Construction and Design of Welded Structures
Domain 3: Construction and design of welded structures
This domain tests whether you can read a welded detail as a designer does: load path, joint type, weld size, symbol and failure mode. Static strength and fatigue are both examinable, and ISO 2553 symbols are assumed knowledge.
Joints, symbols and sizing
Butt, T, corner, lap and edge joints behave differently under tension, shear and bending. A fillet weld's design throat is a = 0.7z for an equal-leg fillet with leg length z, because the critical plane is at 45°. Full-penetration butt welds develop the parent section; partial-penetration and fillet welds do not. ISO 2553 uses a reference line, arrow, elementary symbol, size and a tail for process or specification.
Fatigue
Fatigue cracks usually start at the weld toe or root and depend on stress range, detail geometry and number of cycles, not on static strength alone. Eurocode 3 expresses this through detail categories and S-N curves. Welds should be placed in low-stress regions, transitions should be smooth, and abrupt changes in stiffness avoided.
Worked example
A bracket uses an 8 mm equal-leg fillet. The throat is 0.7 × 8 = 5.6 mm. If the design changes to a 6 mm leg, the throat becomes 4.2 mm, a 25% reduction in throat and therefore roughly 25% less shear capacity per unit length. The drawing may show 'a5.6' or 'z8'; confusing leg with throat overestimates capacity by about 40%.
Traps
Sizing a fillet by leg when the design uses throat. Assuming a partial-penetration weld is full strength. Ignoring fatigue because static stress is low. Misplacing the symbol above or below the reference line, which changes the side of the joint.