This section of IS 802 specifies the permissible stresses for structural steel members and connections (bolts, welds) used in the design of overhead transmission line towers. It is used by structural engineers to size steel angles, evaluate compressive buckling, and determine bolt shear and bearing capacities.
Code of Practice for Use of Structural Steel In Overhead Transmission Line Towers, Part 1: Material,Loads and Permissible Stress, Section 2: Permissible Stress
Quick Reference Values
permissible axial tensionEqual to the minimum guaranteed yield stress (Fy)
max slenderness ratio leg members120
max slenderness ratio other members200
max slenderness ratio redundant members250
modulus of elasticity E200,000 MPa (approx 2.0 x 10^5 N/mm2)
permissible bearing stress bolts1.5 times the ultimate tensile stress of the bolt material
bolt hole clearance1.5 mm for bolts up to 24 mm diameter
Key Formulas
Fa = Fy [1 - 0.5 * (KL/r / Cc)^2] — Permissible compressive stress for KL/r <= Cc
Fa = (π^2 * E) / (KL/r)^2 — Permissible compressive stress for KL/r > Cc
Cc = π * √(2E / Fy) — Limiting slenderness ratio demarcating elastic and inelastic buckling
Practical Notes
Unlike IS 800 working stress design, IS 802 applies the factor of safety (reliability/load factors) on the load side in Section 1. Therefore, the permissible tensile stress in Section 2 is typically taken directly as the yield stress (Fy) of the material.
Accurate calculation of the effective slenderness ratio (KL/r) is critical. The K-factor depends on the end restraint conditions, bolting patterns, and whether the member is a leg, bracing, or redundant.
Using higher grade steel (HT) increases tensile capacity but does not improve the buckling strength of slender members (where KL/r > Cc), as elastic buckling depends only on the Modulus of Elasticity (E).