Butt weld (also called groove weld) is a weld between two pieces of metal in the same plane — joining edges directly. Distinguished from fillet welds (between perpendicular surfaces). Butt welds are used where full strength of the base metal must be developed — connecting plates end-to-end, joining plate-girder web sections, splicing structural columns. Per IS 816:1969 + IS 9595:1996 + IS 800:2007 Cl. 10.7, butt welds are categorised by joint preparation: V-groove (single side bevel), U-groove (deeper bevel for thick plates), square (no bevel for thin plates), double-V (welded from both sides for full penetration).
For thick plates (>10 mm), V-groove or U-groove preparation is needed for full penetration. Single-V welds from one side may have incomplete penetration on the back; back-gouging and capping is required for full strength. Double-V (welded from both sides with back-gouging between) achieves 100% penetration. Square butt welds (no bevel) are used for thin plates (<6 mm) and partial-penetration applications. Design per IS 800:2007 Cl. 10.7: butt weld design strength = 0.7 × leg size × length × fy × γm⁻¹ for full-penetration; reduced for partial-penetration. Inspection: (1) Visual — full bead coverage, no undercut, smooth contour. (2) Radiographic — required for tension butt welds in important structures (IS 822). (3) Ultrasonic — for thick plates, full-penetration verification. (4) Magnetic particle — for ferromagnetic materials.
Per IS 9595:1996 + IS 800 Cl. 10.7: visual inspection 100%; radiographic 10-100% depending on member criticality; acceptance criteria for porosity, undercut, lack of fusion per IS 9595 Annex C; γm = 1.25 (shop weld) or 1.5 (site weld).
Site example
Site reality: a Bangalore plate-girder bridge had butt welds on web splices that were single-V from one side without back-gouging. Radiographic inspection found 30% incomplete penetration. Remediation: re-weld with back-gouging and complete penetration. ₹4.2 lakh re-work cost. Always specify full-penetration butt welds with appropriate groove preparation; partial-penetration is acceptable only if explicitly designed for.
Frequently asked
What is butt weld?
Butt weld (or groove weld) is a weld between two pieces of metal in the same plane — joining edges directly. Used where full strength of the base metal must be developed. Per IS 816:1969 + IS 9595:1996 + IS 800:2007 Cl. 10.7. Categories: V-groove, U-groove, square, double-V — by joint preparation.
What is the difference between fillet and butt weld?
Fillet welds: triangular cross-section weld joining two surfaces at right angles (T-joints, lap joints). Butt welds: weld between two pieces in the same plane (V-groove, U-groove, square preparation). Fillet welds carry shear; butt welds can carry tension/compression at full base-metal strength (if full penetration). Fillet is more common in fabrication; butt is used for strength-critical connections.
How is butt weld inspected?
Per IS 9595 + IS 822: (1) Visual inspection 100% — full bead coverage, no undercut, smooth contour. (2) Radiographic inspection 10-100% (depending on member criticality and tension state). (3) Ultrasonic for thick plates — full-penetration verification. (4) Magnetic particle for ferromagnetic materials. (5) Welder qualification per IS 7307. Acceptance criteria for porosity, undercut, lack of fusion per IS 9595 Annex C.