Course · lesson 6 of 7

Welded connections

Most steel joints are welded somewhere: the fin plate to the column, the beam to its end plate. This lesson sizes fillet welds to SANS 10162-1, follows a weld group under shear and moment, and reads the weld rows of the STATIX fin plate and end plate calculators.

1. Learning outcomes

After this lesson you can:

tell a fillet weld from a butt weld, and a leg from a throat;
find a fillet weld's resistance per millimetre to SANS 10162-1, allowing for the direction of the force;
find the force per millimetre in a weld group under shear and moment;
explain a full-strength weld and why end plate web welds are made that way;
read the weld rows of the STATIX fin plate and end plate calculators.

2. Before you start

Lesson 5: the fin plate and its eccentricity.
The free fin plate and end plate calculators, and your copy of SANS 10162-1.
A pocket calculator and about 55 minutes.

3. Fillet and butt welds

The two kinds of weld. A fillet weld sits in the corner between two parts and is the everyday weld of steel connections. A complete-penetration butt weld fills the whole thickness and, with a matching electrode, is as strong as the plate it joins. The STATIX connection pages check fillet welds.

Leg and throat. A fillet is named by its leg s, the side of the triangle. It breaks across its throat a, the shortest line from the root to the face. For equal legs a = s / √2 ≈ 0.707 s. Always check which one a drawing or a calculator means.

4. The strength of a fillet

SANS checks a fillet twice: the steel it is fused to (the area A_m = s L along the fusion face, with the weaker parent's f_u) and the weld metal itself (the throat area A_w = a L, with the electrode strength X_u). The smaller one is the weld's resistance.

Direction. A fillet pulled across its length is half as strong again as one sheared along it: the weld metal term grows with the angle θ between the weld axis and the force. The base metal check does not grow, so for a strong electrode on ordinary steel the base metal soon governs at large angles.

per mm of weld, SANS 10162-1 13.13.2.2 (φ_w = 0.67): base metal: 0.67 φ_w s f_u (f_u of the weaker part joined) weld metal: 0.67 φ_w (0.707 s) X_u (1 + 0.5 sin^1.5θ) (E70XX: X_u = 480 MPa) the smaller governs

5. Weld groups: shear and moment

A weld group under shear and moment. The fin plate weld carries the shear evenly along its length, and the moment V e like a small beam of the plate's depth: the force across the weld is largest at the top and bottom. STATIX treats each weld as a line: f_par = V / 2h_p, f_perp = V e / (2 h_p² / 6).

Adding the two. At the worst point the weld carries f_par along it and f_perp across it. Their resultant q, at the angle θ to the weld, is checked against the 13.13.2.2 resistance per millimetre at that same angle.

two welds of length h_p, load V at e: f_par = V / 2h_p f_perp = V e / (2 h_p² / 6) q = √(f_par² + f_perp²) at θ = atan(f_perp / f_par)

6. End plate welds

A moment end plate. The beam end is welded all round to a plate, which is bolted to the column. The flange welds carry the moment as a pull at the top and a push at the bottom; the web welds carry the shear and, in practice, are made full strength.

Where the flange weld force comes from. The bolt rows pull on the plate; together they balance the push of the bottom flange. STATIX gives that whole resultant to the flange weld pair (a safe simplification), and the shear to the web welds.

A full-strength weld is as strong as the part it joins, so the weld is never the weak link. The STATIX end plate page designs the beam web welds this way (SCI P398, Step 7): the two fillets must carry the web's own tension and shear strength per millimetre.

7. Two codes, one weld

The same weld in two codes. The STATIX end plate page runs EN 1993-1-8 only, so its weld rows use the EN throat method; the fin plate page and the app's SANS route use 13.13.2.2. For ordinary fillets the two land close together, but never mix their factors.

The STATIX end plate calculator is a single-code page: it runs the EN 1993-1-8 component method, and its weld rows follow EN. This lesson reads those rows as they are and checks the same weld by SANS for comparison.

8. In STATIX

In STATIX the weld is one or two inputs: the fin plate's fillet leg, or the end plate's flange and web legs (both are legs; the throat is 0.7 times the leg). Each page lists its weld rows with the clause it used.