in load, material, workmanship and analysis.
  7. Effects such as residual stress, post-yield strength and redistribution are ignored.
  8. Deformations are small and the principle of superposition applies.

✓ Answer checked by Claude Opus 5.5 (high effort)

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