Allowable average shear stress in an un-stiffened web for beams made of steel of grade 250 N/mm² is:

Concepts:

.As per IS 800:1987,

Maximum permissible direct tensile stress in structural steel is 0.60 fy

Maximum permissible direct compressive stress in structural steel is 0.60 fy

Maximum permissible bending tensile stress in structural steel is 0.66 fy

Maximum permissible bending compressive stress in structural steel is 0.66 fy

Maximum permissible average shear stress in structural steel is 0.40 fy

Maximum permissible bearing stress in structural steel is 0.75 fy

Calculation:

Given: f_y = 250 MPa

τ = 0.4 × 250 = 100 N/mm²

Additional Information The nominal shear yielding strength of webs is based on the Von –Misses yield criterian, which states that for an un-reinforced or unstiffened web of a beam, whose width to thickness ratio is comparatively small so that web buckling is avoided , the shear strength is  given as

 τ = 0.58 fy

However, IS 800 adopted shear strength as:

τ = 0.6 × fy

where fy­ is the yield strength of material

The above relation is only hold when there is no shear buckling of web and this is only possible when depth to thickness ratio of web is very small