Steel Beams – Allowable average shear stress in an unstiffened web (Fe 250 grade steel)

Introduction / Context:
In working-stress design tradition, Indian steel codes specify permissible average shear stress values for webs of rolled beams and plate girders. For unstiffened webs in Fe 250 steel, a conservative allowable shear controls web sizing and prevents shear buckling and yielding under service loads.

Given Data / Assumptions:

• Material: structural steel grade 250 N/mm2 (Fe 250).
• Member: beam with unstiffened web region.
• Design basis: permissible/allowable stress format.

Concept / Approach:

The web primarily resists transverse shear. Average web shear stress τ_avg = V / (b_w * t_w) is checked against the code-specified allowable value. Traditional values adopt about 100 N/mm2 for Fe 250 to maintain adequate safety against shear yielding and instability for typical web slenderness limits.

Step-by-Step Solution:

Verification / Alternative check:

Design tables and classic examples align with the 100 N/mm2 benchmark; higher-grade steels or limit-state methods use different formulas and factors but this question targets the traditional allowable value.

Why Other Options Are Wrong:

250 N/mm2 equals nominal yield strength, not an allowable; 165 or 150 N/mm2 are higher than standard permissible shear for unstiffened webs in Fe 250 under WSD.

Common Pitfalls:

Confusing allowable stresses with limit-state design shear capacities, or using web area of the whole section instead of the clear web area.

Final Answer:

100 N/mm2