Steel Fabrication

Fabrication is the off-site shop process of cutting, drilling, welding, and assembling structural steel members from raw plate and section material into ready-for-erection components. Indian structural steel fabrication is a specialised industry with major players (Larsen & Toubro, BHEL, Tata Projects, Jindal Steel, McNally Bharat, Punj Lloyd, Pratibha, Tecpro) operating large fabrication shops with CNC plate-cutting, automated drilling, and shop-welding facilities. Quality control at fabrication is governed by IS 800:2007 Section 10, IS 7307 (welder qualification), and IS 822 (welding inspection).

The fabrication sequence: (1) Material identification and traceability — each plate and section traced via mill-test certificate (MTC) to its source heat number; (2) Pre-fabrication marking — IS 800 Cl. 16 specifies tolerances on plumb, sweep, twist, and straightness; (3) Cutting — CNC plasma or oxy-fuel cutting for plates, hot-saw or shearing for sections; (4) Drilling and reaming — for bolted connections, drilling at gusset plates and beam flanges, with hole tolerance per IS 800 Cl. 10.2; (5) Pre-assembly — components fitted together with light tack welds; (6) Welding — full-strength welds per the connection design and IS 9595 procedure; (7) Camber correction — heat straightening or mechanical adjustment; (8) Inspection — dimensional check, visual weld inspection, hole tolerance check; (9) Surface preparation — abrasive blasting to SA 2.5 standard, application of corrosion-protection primer; (10) Marking and despatch — each member tagged with mark number for site erection.

The single most critical fabrication tolerance is hole alignment for bolted connections. Per IS 800 Cl. 10.2.4, hole diameter is bolt diameter + 2 mm (for nominal bolts) — but if multiple bolts in a connection are misaligned by >1 mm, site fitting becomes impossible without enlarging the holes (which violates the bolt design assumptions). Pre-assembly (matching) of connections in the fabrication shop, before despatch, prevents 90% of site fit-up problems. Site engineers should refuse despatch of fabricated members that have not undergone matched-bolt-hole verification.