Welded connections

Welding fuses steel parts together with molten filler metal, giving rigid, airtight joints without holes. It is the standard method in fabrication shops, while bolting is usually preferred on site where welding quality is harder to control. Two weld types dominate. Butt (groove) welds join plates edge to edge and, when full penetration, are as strong as the parent plate. Fillet welds sit in the corner between two surfaces at right angles, such as a flange on a base plate or a lap joint.

A fillet weld is defined by its leg size s. Its throat, the shortest distance from root to face, is 0.7 s for a 90° joint, and the weld strength is the throat area times the design weld strength, based on fu of the weaker part divided by √3 and a partial factor larger for site welds than for shop welds. Effective length is the overall length minus allowances for end craters. Minimum leg size depends on plate thickness to avoid cracking from rapid cooling; maximum size is limited at plate edges.

Welds under combined direct and bending stress are checked with an equivalent stress. Quality depends on procedure, welder qualification, preheat for thick plates and inspection: visual checks, dye penetrant, magnetic particle, ultrasonic or radiography. Defects such as lack of fusion, porosity, slag and undercut reduce strength and fatigue life. Distortion and residual stress are side effects that fabricators control by sequencing.