AS 4100 Fillet Weld Design — SP & GP Capacity (Clause 9.7)

Complete AS 4100:2020 fillet weld design reference: simple method vs directional method, SP and GP electrode classifications, throat thickness calculation, longitudinal and transverse weld capacity, and worked examples for 6mm and 8mm fillet welds in common structural connections.

This page covers the full fillet weld design workflow under AS 4100 Clause 9.7. The Steel Calculator WASM engine performs weld capacity checks automatically for AS 4100 (Australia), AISC 360 (USA), EN 1993 (Europe), CSA S16 (Canada), and IS 800 (India).

PRELIMINARY — NOT FOR CONSTRUCTION. All results are for educational and reference use only. Must be independently verified by a Chartered Professional Engineer (CPEng) or RPEQ before use in any project.

Weld Classification — SP vs GP (AS 4100 Table 9.7.3.10(1))

AS 4100 distinguishes between two categories of welding consumables based on their mechanical properties and manufacturing quality control. Only SP electrodes are permitted for structural applications.

SP (Structural Purpose) electrodes are low-hydrogen consumables with controlled toughness and diffusible hydrogen limits. They are mandatory for all structural welds in steelwork to AS 4100.

GP (General Purpose) electrodes are non-low-hydrogen consumables with no guaranteed Charpy impact properties. They may only be used for non-structural attachments, tack welds that are remelted during the final weld pass, or applications outside the scope of AS 4100.

E48XX electrodes (f_uw = 480 MPa) are the default choice for structural fabrication with Grade 300 and Grade 350 steel. E41XX (f_uw = 410 MPa) may be specified for Grade 250 steel or where lower-strength welds are acceptable.

Fillet Weld Geometry — Throat Thickness

The effective throat thickness t_t determines the weld cross-sectional area available to resist load. For a fillet weld with equal leg sizes, the throat thickness depends on the weld profile.

Standard flat-faced fillet weld:

t_t = t_w / sqrt(2) where t_w is the nominal leg length

For a 6mm fillet weld: t_t = 6 / 1.414 = 4.24 mm

For an 8mm fillet weld: t_t = 8 / 1.414 = 5.66 mm

Deep-penetration fillet weld (submerged arc or qualified procedure):

If the welding procedure produces consistent penetration beyond the root, a larger throat may be assumed:

t_t_enhanced = (t_w + p) / sqrt(2) where p is the measured penetration (typically 1-2mm)

For a 6mm fillet with 1.5mm penetration: t_t = 7.5 / 1.414 = 5.30 mm — a 25% increase in throat area.

Penetration must be demonstrated by macro-etch cross-sections in the welding procedure qualification record (PQR) to AS/NZS 1554.1.

Simple Method — Uniform Weld Capacity (AS 4100 Clause 9.7.3.10)

The simple method assumes the applied load direction does not affect the weld strength. This is conservative but straightforward for design.

V_w = 0.60 x f_uw x t_t (capacity per unit length of weld)

where:

• f_uw = nominal tensile strength of the weld metal (MPa)
• t_t = effective throat thickness (mm)

Design Capacity (Simple Method)

phi = 0.80 for fillet welds (AS 4100 Table 3.4)

phi x V_w = 0.80 x 0.60 x f_uw x t_t

For E41XX electrodes, multiply V_w values by 410/480 = 0.854.

Simple Method Worked Example

An 8mm fillet weld, 150mm long, E48XX electrode, simple method:

Total capacity = 1.304 kN/mm x 150 mm = 195.6 kN

This is a conservative estimate — the directional method below will typically produce higher capacities for welds loaded transversely.

Directional Method — Orientation-Dependent Capacity (AS 4100 Clause 9.7.3.10)

The directional method recognises that fillet welds are 30-50% stronger when loaded transverse to the weld axis than when loaded parallel. This is the recommended method for structural design.

Longitudinal Loading (Force Parallel to Weld Axis)

V_w_parallel = 0.60 x f_uw x t_t

phi x V_w_parallel = 0.80 x 0.60 x f_uw x t_t (same as simple method)

Transverse Loading (Force Perpendicular to Weld Axis)

V_w_transverse = 0.85 x f_uw x t_t

phi x V_w_transverse = 0.80 x 0.85 x f_uw x t_t

3.2.1 For combined loading at angle theta to the weld axis, the interaction check is:

(V_parallel* / phi-V_w_parallel)^2 + (V_transverse* / phi-V_w_transverse)^2 <= 1.0

Directional Method — Capacity Table (E48XX)

The transverse capacity is exactly 0.85 / 0.60 = 1.417 times the longitudinal capacity. For welds loaded predominantly in shear transverse to the weld axis (e.g., a fillet weld connecting a beam web stiffener to the end plate), the directional method provides 42% more capacity than the simple method.

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

Problem Statement

A 310UB40.4 beam is connected to a 10mm end plate via two 6mm fillet welds on the web, each 260mm long. The beam reaction V* = 95 kN is transferred through the web welds. Check the weld capacity using both the simple method and directional method.

Weld Geometry

• Two fillet welds (one each side of web), 6mm leg, E48XX electrode
• Length per weld: L_w = 260 mm
• Total weld length: 2 x 260 = 520 mm (two welds, parallel loading along the weld axis)
• Loading: shear force parallel to weld axis (longitudinal loading)

Simple Method Check

Design capacity per unit length: phi x V_w = 0.977 kN/mm

Total capacity: 0.977 x 520 = 508.0 kN

V* = 95 kN << 508 kN. D/C = 0.187 — the web welds are lightly loaded.

Directional Method Check

Since the load is purely longitudinal: same capacity as simple method = 508.0 kN.

Check Minimum Weld Length

AS 4100 Clause 9.7.3.3 requires that the effective length of a fillet weld be at least 4 x t_w (the leg size) and not less than 40 mm. L_w = 260 mm > max(4 x 6, 40) = 40 mm — OK.

The effective length for end returns (weld around corners at the top and bottom of the web) must be added separately. Typically, 2 x t_w = 12 mm end returns at each end add another 4 x 12 = 48 mm of effective weld length (conservatively assuming the end return welds carry longitudinal load only).

Worked Example: Flange-to-End-Plate Weld (Transverse Loading)

Problem Statement

A 310UB40.4 beam is connected to a 10mm end plate via two 8mm fillet welds on each flange, each 150mm long. The design moment at the connection produces a flange force of F* = 180 kN (tension in the top flange, compression in the bottom). Check the weld capacity.

Weld Geometry

• Two fillet welds per flange (each side), 8mm leg, E48XX electrode
• Length per weld: L_w = 150 mm
• Total weld length per flange: 2 x 150 = 300 mm
• Loading: the flange force acts perpendicular to the weld axis (transverse loading towards/away from the end plate face)

Simple Method Check

phi x V_w per mm = 1.304 kN/mm Total capacity per flange = 1.304 x 300 = 391.2 kN F* = 180 kN << 391.2 kN — OK.

Directional Method Check

Since the load is transverse: phi x V_w_transverse = 1.847 kN/mm

Total capacity per flange = 1.847 x 300 = 554.1 kN F* = 180 kN << 554.1 kN — OK, with significant reserve.

The directional method shows 554.1 / 391.2 = 42% more capacity. In a production design, this additional capacity could allow a 6mm weld instead of 8mm, saving weld metal and reducing distortion.

Weld Group Capacity Under Combined Loading

For a connection with welds oriented at various angles to the applied load, the weld group capacity must be analysed point by point. The general approach:

1. Decompose forces at each point on the weld into components parallel (V_parallel) and perpendicular (V_transverse) to the weld axis.

2. For a fillet weld with a known leg size and electrode classification, compute phi-V_w_parallel and phi-V_w_transverse per mm.

3. Apply the interaction equation at the most heavily loaded point:

   (V_parallel* / phi-V_w_parallel)^2 + (V_transverse* / phi-V_w_transverse)^2 <= 1.0

For a circular fillet weld (e.g., CHS-to-end-plate), the angle theta changes continuously around the circumference. A numerical integration or worst-point check at 2-4 locations typically suffices for design.

Weld Capacity Comparison — AS 4100 vs International

AS 4100 provides the cleanest two-path approach: a simple conservative method and a directional method with clear 0.60/0.85 split. The AISC method uses a continuous theta-based strength equation with a 1.0 + 0.5 x sin^1.5(theta) multiplier on the base 0.60 x F_EXX stress. At theta = 90 degrees (transverse), AISC gives 1.5 x 0.60 x F_EXX = 0.90 x F_EXX, compared to AS 4100's 0.85 x f_uw — slightly different but consistent within the respective phi factor frameworks.

Frequently Asked Questions

What is the design capacity of a 6mm fillet weld per AS 4100?

Per AS 4100 Clause 9.7.3.10, the design capacity of a 6mm fillet weld using the simple method is phi-Vw = 0.80 x 0.60 x fuw x tt = 0.80 x 0.60 x 480 x 4.24 = 0.977 kN/mm. For a 6mm SP weld using the directional method, the capacity is phi-Vw = 1.311 kN/mm for transverse loading — 34% higher than the simple method. The directional method distinguishes between longitudinal and transverse loading.

What does the SP vs GP designation mean for AS 4100 welding electrodes?

AS 4100 Table 9.7.3.10(1) classifies welding consumables as SP (Structural Purpose) or GP (General Purpose). SP electrodes (E41XX, E48XX to AS/NZS 4855) have a minimum tensile strength of 410 MPa or 480 MPa and are mandatory for all structural welds. GP electrodes (E41X0 to AS/NZS 1553.1) share the same tensile strength but lack the toughness and hydrogen-control requirements of SP electrodes. GP electrodes are restricted to non-structural applications.

How does AS 4100 fillet weld capacity compare to AISC 360?

AS 4100 uses phi = 0.80 for fillet welds (directional method) vs AISC phi = 0.75, making AS 4100 slightly less conservative. Both codes use the 0.60 x fuw base strength, but AS 4100 provides two paths: the simple method (0.60 x fuw for all load directions) and the directional method (0.85 x fuw transverse, 0.60 x fuw longitudinal, with an interaction check). AISC uses a 1.0 + 0.50 x sin^1.5(theta) directional factor.

What minimum fillet weld size does AS 4100 require?

AS 4100 does not prescribe minimum fillet weld sizes directly — these are governed by AS/NZS 1554.1 (Structural Steel Welding). The general rule is that the fillet weld leg size should not exceed the thickness of the thinner connected part. For structural applications, a 6mm fillet weld is the practical minimum for manual metal arc welding. For thinner materials (<5mm), 4mm or 5mm fillet welds may be used with appropriate welding procedure qualification.

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Related Pages

• AS 4100 Beam Design — 310UB40.4 Worked Example — Flexure, shear, LTB
• AS 4100 Column Design — 200UC46 Worked Example — Axial compression and buckling
• AS 4100 Bolt Design Guide — Clause 9.3 — Shear, bearing, tearout
• AS 4100 Load Combinations — AS 1170.0 — ULS/SLS/STB combinations
• Australia AS 4100 Steel Design Guide — Complete standards reference
• Fillet Weld Size Chart — Multi-Code Reference — International weld size comparison
• Australian Weld Electrodes — SP & GP Guide — Electrode selection guide

This page is for educational reference. All resistance formulae are per AS 4100:2020 with AS/NZS 4855 electrode classifications. Verify the applicable edition of AS/NZS 1554.1 for your project welding requirements. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent review by a registered structural engineer (CPEng/RPEQ).