Steel angle bar is the dominant structural shape for transmission and telecom tower fabrication. This guide explains the grade choice, the standard sizes, the galvanizing requirement and the typical specification for export tower orders.
Steel angle bar (also called angle iron or L-section) is the structural backbone of lattice transmission towers, telecom towers and substation structures. A typical 110 kV transmission tower uses 5–15 tonnes of equal-leg and unequal-leg angle, while a 500 kV or 765 kV EHV tower may use 30–60 tonnes. The tower is hot-dip galvanized after fabrication and is designed for a 50+ year service life in the most corrosive environments, so the angle specification must balance strength, weldability and galvanizing quality.
Standard Grades and Designations
The most common grade for transmission tower angle is ASTM A36 for North American projects and EN 10025 S275JR or S355JR for European and international projects. Higher-strength grades (ASTM A572 Grade 50, EN 10025 S355J0/J2) are used for the most heavily loaded members and for EHV towers. The angle is produced in equal-leg (e.g., 50x50x5 mm, 75x75x6 mm, 100x100x10 mm) and unequal-leg (e.g., 75x50x6 mm, 100x75x8 mm) configurations, with the unequal-leg shape often used for bracing members where the geometry of the connection favors it.
Equal vs Unequal Leg Selection
Equal-leg angle (e.g., L75x75x6) is the workhorse for tower leg members, main bracing, and most secondary members. It is the most widely stocked and the cheapest per kilogramme. Unequal-leg angle (e.g., L100x75x8) is used where the connection geometry favors the wider leg on one face (for example, when the wider leg bolts to a gusset plate on a horizontal face), and where the asymmetric section optimizes the structural efficiency of a member with different bending demands on each axis.
Galvanizing and Long Service Life
Transmission tower angle is hot-dip galvanized after fabrication to ASTM A123 (North America) or ISO 1461 (international), with a minimum coating weight of about 85 μm (610 g/m²) for standard atmospheric exposure. In coastal or industrial environments, heavier coatings (100–130 μm) may be specified. The silicon content of the steel must be controlled (typically below 0.04% or above 0.15%) to avoid the Sandelin effect, which produces a rough, brittle coating. Most tower-grade angle is supplied as silicon-killed steel to ensure good galvanizing quality.
Hole Punching, Tolerance and Bundle Detail
Tower angle is supplied with holes punched or drilled per the tower detail drawing. The hole diameter is typically 17.5 mm for 16 mm bolts, 21.5 mm for 20 mm bolts, and the hole spacing and edge distance must follow the tower design code. The angle is supplied in standard lengths of 6 m, 9 m, or 12 m, with the cutting, punching and bundling done by the mill or the angle service center. For export orders, the angle is usually bundled with steel strapping and wooden spacers, and shipped in 20' or 40' containers with desiccant to prevent wet-storage stain.
What to Specify on the Tower Angle RFQ
State the standard (ASTM A36, A572, EN 10025 S275JR/S355JR, JIS G3101 SS400, or GB/T 706 Q235/Q355), the section designation (e.g., L75x75x6, L100x75x8), the length, the hole pattern (if pre-punched), the galvanizing requirement (ASTM A123 or ISO 1461), the silicon content limit (to control coating quality), and any Charpy impact requirement for cold-climate projects. For EHV and critical towers, also specify Charpy impact at the design temperature and 100% ultrasonic testing of the leg material.
Related Resources from Baobin Steel
- Steel bar catalog – angle, channel and beam
- Structural steel overview – grades and standards
- Request a quote – send your tower member list for angle pricing