ISA (Indian Standard Angle)

ISA (Indian Standard Angle) is a hot-rolled L-shaped steel section used extensively in Indian construction for trusses, bracing, lacing, towers, and connections. Standardised in IS 808:1989 with both equal angles (e.g., ISA 65×65×6) and unequal angles (e.g., ISA 100×75×8), ISA designations specify leg lengths in mm and thickness in mm. The smallest standard angle is ISA 20×20×3 (0.9 kg/m) and the largest commonly stocked is ISA 200×200×25 (75.4 kg/m). For tower and transmission-line applications, even larger angles up to ISA 250×250×35 are produced.

Design per IS 800:2007 covers angles in tension (governed by net section, single-bolt connection efficiency, and block shear), compression (with reduction for slenderness about minor principal axis), and as members of trusses (where multiple angles connect at gusset plates). The asymmetric section produces principal axes inclined to the leg axes — IS 808 tabulates rzz (the minimum radius of gyration) for slenderness check. Single-angle struts are typically eccentrically loaded — IS 800 Cl. 7.5.1 provides effective slenderness ratios depending on connection arrangement (bolted/welded, single/double row).

The most common Indian uses: (1) Roof trusses — 4-12 m span warehouses use 2 × ISA back-to-back members at top and bottom chord, 1 × ISA for diagonals. (2) Bracing in steel buildings — single or paired angles in vertical / horizontal bracing systems. (3) Transmission-line towers — virtually 100% of 33 kV to 765 kV power transmission towers use angles per ISIN 6533 (lattice tower code). (4) Lift wells, stair stringers, walkway grating supports. (5) Lacing of built-up columns and beams. The common fabrication error is connecting tension angles via a single bolt — IS 800 imposes a 50-65% reduction factor for single-bolt connection that often makes the section inadequate; always design for double-bolt or welded connection.