Construction and Design
This module tests whether you can read a design intent, size a weld for static load, recognise when fatigue rather than strength governs, and design details that can actually be made and inspected.
Joints, symbols and static sizing
ISO 2553 governs weld symbols: arrow line, reference line, dashed identification line, supplementary symbols for all-round, site and flush welds. For fillet welds the design value is the throat, not the leg: a = 0.707 × z.
Fatigue
Fatigue strength is set by the stress range and the detail category, not by static strength or weld size. S-N curves in the IIW Recommendations assign FAT classes to details (FAT 90 for a transverse butt weld with root imperfections, FAT 36 for an unstiffened fillet toe). Adding weld metal does not raise the fatigue class; improving toe geometry (grinding, TIG dressing, peening) does.
Brittle fracture and access
Thickness, service temperature, restraint and toughness interact; a design is checked with fracture-mechanics or Charpy-based methods. Allow physical access for the torch, for NDT probes and for visual inspection; a weld that cannot be inspected cannot be accepted.
Worked example: fillet capacity
An 8 mm leg fillet: throat a = 0.707 × 8 = 5.66 mm. Using a design shear strength fvw,d = 208 N/mm², capacity per unit length = 5.66 × 208 ≈ 1177 N/mm, i.e. about 1.18 kN per mm of weld. A joint carrying 1.5 kN/mm therefore needs a leg of roughly 8 × (1.5/1.18) since capacity scales linearly with leg - approximately 10 mm, subject to minimum and maximum leg limits.
Traps
- Using leg length where the specification asks for throat.
- Assuming a larger fillet improves fatigue life.
- Overlooking that a detail which is strong in static tension may be the worst fatigue detail on the structure.