Australian Weld Capacity — AS 4100 Clause 9.7 Design Guide

Complete reference for welded connection design to AS 4100:2020 Clause 9.7. Covers fillet weld capacity per mm for GP and SP categories, parent metal capacity checks, electrode matching for Australian steel grades (Grades 300 through 450), butt weld capacity, intermittent welds, fatigue categories, and practical weld sizing.

Related pages: AS 4100 End Plate Connections | Weld Inspection Guide | Weld Symbols (AWS) | Welded Connection Calculator

Weld Design Philosophy — AS 4100 Clause 9.7

Welded connections in Australian steel structures rely predominantly on fillet welds for their simplicity and economy. Clause 9.7 covers both fillet welds and butt welds, with the capacity determined by the weld metal strength category (GP or SP) and the loading direction relative to the weld axis.

The design capacity of a fillet weld per unit length is:

phi_vw = phi x 0.60 x fuw x tt x kr (Clause 9.7.3.10)

Where phi = 0.80 for all weld types (AS 4100 Table 3.4), fuw = nominal tensile strength of the weld metal, tt = design throat thickness = 0.707 x leg_size (for equal-leg 90-degree fillets), and kr = reduction factor depending on the angle between the weld axis and the line of action of the design force.

GP vs SP Weld Categories

AS 4100 distinguishes between General Purpose (GP) and Structural Purpose (SP) weld categories, a classification unique to the Australian code:

For GP category, kr = 1.0 regardless of load angle. This is conservative for transverse welds but simplifies design. GP welds require only visual inspection to AS/NZS 1554.1 Category GP.

For SP category, kr accounts for the increased strength of transversely loaded fillet welds: kr = 1.0 / (0.60 x sin^2(theta) + 0.72 x cos^2(theta))^0.5

Where theta is the angle between the weld axis and the line of force. For theta = 90 degrees (transverse): kr = 1.0 / sqrt(0.60) = 1.29. For theta = 0 degrees (longitudinal): kr = 1.0 / sqrt(0.72) = 1.18.

Paradoxically, SP welds can have LOWER capacity than GP welds for longitudinal loading because the SP quality requirements mandate more stringent inspection but the calculated kr can be < 1.0. Check both GP and SP and use the more favourable result if the quality requirements can be met.

Fillet Weld Capacity Table — GP Category, E48XX Electrodes

Capacity = 0.80 x 0.60 x 490 x tt x 1.0 / 1000. E48XX (E4818, E4824) electrodes are standard for all Grades 300 and 350 base metal.

For SP category transverse welds (theta = 90 deg, kr = 1.29), multiply the above values by 1.29. An 8 mm SP transverse fillet delivers 1.72 kN/mm (vs 1.33 kN/mm GP).

Electrode Selection for Australian Steel Grades

Per AS 4100 Table 9.7.3.10 and AS/NZS 1554.1:

E48XX is the default electrode for all Grade 300 and Grade 350 construction, covering approximately 85% of structural steel connections in Australia. The slight undermatch for Grade 350 (fu = 490 MPa weld vs fu = 480 MPa parent metal) is within the acceptable range per AS 4100.

Parent Metal Capacity Check — Clause 9.7.3.10

The weld may be adequate, but the connected plate must also be able to transfer the force into the weld. The parent metal capacity per unit width of connected plate is:

phi_vw_parent = phi x 0.60 x fyp x tp

This represents the shear capacity of the plate material immediately adjacent to the weld. For a 10 mm Grade 300 plate: phi_vw_parent = 0.90 x 0.60 x 300 x 10 / 1000 = 1.62 kN/mm along the weld.

For thin plates (< 8 mm), an 8 mm fillet weld may have higher capacity than the plate can transfer, and reducing the weld to 6 mm (capacity 1.00 kN/mm) may be acceptable. Always check both weld metal AND parent metal.

Butt Weld Capacity — Clause 9.7.2

Full-penetration butt welds are designed as if they are the parent metal — there is no separate weld metal capacity check:

phi_Nw = phi x fyp x An (tension) and phi_Vw = phi x 0.60 x fyp x Aw (shear)

The resistance factor phi = 0.90 (same as base metal). However, the weld must be made with a matching or overmatching electrode and must be fully inspected (UT or radiography for tension butt welds).

Partial-penetration butt welds are designed as deep fillet welds, with the throat depth equal to the preparation depth specified on the welding symbol.

Minimum and Maximum Weld Sizes

Minimum fillet weld size per AS/NZS 1554.1 depends on the thicker connected part:

Maximum fillet weld size: Along the edge of a plate of thickness t, the maximum leg size = t (for t <= 6 mm) or t - 2 mm (for t > 6 mm). This prevents the weld from melting away the plate edge.

Intermittent Fillet Welds — Clause 9.7.3.11

Intermittent fillet welds may be used where the required continuous weld capacity is low. The effective length of an intermittent weld segment must be at least 40 mm or 4 x leg size, whichever is greater. The clear distance between segments must not exceed 300 mm or 16 x thickness of the thinner plate.

Intermittent welds are common for stiffeners, gusset plates, and lightly loaded connections where a continuous weld would be wasteful. However, they should not be used in fatigue-loaded or corrosion-exposed applications.

Fatigue Considerations — Clause 9.8

Welded connections are particularly susceptible to fatigue because the weld toe creates a geometric stress concentration. AS 4100 Clause 9.8 provides fatigue strength curves for different weld detail categories, following the approach of AS 4100 Appendix K.

Key fatigue-sensitive weld details and their detail categories:

• Continuous fillet weld parallel to stress direction: Category 80
• Transverse fillet weld (load-carrying): Category 63
• Fillet weld attachment end (non-load-carrying): Category 50
• Butt weld, ground flush, UT inspected: Category 112

For crane runway girders, vibrating machinery supports, and highway bridge components, fatigue is the governing limit state and SP weld quality is mandatory.

Worked Example — Welded Bracket Connection

Problem: A bracket plate (16 mm Grade 300) connects a beam to a column flange (20 mm Grade 300). The bracket carries V* = 180 kN at e = 200 mm from the weld group centroid. Weld layout: two vertical fillet welds at 250 mm long each, one horizontal weld at 200 mm. E48XX GP electrodes.

Step 1 — Weld group centroid (from top of bracket): Two side welds (vertical, 250 mm long), one end weld (horizontal at bottom, 200 mm). Centroid y_c = (2 x 250 x 125 + 200 x 250) / (2 x 250 + 200) = (62500 + 50000) / 700 = 112500/700 = 160.7 mm from top.

Step 2 — Eccentricity and weld group properties: Eccentricity of V* from centroid: e' = 200 + (250 - 160.7) = 200 + 89.3 = 289.3 mm. Moment: M* = V* x e' = 180 x 0.2893 = 52.1 kNm.

Polar moment of inertia of weld group about centroid (treating welds as lines): I_p = I_x + I_y for the three welds.

Side welds: Each contributes d^3/12 + d x (d/2 - y_c)^2 = 250^3/12 + 250 x (125 - 160.7)^2 = 1,302,083 + 250 x 1274 = 1,302,083 + 318,500 = 1,620,583 mm^3 per weld (x 2 = 3,241,166).

End weld: 200^3/12 + 200 x (250 - 160.7)^2 = 666,667 + 200 x 7975 = 666,667 + 1,595,000 = 2,261,667 mm^3.

I_y contributions negligible for vertical welds.

Total I_p ~ 3,241,166 + 2,261,667 = 5,502,833 mm^3 (line units).

Step 3 — Weld stresses at critical point (top of vertical welds): Distance from centroid to top: r_top = sqrt((125)^2 + (250 - 160.7)^2) = sqrt(15625 + 7975) = sqrt(23600) = 153.6 mm.

Torsional stress (from moment): f_t = M* x r_top / I_p = 52.1 x 10^6 x 153.6 / 5,502,833 = 1454 N/mm = 1.454 kN/mm (per unit length of weld).

Direct vertical shear stress: f_v = V* / L_total = 180 / (250 + 250 + 200) = 180 / 700 = 0.257 kN/mm (vertical direction).

Vector addition at the critical point: f_res = sqrt(f_t^2 + f_v^2 + 2 x f_t x f_v x cos(angle)).

The angle between f_t (perpendicular to r) and f_v (vertical) is approximately 55 degrees. f_res ~ sqrt(1.454^2 + 0.257^2 + 2 x 1.454 x 0.257 x cos(55)) = sqrt(2.114 + 0.066 + 0.429) = sqrt(2.609) = 1.615 kN/mm.

Step 4 — Weld size selection: phi_vw for GP 6 mm fillet = 1.00 kN/mm. Required length per kN = 1.0. We need 1.615 kN/mm, so 6 mm is inadequate.

Try 8 mm: phi_vw = 1.33 kN/mm. 1.615 > 1.33 — still inadequate.

Try 10 mm: phi_vw = 1.66 kN/mm. 1.615 < 1.66 — OK.

Step 5 — Parent metal check: For 16 mm plate: phi_vw_parent = 0.90 x 0.60 x 300 x 16 / 1000 = 2.59 kN/mm > 1.66 kN/mm. OK. (Weld governs.)

Alternative: Use SP category 8 mm fillet, kr = 1.29 (transverse end weld) and kr = 1.18 (side welds). Mixed kr values complicate the analysis but can save a weld size increment. For simplicity, specify 10 mm GP fillet.

Result: 10 mm fillet welds, E48XX GP, all three sides of bracket.

Weld Volume Estimation

For estimating welding consumables and labour:

Weld metal volume (mm^3) = (leg size)^2 / 2 x total weld length (for equal-leg fillets).

For our bracket with 10 mm fillet, 700 mm total length: Volume = 100/2 x 700 = 35,000 mm^3 = 35 cm^3. At steel density, approximately 0.27 kg of weld metal. Adding 20% for reinforcement and spatter, allow ~0.33 kg of electrode per bracket.

Frequently Asked Questions

How do I choose between an 8 mm and 10 mm fillet weld? The 8 mm fillet is the default for connections with 10-16 mm plate and is the most commonly specified weld in Australian steel structures. It provides 1.33 kN/mm in GP and can be deposited in a single pass. A 10 mm weld (1.66 kN/mm) may require two passes in the flat position and carries a higher heat input, increasing distortion. Use 8 mm as the baseline and upgrade to 10 mm only when required by capacity. Avoid specifying both 8 mm and 10 mm on the same assembly — standardise to one size where possible.

What is the difference between E48XX and E49XX electrodes? E48XX (fuw = 490 MPa) is the Australian classification for low-hydrogen SMAW electrodes per AS/NZS 1553.1. E49XX (fuw = 490 MPa) is the ISO/EN equivalent. The numerical difference (48 vs 49) reflects different classification systems, not different strength. In Australian practice, "W50X" electrodes are specified, providing 490 MPa tensile strength. For GMAW (MIG) wire, ER70S-6 provides 490 MPa and is the standard solid wire for structural work.

When are butt welds required instead of fillet welds? (a) When the design force exceeds the capacity of a practical fillet weld (typically > 16 mm fillet on thick plate). (b) For tension splices where a flush surface is required. (c) For full-strength connections (e.g., SMF beam-to-column connections). (d) Where fatigue governs and the higher Category 112 of ground-flush butt welds is needed. (e) For column splices in multi-storey construction where the weld must transmit the full column capacity.

Can I use intermittent fillet welds in primary connections? Intermittent welds are acceptable for stiffeners, gusset plates, and light bracing connections where the stress is low and a continuous weld would provide far more capacity than needed. They should NOT be used for primary beam-to-column connections, for fatigue-loaded members, or for compression members where the intermittent nature could induce local buckling between weld segments.

This page is for educational reference. Weld design per AS 4100:2020 Clause 9.7 and AS/NZS 1554.1. Verify electrode selection and inspection categories. All structural designs must be independently verified by a licensed Professional Engineer or Structural Engineer. Results are PRELIMINARY — NOT FOR CONSTRUCTION.