Designing welded steel connections in accordance with AS 4100: Steel Structures is critical to ensuring structural safety and performance in buildings, bridges, and industrial structures.

This guide provides a practical overview of how to design compliant welded connections, particularly fillet and butt welds, in line with Australian Standards. You’ll learn when to use welded connections, how to size and assess them, and how to apply relevant code provisions.

To streamline the process, we’ll also demonstrate how to verify your designs using the ClearCalcs Steel Weld Calculator.

Introduction

Welded connections are a fundamental element of structural steel connection design, offering strength, rigidity, and efficiency across a wide range of engineering applications.

AS 4100, published by Standards Australia, sets out the design requirements for welded steel connections, including sizing rules, throat thickness, and capacity checks. It ensures safety and reliability across various steel structures.

This guide walks through key design considerations such as when to use welds, available weld types, and applicable standards. We’ll then break down the design methodology in AS 4100 (2020) and show how engineers can quickly validate their welds using ClearCalcs.

Note: This guide is a technical overview, not a substitute for official standards. Engineers must remain qualified and follow the appropriate codes in their design jurisdiction.

When to use a welded connection

Welded connections are preferred over bolted connections in various situations, including:

High-strength requirements – Welds create continuous joints with high load-carrying capacity.

Sealed joints – Ideal for fluid-tight and airtight structures such as pressure vessels and tanks.

Compact and aesthetic Design – Provides a clean, seamless finish without protruding fasteners. Welds are often used in simple connections where visual appearance or compact geometry is preferred over bolted alternatives.

Complex geometry – Suitable for connections that require intricate shapes or custom fabrication.

Fatigue-resistant designs – Properly designed welds can offer better fatigue performance compared to bolted connections.

Permanent connections – Best used when disassembly is not required.

While welds offer strength and continuity, bolted connections may be preferred when modularity, ease of inspection, or on-site flexibility is needed.

For more information on bolted connections, check out our Design Guide for Steel Bolts.

Types of welded connections

Welded connections can be classified based on the joint type and the welding method used.

Types of weld joints

• Butt welds – Two members are aligned in the same plane and joined edge-to-edge. Butt welds are frequently used to join steel members in axial tension or compression, particularly in portal frames and continuous trusses.
• Fillet welds – Used for joining steel members in lap, T-joint, and corner connections with a triangular cross-section.
• Plug and slot welds – Used for attaching plates by welding within pre-drilled holes or slots.

Types of welding methods

• Manual metal arc welding (MMAW or stick welding) – Common in site welding, versatile but slower.
• Gas metal arc welding (GMAW or MIG welding) – Fast and widely used in fabrication.
• Flux-cored arc welding (FCAW) – Suitable for heavy-duty applications.
• Submerged arc welding (SAW) – Used for large-scale, automated welding with high penetration.

Each method has its advantages and is chosen based on project requirements, accessibility, and material thickness.

Advantages and disadvantages of welded connections

Welded connections offer several structural benefits but also come with limitations that engineers must consider in design decisions. Below is a comparison of the main advantages and disadvantages of welds in structural steelwork.

Advantages:

• High strength and efficiency – Welds provide excellent load transfer with minimal material usage.
• Continuous and rigid connection – No gaps, reducing potential movement or slip.
• Clean and space-saving – Eliminates protruding bolts and plates, creating a streamlined appearance.
• Greater design flexibility – Can accommodate complex geometries.

Disadvantages:

• Permanent and difficult to modify – Unlike bolts, welded joints cannot be easily removed.
• Quality control issues – Requires skilled labor and proper inspection to avoid defects like cracks and porosity.
• Heat-affected zone (HAZ) weakening – Nearby material may weaken due to high temperatures.
• On-site welding challenges – Environmental factors such as wind, rain, or limited access can affect weld quality.

While welded connections offer significant benefits, careful consideration is needed to avoid defects and ensure compliance with standards.

Applicable Australian standards for welds

Designing and specifying welded steel connections in Australia requires compliance with several interrelated standards. These documents outline everything from connection strength requirements to welding execution, inspection, and quality control.

Below are the key Australian Standards that govern structural welding design:

AS 4100 – Steel Structures

The primary design standard for steel structures in Australia. It provides:

• Design provisions for welded connections
• Stress limits, throat thickness, and weld sizing rules
• Limit state design methodology for strength, stability, and serviceability

AS/NZS 1554 – Structural Steel Welding (Main welding standard)

AS/NZS 1554 defines the necessary weld procedures, including welder qualifications, joint configurations, and quality assurance processes for structural welds.

This suite of standards defines how welding is executed and qualified in different scenarios:

• Part 1: General steel welding requirements.
• Part 2: High-strength steel welding.
• Part 3: Welding of stainless steel.
• Part 4: Welding of steel structures subject to dynamic loading.

These parts support weld execution in both shop and field environments, and they complement AS 4100’s design requirements.

AS/NZS 5131 – Structural Steelwork – Fabrication and Erection

Specifies welding quality requirements, welder competency, inspection protocols, and fabrication tolerances. This standard aligns closely with the Steelwork Compliance Australia (SCA) certification scheme.

AS 2214 – Certification of Welding Supervisors – Structural Steelwork

Defines minimum qualification requirements for welding supervisors overseeing structural projects.

These standards are supported by guidance from organisations such as the Australian Steel Institute (ASI), which publishes technical notes and fabrication guidance on structural welding best practices. Ensuring compliance with these standards is essential to achieve high-quality, durable welded connections.

Butt welds

AS 4100 (2020) defines two types of butt welds:

• Complete penetration butt weld – Fusion exists throughout the full depth of the joint.
• Incomplete penetration butt weld – Fusion is achieved over less than the full depth of the joint.

These welds are commonly used for connecting members in axial tension or compression and require accurate joint preparation and inspection to ensure structural reliability.

The following sections summarise the key design rules in AS 4100 (2020) for calculating butt weld capacities, including throat thickness, effective length, and strength checks.

Size of butt welds

As per Clause 9.6.2.2 of AS 4100 (2020), the size of a complete penetration butt weld, other than a complete penetration butt weld in a T-joint or a corner joint, and the size of an incomplete penetration butt weld shall be the minimum depth to which the weld extends from its face into a joint, exclusive of reinforcement.

The size of a complete penetration butt weld for a T-joint or a corner joint shall be the thickness of the part whose end or edge butts against the face of the other part.

Design throat thickness of butt welds

Clause 9.6.2.3 of AS 4100 (2020) defines the design throat thickness as:

1. Complete penetration butt weld — The design throat thickness for a complete penetration butt weld shall be the size of the weld.
2. Incomplete penetration butt weld — The design throat thickness for an incomplete penetration butt weld varies depending on the weld preparation method as follows:
   • Prequalified preparation for incomplete penetration butt weld as specified in AS/NZS 1554.1 or AS/NZS 1554.4.
   • Non-prequalified preparation for incomplete penetration butt weld
     
     • where θ ≤ 60° (d – 3) mm, for single V weld; [(d3 + d4) − 6] mm, for double V weld
     • where θ > 60° d mm, for single V weld; (d3 + d4) mm, for double V weld, where, d = depth of preparation (d3 and d4 are the values of d for each side of the weld) and θ = angle of preparation
3. For an incomplete penetration butt weld made by an automatic arc welding process for which it can be demonstrated by means of a macro test on a production weld that the required penetration has been achieved, an increase in design throat thickness up to the depth of preparation may be allowed. If the macro test shows penetration beyond the depth of preparation, an increase in design throat thickness up to that shown in Figure 9.6.3.4 may be allowed.

Effective length of butt welds

As per Clause 9.6.2.4 of AS 4100 (2020), the effective length of a butt weld is be the length of the continuous full size weld.

Effective area of butt welds

As per Clause 9.6.2.5 of AS 4100 (2020), the effective area of a butt weld shall be the product of the effective length and the design throat thickness.

Transition of thickness or width in butt welds

As per Clause 9.6.2.6 of AS 4100 (2020), butt welded joints between parts of different thickness or unequal width that are subject to tension shall have a smooth transition between surfaces or edges. The transition shall be made by chamfering the thicker part or by sloping the weld surfaces or by any combination of those, as shown in Figure 9.6.2.6 in AS 4100 (2020).

Strength assessment of a butt weld

The assessment of a butt weld for the strength limit state shall be as follows:

• The design capacity of a complete penetration butt weld shall be taken as equal to the nominal capacity of the weaker part of the parts joined, multiplied by the appropriate capacity factor (ϕ) for butt welds given in Table 3.4 of AS 4100 (2020) (exerpt below). The butt weld shall be made using a welding consumable which will produce butt tensile test specimens in accordance with AS 2205.2.1 for which the minimum strength is not less than that given in Table 2.1 of AS 4100 (2020) for the parent material.

• The design capacity of an incomplete-penetration butt weld shall be calculated as for a fillet weld (see Clause 9.6.3.10 of AS 4100 (2020)) using the design throat thickness determined in accordance with Clause 9.6.2.3(b) of AS 4100 (2020) which is summarised above.

This section provides the groundwork for calculating weld capacity and verifying compliance. In the next part, we’ll walk through fillet weld provisions, which are more common in fabrication and field connections.

Fillet welds

Fillet welds are among the most widely used connection types in steel structures, especially for lap, T-joint, and corner configurations. AS 4100 provides detailed rules for sizing, throat thickness, and strength design of fillet welds.

This section summarises AS 4100 (2020) design rules for fillet welds, covering size, throat thickness, length, and capacity checks.

Size of fillet welds

As per Clause 9.6.3.1 of AS 4100 (2020), the size of a fillet weld shall be specified by the leg lengths. The leg lengths shall be defined as the lengths (tw1, tw2) of the sides lying along the legs of a triangle inscribed within the cross-section of the weld (see Figures 9.6.3.1(a) and (b) of AS4100 (2020)).

When the legs are of equal length, the size shall be specified by a single dimension (tw).

Where there is a root gap, the size (tw) shall be given by the lengths of the legs of the inscribed triangle reduced by the root gap as shown in Figure 9.6.3.1(c) of AS4100 (2020).

The preferred sizes of a fillet weld less than 15 mm are — 3, 4, 5, 6, 8, 10 and 12 mm.

The minimum size of a fillet weld shall be as per Table 9.6.3.2 of AS4100 (2020) except that the size of the weld need not exceed the thickness of the thinner part joined as per Clause 9.6.3.2 of AS 4100 (2020).

The maximum size of a fillet weld (tw) along an edge is dependent on the thickness of the material (t) as per Figure 9.6.3.3 of AS4100 (2020) below.

Design throat thickness of fillet welds

The design throat thickness (tt) of a fillet weld shall be as shown in Figure 9.6.3.1 of AS4100 (2020), which is shown in the Size of Welds section of this article.

As per Clause 9.6.3.4 of AS 4100 (2020) a weld made by an automatic arc welding process may have its design throat thickness increased as per Figure 9.6.3.4 AS 4100 (2020), provided that it can be demonstrated by means of a macro test on a production weld that the required penetration has been achieved. Where such penetration is achieved, the size of weld required may be correspondingly reduced to give the specified design throat thickness.

Effective length of fillet welds

As per Clause 9.6.3.5 of AS 4100 (2020), the effective length of a fillet weld shall be the overall length of the full-size fillet, including end returns.

The minimum effective length of a fillet weld shall be 4 times the size of the weld. However, if the ratio of the effective length of the weld to the size of the weld does not conform to this requirement, the size of the weld for design purposes shall be taken as 0.25 times the effective length.

The minimum length requirement shall also apply to lap joints. Any segment of intermittent fillet weld shall have an effective length of not less than 40 mm or 4 times the nominal size of the weld, whichever is the greater.

The effective area of a fillet weld is then given by the product of the effective length and the design throat thickness.

Transverse spacing of fillet welds

As per Clause 9.6.3.7 of AS 4100 (2020), if two parallel fillet welds connect two components in the direction of the design action to form a built up member, the transverse distance between the welds shall not exceed 32t_p, except that in the case of intermittent fillet welds at the ends of a tension member, the transverse distance shall not exceed either 16t_p or 200 mm, where t_p is the thickness of the thinner of the two components connected.

It shall be permissible to use fillet welds in slots and holes in the direction of the design action in order to satisfy this Clause.

Intermittent fillet welds

As per Clause 9.6.3.8 of AS 4100 (2020), except at the ends of a built-up member (which are detailed in Clause 9.6.3.9 of AS 4100 (2020)), the clear spacing between the lengths of consecutive collinear intermittent fillet welds shall not exceed the lesser of:

1. for elements in compression, 16t_p and 300 mm; and
2. for elements in tension, 24t_p and 300 mm

Strength limit state for fillet welds

As per Clause 9.6.3.10 of AS 4100 (2020), a fillet weld subject to a design force per unit length of weld (v_w) shall satisfy:

$v^*_w \le \phi v_w= \phi0.6f_{uw}t_tk_r$

Where:

$\phi$ = 0.7 if longitudinal, 0.8 for other fillet weld, capacity factor as per Table 3.4 of AS 4100 (2020)

$f_{uw}$ = nominal tensile strength of weld metal as per Table 9.6.3.10(A) of AS 4100 (2020)

$t_t$ = design throat thickness

$k_r$ = 1.0 or as per Table 9.6.3.10(B) of AS 4100 (2020) for welded lap connections

Combination of weld types

In some structural situations, different weld types may be used together to form a composite connection.

As per Clause 9.7.4 of AS 4100 (2020), if two or more types of weld are combined in a single connection, the design capacity of the connection shall be the sum of the design capacities of each type.

Each weld must be checked for its individual contribution based on location, direction of force, and section geometry.

Weld design by hand calculation

Let’s take the example from our Design Guide for Steel Bolts to Australian Standards. This worked example illustrates how to determine weld size and capacity for standard connection details, such as end-plate beam-column joints.

An end plate is welded to the end of a universal beam that is to be bolted to the flange of a universal column to form a beam-column connection.

In our previous design guide on bolts we designed the bolts to be a 2 x 2 bolt M12 8.8/S bolt arrangement.

Now we must design the weld for the end plate to the universal beam. As previously, the end plate is 370mm x 150mm to fit onto the flange of the column and is 12mm thick with 440MPa tensile strength. The welded end plate connection is exerting a shear force of 80kN on the bolt group as well as a 20kNm out of plane moment. The beam is a 200UB18.2 section with a steel grade of 300MPa.

Complete penetration butt weld design by hand calculation

There is a 20kNm out of plane moment exerted on the connection which causes a tensile force on the weld connecting the top of the UB to the end plate.

Moment = Force * Eccentricity

Tensile force on top flange weld = $\frac{\text{Moment}}{\text{Eccentricity}}$ = $\frac{20kNm}{0.2m}$ = $100kN$

The design capacity of a complete penetration butt weld shall be taken as equal to the nominal capacity of the weaker part of the parts joined, multiplied by the appropriate capacity factor (ϕ). The capacity factor for a SP complete penetration buttweld is 0.9.

As per Clause 9.6.3.10 of AS 4100 (2020), the design force per unit length (v^*_w) shall be the vectorial sum of the design forces per unit length on the effective area of the weld.

The effective area of a butt weld shall be the product of the effective length and the design throat thickness. The design throat thickness for a complete penetration butt weld shall be the size of the weld.

The size of a complete penetration butt weld shall be the minimum depth to which the weld extends from its face into a joint, exclusive of reinforcement.

The minimum thickness of the joined members (between the UB and the plate itself) is 4.5mm which is the web thickness of the 200UB18.2.

v^*_w = $\frac{\sqrt{(V^*)^2+(N^*)^2}}{l_w}$ = $\frac{\sqrt{(100*)^2+(80)^2}}{0.2^2}$ = 640kN/m = 0.64kN/mm

$\phi v_w$ = $\phi f_yt_t$ = 0.9*300*4.5 = 1.15kN/mm

$v^*_w \le \phi\ v_w$

Therefore, the complete penetration buttweld design is safe.

Fillet weld design by hand calculation

Let’s assume that the builder completed manual metal arc welding type E43XX with equal leg fillet welds, which have a weld thickness of 6mm.

As per Clause 9.6.3.10 of AS 4100 (2020), a fillet weld subject to a design force per unit length of weld (v_w) shall satisfy:

$v^*_w \le \phi v_w = \phi0.6f_{uw}t_tk_r$

Where:

$\phi$ = 0.8 capacity factor as per Table 3.4 of AS 4100 (2020)

$f_{uw}$ = 430MPa nominal tensile strength of weld metal as per Table 9.6.3.10(A) of AS 4100 (2020)

$t_t$ = 4.24mm design throat thickness = $\sqrt{t^2_w+t^2_w}$ where t_w is 6mm based on the builders welding practices

$k_r$ = 1.0 as per Table 9.6.3.10(B) of AS 4100 (2020)

v^*_w = $\frac{\sqrt{(V^*)^2+(N^*)^2}}{l_w}$ = $\frac{\sqrt{100^2+80^2}}{0.2}$ = 640kN/m = 0.64kN/mm

$\phi v_w$ = $\phi 0.6f_{uw}t_tk_r$ = 0.8 *0.6 * 430 * 4.24 * 1.0 = 875.13N/mm = 0.875kN/mm

$v^*_w \le \phi v_w$

Therefore, the fillet weld design is safe..

Weld design with ClearCalcs

Let’s run the same example using the ClearCalcs Steel Weld Calculator to compare.

Step 1: Input the loads on the welded connection

As per our example “The welded end plate connection is exerting a shear force of 80kN on the bolt group as well as a 20kNm out of plane moment”.

There is a 20kNm out of plane moment exerted on the connection which causes a tensile force on the top of the weld and a compressive force on the bottom of the weld. This tensile value is calculated using stress equations to determine the axial loads on the bolts, caused by the out-of-plane moment.

Tensile Force N = $\sigma$A

N = $\frac{M_x(y_i)}{I_x}A$ = $\frac{20,000,000*(100)}{20,000}*1$ = 100,000N = 100kN

Where:

M_x = 20kNm = 20,000,000Nmm

eccentricity (y_i) = 100mm (distance from centroid to flange)

I_x= moment of inertia = $\sum_{}^{}(y_i^2)$ = 100² (eccentricity to top flange of UB) + 100² (eccentricity to bottom flange of UB) =20,000mm²

We can then input the shear force and tensile force on the weld into the Loads section of the calculator as per the below.

Step 2: Define the key properties

The type of weld in the problem is a complete penetration butt weld and we will specify a structural purpose (SP) weld as a beam-column connection is a critical structural weld.

We can determine the throat thickness based on an initial weld thickness of 6 mm. If the weld is overcapacity, this can be increased to meet design demands.

t_t = 4.24mm design throat thickness = $\sqrt{t^2_w+t^2_w}$ where t_w is 6mm based on the builders welding practices

We will specify the length of the weld as 200mm which is the length of the web of the 200UB18.2 beam that is being welded to the end plate. The length of the weld could be increased if we decide to weld the flanges of the beam to the end plate as well.

The nominal tensile strength of the weld metal is 430MPa as per Table 9.6.3.10(A) of AS 4100 (2020), and the base metal’s tensile strength is 300MPa as is the grade of the 200UB18.2.

Step 3: Assess weld utilisation

The summary outputs instantaneously specify that the specified weld is safe, with a utilisation of 56%.

If the utilisation was 100% or more, we would return to step 2, select a different type of welded connection, increase the throat thickness or the length of the weld until we could achieve a safe connection.

With ClearCalcs, we can get instant feedback on the changes these parameter changes make without having to perform the rigorous hand calculations as per the previous example.

Fillet weld design with ClearCalcs

If we wanted to design a fillet weld, all we need to do is change the weld type in step 2 under the Key Properties heading to other fillet weld and complete the same steps as detailed in the full penetration butt weld example.

Conclusion

Welded connections play a crucial role in steel construction, offering strength, efficiency, and design flexibility. However, careful consideration must be given to material properties, welding techniques, and quality control to ensure compliance with AS 4100 and related standards.

For accurate and efficient weld design calculations, we recommend using ClearCalcs Steel Weld Calculator, a powerful structural engineering platform that streamlines compliance with Australian Standards and simplifies complex structural calculations.

By leveraging ClearCalcs, engineers can save hours of manual checking, reduce the risk of errors, and confidently validate welds, bolts, and steel members in minutes.

‍