Weld Categories — AS/NZS 1554.1

AS/NZS 1554.1 defines three weld categories:

All structural steel connections to AS 4100 must use SP category as a minimum. GP is only suitable for stair treads, handrails, and non-structural attachments. FP is required for crane girders, bridges, and dynamically loaded structures per AS 5100.6.

Fillet Weld Design — AS 4100 Clause 9.7.3

Design throat thickness (equal leg 90-degree fillet):

t_t = 0.707 x s (where s = leg length in mm)

The 0.707 factor assumes a 45-degree fillet face with slight convexity. For a concave fillet (higher quality, less stress concentration), t_t = 0.75 x s may be used subject to fabricator agreement.

Design capacity of fillet weld per unit length (Clause 9.7.3.10):

phi_v_w = phi x 0.6 x f_uw x t_t

Where:

• phi = 0.80 (welds in SP category — Table 3.4)
• 0.6 = shear factor (weld metal treated as shear-critical)
• f_uw = nominal tensile strength of weld metal (MPa)
• t_t = design throat thickness (mm)

Weld metal strengths (AS/NZS 1554.1):

E48XX (W50X) is the standard electrode for structural steel in Australia. The 48 indicates 480 MPa minimum tensile strength. E41XX is an alternative for thin materials or where matching undermatching weld metal to Grade 250/300 plate is desired.

Weld Capacity Table — E48XX, SP Category (phi = 0.80)

Formula: phi_v_w = 0.80 x 0.6 x 480 x 0.707 x s / 1000 kN/mm

Directional Method — Clause 9.7.3.10

For fillet welds loaded at an angle theta to the weld axis, the capacity may be increased using the directional method:

phi_v_w_theta = phi_v_w / (sin^2(theta) + 0.75 x cos^2(theta))^0.5

For theta = 0 degrees (longitudinal shear along weld axis): Factor = 1 / sqrt(0.75) = 1.155 For theta = 90 degrees (transverse to weld axis): Factor = 1 / sqrt(1.0) = 1.0 For theta = 45 degrees: Factor = 1 / sqrt(0.5 + 0.75 x 0.5) = 1 / sqrt(0.875) = 1.069

The directional method provides up to 15.5% enhancement for longitudinal loading. In Australian practice, the enhancement is modest — many designers apply the standard capacity without directional factors for simplicity, treating it as an additional reserve.

Worked Example 1: Beam Web-to-End Plate Weld

Problem: A 410UB59.7 beam is fillet welded to a 10 mm end plate. Factored shear V* = 200 kN (beam reaction). Beam web thickness = 7.8 mm. End plate thickness = 10 mm. Determine required fillet weld size and length. E48XX electrode, SP category.

Step 1 — Minimum fillet size (AS/NZS 1554.1 Table 5.1): For 7.8 mm web: minimum fillet = 5 mm. For 10 mm end plate: minimum fillet = 5 mm. Use 6 mm fillet weld (standard minimum for structural connections).

Step 2 — Weld capacity (6 mm fillet, E48XX): phi_v_w = 0.80 x 0.6 x 480 x 0.707 x 6 / 1000 = 0.98 kN/mm

For two welds (both sides of web): phi_v_w_total = 0.98 x 2 = 1.96 kN/mm

Step 3 — Required weld length (shear only, ignoring eccentricity for flush end plate): L_w = V* / phi_v_w_total = 200 / 1.96 = 102 mm

Add 2 x s = 12 mm for start/stop crater allowance per AS/NZS 1554.1: effective length = 102 + 12 = 114 mm. Use 120 mm minimum.

Step 4 — Check base metal (web shear rupture): For web plate (t_w = 7.8 mm, f_u = 440 MPa, Grade 300): phi_V_base = 0.90 x 0.6 x 440 x 7.8 x 120 / 1000 = 222 kN >> 200 kN. OK.

Step 5 — Eccentricity consideration: For a flush end plate, the eccentricity from bolt line to weld is the end plate thickness (10 mm) plus half the setback — typically 50 mm total. The eccentric moment M*_e = 200 x 0.050 = 10 kNm.

For the weld group (L = 120 mm, two vertical welds): section modulus S_w = 2 x 120^2 / 6 = 4,800 mm^2. Bending stress in weld: f_bending = M*_e / S_w = 10 x 10^6 / 4800 = 2,083 N/mm = 2.08 kN/mm.

Resultant per mm: sqrt((200/120)^2 + 2.08^2) = sqrt(1.67^2 + 2.08^2) = sqrt(2.79 + 4.33) = 2.67 kN/mm >> 1.96 kN/mm. NOT OK.

Weld requires redesign. Increase weld length to 180 mm: S_w = 2 x 180^2 / 6 = 10,800 mm^2 f_bending = 10 x 10^6 / 10,800 = 926 N/mm = 0.93 kN/mm Direct shear = 200/180 = 1.11 kN/mm Resultant = sqrt(1.11^2 + 0.93^2) = 1.45 kN/mm < 1.96 kN/mm. OK.

Final design: 6 mm fillet weld, 180 mm length both sides. E48XX electrode (W50X). SP category.

Note: The eccentricity from flush end plate connections often governs the weld design. The weld length must be sufficient to resist the combined direct shear + eccentric moment. A deeper end plate with longer welds reduces the bending stress. If end plate depth is constrained, increase fillet size to 8 mm.

Worked Example 2: Column Stiffener Weld

Problem: A 310UC158 column requires 10 mm full-depth stiffeners to resist a beam flange force N* = 650 kN from a moment connection. Column web thickness = 15.7 mm. Stiffener = 10 mm Grade 300 plate. Weld stiffeners to column web with fillet welds. E48XX, SP category.

Step 1 — Available weld length: Stiffener depth = d_col - 2 x t_f_col = 311 - 2 x 25 = 261 mm Two stiffeners (each side of web): total weld length = 261 x 2 x 2 sides = 1,044 mm (weld both sides of each stiffener).

Step 2 — Weld size: Minimum for 10 mm stiffener plate: 5 mm per AS/NZS 1554.1. Maximum for single pass: 8 mm. Use 6 mm fillet weld.

Step 3 — Weld capacity: phi_v_w per mm = 0.98 kN/mm (from table above) Total capacity = 0.98 x 1044 = 1,023 kN >> 650 kN. OK (0.64 utilization).

Step 4 — Check base metal (column web): The stiffener force must be transferred through the weld into the column web. Check web shear: Web shear area = d_1 x t_w = (311 - 50) x 15.7 = 261 x 15.7 = 4,098 mm^2 phi_V_web = 0.90 x 0.6 x 300 x 4098 / 1000 = 664 kN > 650 kN. OK (marginal — stiffeners adequate).

Actually, the column web panel zone must also resist the shear from the beam moment. The combined panel zone shear from both beams plus the stiffener force must be checked per Clause 5.12. For a typical moment frame, panel zone doubler plates may be required.

Final design: 6 mm fillet weld both sides, full stiffener depth (261 mm). E48XX (W50X), SP category. Weld both sides of each stiffener to the column web.

Minimum and Maximum Fillet Weld Sizes — AS/NZS 1554.1

Maximum single pass for SP category: 12 mm. For larger fillets, use multi-pass welds. Multi-pass fillet capacities are additive: a 16 mm multi-pass fillet (two 8 mm passes) has a throat of 0.707 x 16 = 11.3 mm.

Australian Welding Symbols — Quick Reference

Use AS 1101.3 welding symbols on fabrication drawings:

• Fillet weld arrow side: Triangle on reference line, size to left (e.g., 6)
• Fillet weld other side: Triangle above reference line, size to left
• Fillet weld both sides: Triangle both sides of reference line
• Site weld: Flag at the junction of arrow and reference line
• All-around weld: Circle at the junction

Factory note: "ALL WELDS TO AS/NZS 1554.1 SP CATEGORY. ALL FILLET WELDS E48XX/W50X UNLESS NOTED OTHERWISE. MINIMUM FILLET SIZE PER AS/NZS 1554.1 TABLE 5.1."

Frequently Asked Questions

What is the difference between SP and GP weld categories? SP (Structural Purpose) requires: (a) qualified welding procedures per AS/NZS 1554.1 Section 4, (b) welder qualification to AS 1796 or equivalent, (c) visual inspection of all welds, (d) NDT sampling (typically 10% MT or UT for full-penetration butt welds). GP (General Purpose) requires only visual inspection and is not permitted for structural connections to AS 4100. The cost premium for SP is approximately 5-10% over GP due to documentation and inspection requirements.

E41XX vs E48XX — when is E41XX acceptable? E41XX (410 MPa) weld metal may be specified for Grade 250 plate connections where the lower weld strength matches the lower base metal strength. However, for Grade 300 and above, E48XX (480 MPa) is standard. The practical difference is small (E48XX requires E70XX electrodes, which are the industry standard for structural steel), so standardizing on E48XX avoids mix-ups and is the recommended practice.

How do I verify weld sizes during inspection? For fillet welds, use a fillet weld gauge (bridge cam gauge) to measure leg length and throat thickness. The gauge reads both dimensions directly. AS/NZS 1554.1 Table 6.1 provides acceptance criteria: leg length may be up to 10% undersize for individual measurements, and throat thickness must be >= 0.7 x leg length. Reject criteria: undersized weld > 10%, undercut > 1 mm deep, or overlapping starts/stops at critical locations noted on drawings.

This page is for educational reference. Weld design per AS 4100:2020 Clause 9.7 and AS/NZS 1554.1. Verify weld capacities per ASI Welded Connection Design Tables. All structural designs must be independently verified by a licensed Professional Engineer or Structural Engineer. Results are PRELIMINARY — NOT FOR CONSTRUCTION.