Structural Engineering & Statics
where the load goes
Concept

Throat stress — where it appears

The stress on a fillet weld's throat — the shortest plane through it, at 45° to both legs and about 0.7 of the leg size. Every force on a weld is resolved onto that plane before it is compared with anything, which is why a weld's capacity depends on the direction of the force and why the leg size on a drawing is not the dimension the check uses.

Named by 3 essays across one field — each of them below, with the objects they name alongside it.

Throat stress round a fillet weld group. A l shape weld group carrying 150 kN at 200 mm from its centroid. The peak throat stress is 2.17 kN per mm of throat, at (139.53, 100); the worst point at maximum radius from the centroid carries 1.99. Checking by radius is wrong here by 9.17%, and points at identical radius differ by a factor of 1.

The radius rule, and where it fails

A weld group under an eccentric load is checked at the point furthest from its centroid, on the reasoning that the stress from the twist grows with the radius. That reasoning ignores the direction the two stresses point in, and for one common shape it misses the peak by nine per cent.

connections · Weld group The whole line works, and the axis is not a choice. A weld group 200 mm deep carrying 15 kNm about an axis in its own plane, together with 100 kN of vertical shear. The bending force per unit length runs linearly from 1125 N/mm at one extreme to the same the other way at the other, through zero at the group's own centroid — and the centroid is where the neutral axis is because a weld carries compression across its throat as readily as tension. The 250 N/mm of shear runs along the weld and is uniform over the whole 400 mm, so the two components are perpendicular to one another and are combined on the throat rather than added as vectors in the plane.

The eccentricity at right angles to the drawing

A bracket's load stands off the plane of its welds as well as being offset within it, and the second eccentricity produces a completely different object — bending about an axis through the group rather than torsion about a point in it. The weld line has no compression zone to argue about, so its neutral axis is its own centroid, and the whole length works.

connections · Weld group Where each check bites. The c shape group with a 5.0 mm throat drawn to scale, loaded 1 mm from its vertical run. Offset from each weld, the use of each point: solid for the static directional check, dashed for fatigue at 2 × 10⁶ cycles, each scaled to its own worst point, which is marked. The static check's worst point is where the force runs most nearly along a weld; the fatigue check's is where it runs most nearly across one. Here the two checks are governed at different points, and a fatigue check made only at the static peak allows a load range 2.25 times the group's.

Strongest across, and first to crack across

A fillet weld pulled across its length is √1.5 stronger than one pulled along it, and every static check of a weld group uses that. In fatigue the same two directions are rated the other way round: the part of the force across the weld is checked against a detail category of 36 and the part along it against 80. The point of a group that governs the static check need not govern the fatigue one, and checking fatigue there can allow more than twice the load range the weld will take.

connections · Weld group

Named alongside it

The objects these essays reach for when they reach for this one.

EccentricityFillet weldWeld groupBolt groupFree bodyBolt tensionCentroidConnectionCut-off limitDetail categoryElastic methodFatigue

All concepts