AS 4100 Fillet Weld Design — SP/GP Capacity Per mm Run

Fillet welds are the most common weld type in structural steelwork, and AS 4100:2020 governs their design across Australia and New Zealand. This guide covers throat area calculation, SP vs GP capacity differences, minimum weld sizes, and a complete worked example with an 8 mm SP fillet using E48XX electrodes. Verify every number with our free welded connection calculator.

What you will learn

• How to calculate effective throat area for fillet welds
• The difference between SP and GP weld categories and when each applies
• Minimum fillet weld sizes based on plate thickness
• A full worked example: 8 mm SP fillet weld, E48XX electrode, 200 mm long
• How to check both weld metal and base metal capacity
• Directional strength enhancement for transverse welds

Copyright and standards notice

This site does not reproduce copyrighted code clauses or proprietary tables verbatim. Discussion of AS 4100 here is high-level and intended to help you understand verification workflows. Always consult the official published standard (AS 4100:2020 and AS/NZS 1554) for authoritative requirements.

Fillet weld geometry — throat vs leg size

The most common error in weld design is confusing the leg size with the effective throat thickness. They are not the same:

• Leg size (tw): The dimension measured along each face of the joint. This is what you specify on drawings (e.g., "8 mm fillet weld").
• Design throat thickness (tt): The shortest distance from the root to the face of the weld. For an equal-leg fillet: tt = tw / sqrt(2) = 0.707 x tw.

For an 8 mm fillet weld:

• Leg size: tw = 8 mm
• Design throat: tt = 8 / 1.414 = 5.66 mm

The effective throat is what resists the applied load. Using the leg size instead of the throat overestimates capacity by 41% — a dangerous error.

SP vs GP weld categories

AS 4100 defines two fabrication quality categories that directly affect design capacity through the capacity reduction factor:

Key insight: Choosing GP instead of SP reduces weld capacity by 25%. This is not a minor difference — it can mean the difference between an adequate and an undersized connection. The category must be specified on drawings before fabrication begins.

Minimum fillet weld sizes

AS 4100 Cl. 9.7.3.2 specifies minimum fillet weld sizes based on the thickness of the thicker part being joined. This ensures adequate heat input and avoids rapid cooling that causes cracking:

For example, welding a 10 mm gusset plate to a 16 mm thick column flange requires a minimum 6 mm fillet weld. Starting your design at the minimum size and checking if it provides enough capacity is the standard workflow.

Worked example: 8 mm SP fillet weld, E48XX electrode

Problem statement

Check 1: Weld metal capacity

Step 1 — Effective throat thickness: tt = tw / sqrt(2) = 8 / 1.414 = 5.66 mm

Step 2 — Effective weld area per mm run: A_w = tt x 1 mm = 5.66 mm² per mm

Step 3 — Design capacity per mm run (Cl. 9.7.3.10): phi_w x v_w = phi_w x 0.6 x fuw x tt = 0.80 x 0.6 x 480 x 5.66 = 1303 N/mm = 1.303 kN/mm

Step 4 — Total weld capacity (two 200 mm runs): V_w = 1.303 x 200 x 2 = 521.2 kN

Utilization: 180 / 521.2 = 0.35 (OK)

Check 2: Base metal capacity

The parent metal adjacent to the weld must also be checked. For a 10 mm plate with Grade 300 steel:

Base metal shear capacity per mm along the weld (Cl. 9.7.3.9): phi x 0.6 x fu x tp = 0.90 x 0.6 x 440 x 10 / 1000 = 2.376 kN/mm per side

Total for two sides over 200 mm: V_bm = 2.376 x 200 x 2 = 950.4 kN

Utilization: 180 / 950.4 = 0.19 (OK — base metal does not govern)

Check 3: GP comparison

If this same weld were fabricated to GP category instead of SP:

phi_w x v_w (GP) = 0.60 x 0.6 x 480 x 5.66 = 977 N/mm = 0.977 kN/mm

Total: 0.977 x 200 x 2 = 390.9 kN

Utilization (GP): 180 / 390.9 = 0.46 — still adequate, but 11% higher utilization than SP.

Results summary

Weld metal capacity governs for both categories. The 8 mm SP fillet is adequate with good margin.

Try this example: Welded Connection Calculator -- enter these values and compare your results.

Capacity per mm run — quick reference

For common fillet weld sizes with E48XX electrode, the design capacity per mm of weld run is:

These values assume longitudinal loading and E48XX electrode (fuw = 480 MPa). For transverse loading, AS 4100 permits a directional strength enhancement of up to 1.5 times the longitudinal capacity, but this must be applied carefully — see the next section.

Directional strength enhancement

AS 4100 Cl. 9.7.3.10 recognizes that fillet welds loaded transversely (perpendicular to their axis) are stronger than those loaded longitudinally (parallel to their axis). The enhancement factor depends on the angle of loading:

• 0 degrees (longitudinal): factor = 1.00
• 45 degrees: factor = 1.22
• 90 degrees (transverse): factor = 1.50

Practical example: A 6 mm SP fillet weld loaded transversely has an effective capacity of: 0.977 x 1.50 = 1.466 kN/mm — equivalent to an 8 mm longitudinally loaded weld.

This means a transversely loaded weld can be smaller than a longitudinally loaded weld for the same demand. But be careful: the enhancement only applies when you can guarantee the load direction. For weld groups with combined loading, use the vector summation method per AS 4100.

Common mistakes in AS 4100 weld design

1. Using leg size instead of throat thickness. This overestimates capacity by 41%. The throat is tw x 0.707 for equal-leg fillets.

2. Applying SP phi factor to GP welds. If the fabrication quality is GP, you must use phi = 0.60, not 0.80. This 25% capacity reduction is significant.

3. Ignoring minimum weld size requirements. A 3 mm fillet on a 25 mm plate will not achieve adequate fusion. Check the minimum size table before finalizing.

4. Forgetting effective length reductions. The effective length of a fillet weld may be less than the physical length due to start/stop craters. AS 4100 requires deducting twice the leg size from each end if returns are not provided.

5. Not checking base metal. Weld metal capacity means nothing if the parent plate tears out first. Always check base metal shear capacity, especially for thin plates.

6. Ignoring weld group eccentricity. When a weld group is loaded eccentrically, the resultant stress varies along the weld. The peak stress at the extreme point governs, not the average stress.

AS 4100 vs other standards — weld capacity comparison

For engineers working across multiple codes, here is how fillet weld capacity frameworks compare:

The main takeaway: all three standards use the same throat geometry, but the safety factors and quality frameworks differ. You cannot mix factors between codes.

Frequently Asked Questions

What electrode should I specify for Grade 300 steel? E48XX is the standard match for Grade 300 and 350 base metal. The "48" indicates 480 MPa nominal tensile strength of the weld metal, which exceeds the 440 MPa ultimate strength of Grade 300 plate.

When is GP weld quality acceptable? GP is acceptable for secondary members, non-structural attachments, and connections where the consequence of failure is low. For primary structural connections, moment connections, and fatigue-sensitive details, SP is almost always required.

How do I handle intermittent fillet welds? Intermittent welds are designed by computing the required weld length per unit of connection, then distributing discrete weld segments with gaps. The capacity is based on the total effective weld length only — gaps do not contribute.

Can I use E41XX electrode with Grade 300 steel? E41XX has a lower tensile strength (410 MPa) than E48XX (480 MPa). This reduces weld capacity proportionally. It is acceptable if the capacity is adequate, but E48XX is the standard specification for Grade 300/350 work.

What is the maximum fillet weld size? The maximum leg size should not exceed the thickness of the thinner plate being joined. For plate edges, it should be 1-2 mm less than the plate thickness to avoid melting the edge.

Key Takeaways

• Throat thickness, not leg size, determines fillet weld capacity. For equal-leg fillets: tt = 0.707 x tw.
• SP welds are 25% stronger than GP for the same weld size due to higher phi factor (0.80 vs 0.60).
• Always check both weld metal and base metal capacity — thin plates can govern over the weld throat.
• Minimum weld sizes exist to ensure fusion and prevent cracking. Check the table before designing.
• Transverse welds are up to 1.5x stronger than longitudinal welds — use this to optimize weld groups.
• The capacity-per-mm table (5 mm to 12 mm, SP and GP) lets you quickly size welds for any demand.

Run This Calculation

Welded Connection Calculator — fillet and groove weld capacity per AS 4100, AISC 360, EN 1993, and CSA S16 with SP/GP category selection.

Bolted Connection Calculator — for mixed weld-and-bolt connection groups or alternative connection designs.

Further Reading

• Weld design checklist — step-by-step verification guide
• Minimum fillet weld size — AISC J2.4 and AS 4100 table
• Fillet weld size chart — capacity per unit length
• Weld electrode reference — E60XX, E70XX, E80XX classifications
• Weld symbol chart — AWS A2.4 reference
• Steel Fy and Fu reference — yield and tensile strength by grade
• AS 4100 code notes
• AS 4100 bolt group design — M20 8.8 worked example
• How to verify calculator results

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