Last reviewed 16 Sept 2026 · 7 min read
Welding in steel structures
Welding joins metal parts by fusing them, usually with a filler metal, forming a continuous connection. Most structural welding is electric arc welding: shielded metal arc (SMAW, manual with coated electrodes), submerged arc (SAW, automatic, for long girder welds), gas metal arc (MIG/MAG) and flux-cored arc welding.
Advantages over bolting: no holes (full gross section effective); lighter joints (no cover plates or angles); rigid, continuous connections; airtight and watertight; neat appearance; easier alterations and additions.
Disadvantages: needs skilled welders and inspection; residual stresses and distortion from uneven heating; brittle fracture risk; defects not always visible; poor fatigue behaviour of some details; field welding needs good conditions.
Common weld defects
Incomplete penetration, lack of fusion, porosity (gas pockets), slag inclusion, undercut (groove melted into base metal), cracks (hot or cold), overlap. Inspection: visual, dye penetrant, magnetic particle, ultrasonic and radiographic tests.
Types of welds
| Weld | Description | Typical use |
|---|---|---|
| Fillet weld | Triangular section in the corner of two surfaces at about right angles | Lap joints, T-joints, most connections |
| Butt (groove) weld | Fills a groove between plates in the same plane; single-V, double-V, U, J, bevel | Splicing plates, girder flanges, full-strength joints |
| Plug and slot welds | Weld in a hole or slot in one plate | Supplementing fillet welds, preventing buckling of wide plates |
| Spot/seam welds | Resistance welds | Light-gauge work |
Types of joints: butt, lap, tee, corner and edge.
Fillet welds
The size of a fillet weld is the leg length (for equal legs). Failure is taken along the throat, the shortest dimension from the root to the face.
= 0.70 for faces at 90° (the usual case). For other angles between fusion faces: 60°–90° → 0.70; 91°–100° → 0.65; 101°–106° → 0.60; 107°–113° → 0.55; 114°–120° → 0.50.
Minimum size based on the thicker part joined:
| Thicker part (mm) | Up to 10 | 10–20 | 20–32 | 32–50 |
|---|---|---|---|---|
| Minimum size (mm) | 3 | 5 | 6 (first run) | 10 (8 for first run) |
Maximum size: along a square edge, thickness minus 1.5 mm; along a rounded toe of a rolled section, three-quarters of the thickness at the toe.
Effective length: actual length minus twice the weld size (for end craters) unless end returns are provided; effective length not less than four times the size.
End returns: side fillet welds terminating at the end of a member should be returned round the corner for a distance not less than twice the size.
Lap length: not less than four times the thickness of the thinner part (or 40 mm).
Design strength of welds
Design capacity of a fillet weld .
= the smaller of the ultimate stresses of the weld metal and the parent metal.
For E250 steel ( = 410 N/mm²), shop weld: N/mm².
Butt welds: designed like the parent metal — full-penetration butt welds are taken as strong as the plate (effective throat = thickness of the thinner part); for partial penetration, the effective throat is the depth of penetration (with limits).
Combined stresses in welds: for a fillet weld carrying normal stress and shear , the equivalent stress must not exceed .
Welded connection of an angle (balancing welds)
An angle connected by one leg to a gusset carries its load along its centroidal axis, which is nearer the connected leg's back. To avoid eccentricity, the side welds are proportioned so that their resultant passes through the centroid:
= force in the weld along the heel (nearer the centroid, carries more), = force at the toe, and = distances of the centroid from the heel and toe edges; = force in an end weld if provided.