Australian Steel Connection Guide — AS 4100 Clause 9 & Standardised AU Details

Comprehensive guide to Australian structural steel connection design per AS 4100:2020 Clause 9 and the ASI Design Guide series. Covers the standardised Australian connection types (flexible end plates, web side plates, angle cleats, bolted moment end plates, column splices, base plates), bolt design per Clause 9.3 (shear, tension, combined actions, bearing, tear-out), weld design for connections per Clause 9.7, standardised detailing dimensions from ASI Design Guides 4, 5, and 7, and a fully worked web side plate connection example in Grade 300PLUS steel with M20 8.8/S bolts.

Quick access: AS 4100 Bolt Design | AU Connection Design | AS 4100 Weld Design | AU Base Plate Design | All AU References

1. The Australian Steel Connection Ecosystem

Australian steel connection design operates within a standardised framework developed by the Australian Steel Institute (ASI):

In Australian practice, the structural engineer specifies member sizes and the overall framing arrangement. The steelwork contractor's engineer (or the fabricator's nominated engineer) designs and details the connections, typically using the ASI standardised design guides for simple connections and performing specific design only for non-standard connections.

The key distinction from UK practice: ASI Design Guide 10 (Specification of Standardised Structural Connections) provides pre-engineered connection details unlike any other national system — the detailer selects the standard connection from the table based on the beam size and design shear force, and the capacity is pre-verified. This eliminates project-specific connection calculations for simple shear connections, massively reducing design time and cost.

2. AS 4100 Clause 9 — Bolt Design

2.1 Bolt Shear Strength (Clause 9.3.2.1)

The nominal shear capacity of a single bolt:

V_f = 0.62 x f_uf x (n_n x A_c + n_x x A_o)

Where:

• f_uf = minimum tensile strength of bolt (830 MPa for Grade 8.8)
• n_n = number of shear planes with threads intercepting the shear plane
• n_x = number of shear planes with the plain shank in the shear plane
• A_c = bolt core area (tensile stress area for threads)
• A_o = bolt shank area (plain diameter)

For a typical M20 8.8 bolt with the shear plane through the thread (n_n = 1, n_x = 0, A_c = 245 mm^2):

V_f_unfactored = 0.62 x 830 x 245 = 126.2 kN (per bolt, per shear plane)
phi_V_f = 0.80 x 126.2 = 101.0 kN (design capacity)

For a double-shear connection (n_n = 2), the capacity doubles to 202.0 kN.

2.2 Bolt Tension Strength (Clause 9.3.2.2)

The nominal tensile capacity of a bolt:

N_tf = A_s x f_uf

For an M20 8.8 bolt, A_s = 245 mm^2:

N_tf_unfactored = 245 x 830 = 203.4 kN
phi_N_tf = 0.80 x 203.4 = 162.7 kN

Important: For bolts subject to applied tension, Clause 9.3.2.4 requires the prying action check. The bolt tension capacity may be reduced depending on the connected plate flexibility. The ASI standardised connections account for prying in their tabulated capacities, so the designer using the Design Guide tables does not need to compute prying separately.

2.3 Combined Shear and Tension (Clause 9.3.2.3)

When a bolt resists both shear V* and tension N* simultaneously:

(V* / phi_V_f)^2 + (N* / phi_N_tf)^2 <= 1.0

This quadratic interaction is more onerous than a linear check. A bolt at 70% shear utilisation can only handle approximately 71% of its tension capacity.

2.4 Ply Bearing (Clause 9.3.2.4)

The nominal bearing capacity per bolt:

V_b = 3.2 x d_f x t_p x f_up    (for edge bolts with standard clearance)
V_b = a_e x t_p x f_up          (alternative formulation using a_e = minimum edge distance)

Where:

• d_f = bolt diameter (20 mm for M20)
• t_p = thickness of the connected ply (use the thinner ply at each shear plane)
• f_up = tensile strength of the ply material (440 MPa for Grade 300PLUS plate)
• a_e = minimum distance from bolt hole centre to plate edge in direction of force

For an M20 bolt bearing on an 8 mm Grade 300 plate, edge condition:

V_b_unfactored = 3.2 x 20 x 8 x 440 = 225.3 kN (per bolt, per shear plane)
phi_V_b = 0.90 x 225.3 = 202.8 kN

The bolt shear (101.0 kN) governs over bearing (202.8 kN) in this case.

For inner bolts (not near an edge), the bearing capacity uses a_e = 3.0 x d_f = 60 mm for standard holes at 2.5 x d_f pitch, giving:

V_b = 60 x 8 x 440 = 211.2 kN (unfactored)

The 3.2 coefficient for edge bolts is a key difference from AISC 360 which uses 2.4 for standard holes — AS 4100 is approximately 33% higher for bearing strength at the same geometry, reflecting the different Australian calibration.

2.5 Tear-Out (Clause 9.3.2.4)

When the edge distance is small relative to bolt diameter, tear-out (block shear of the ply material ahead of the bolt) may govern over bearing:

V_b = a_e x t_p x f_up   (with a_e limited to 2.0 x d_f for tear-out check)

For M20 bolts at 30 mm edge distance: a_e = 30 mm < 2 x 20 = 40, so tear-out governs:

V_b = 30 x 8 x 440 = 105.6 kN (unfactored)
phi_V_b = 0.90 x 105.6 = 95.0 kN — now lower than bolt shear!

This demonstrates why minimum edge distances (1.5 x d_f = 30 mm for M20) are specified — to ensure the transition from bearing to tear-out occurs above the bolt shear capacity.

3. Standardised Australian Connection Types

3.1 Flexible End Plate (ASI Design Guide 4)

The most common Australian beam-to-column shear connection. An 8 or 10 mm plate is shop-welded to the beam web with a continuous fillet weld. The projecting plate is site-bolted to the supporting column flange or web.

Standard detail for a 310UB40.4 beam:

Design philosophy: The plate is designed as a vertical cantilever in bending between the bottom row of bolts (compression under rotation) and the weld group at the beam web. The plate bending check ensures the plate can transfer the eccentric moment V* x e where e is the distance from the bolt group centroid to the weld group face. The ASI tables account for this bending plus bolt shear and bearing.

3.2 Web Side Plate (ASI Design Guide 5)

A single plate bolted to the beam web and connected to the support by either bolting (to a column web or flange) or welding (to a column flange). Preferred where:

• The beam frames into a column web and access for end plate bolting is restricted
• The beam needs site adjustment tolerance (plate can be shimmed)
• The connection must be demountable

Standard detail for a single-sided web side plate, 200UB25.4:

3.3 Angle Cleats (Part of ASI Design Guide 4)

Two angles (typically 90x90x8 EA or 100x100x10 EA in Grade 300) bolted to the beam web and the supporting face. Less common than flexible end plates in modern Australian construction but still used for:

• Light beams (< 250UB range) where end plates would be disproportionately heavy
• Retrofit connections adding new beams to existing steelwork
• Situations where the beam must be erected from below (angles can be pre-bolted to the column)

3.4 Bolted Moment End Plate (ASI Design Guide 10)

For moment-resisting connections in portal frame buildings and moment frames. A full-depth end plate (extending above and below the beam flanges) with bolts in the tension zone outside the beam flanges, designed to transfer both moment and shear.

Standard detail for a 460UB67.1 portal frame eave connection:

4. Column Splices — Australian Standard Detail

Australian column splices per ASI standard practice are located at a convenient height above floor level (typically 1.0–1.2 m above finished floor), above the beam-to-column connections. The splice must transmit the axial load plus the nominal moment from a 100 mm eccentricity (unless a more precise analysis is performed).

Standard splice for a 310UC118 to 250UC89.5 step:

For columns where both sections are the same serial size (non-stepped splice), a simpler detail with only external flange cover plates and web splice plates suffices.

5. Base Plate Design — Australian Practice

Australian base plates are designed per ASI Design Guide 7 or directly to AS 4100 Clause 9. The standard Australian base plate consists of a flat steel plate welded to the column end with cast-in holding-down bolts (typically Grade 4.6 or 8.8 anchor bolts threaded at the top).

Typical base plate for a 200UC46.2, pinned base:

For fixed-base columns (moment-resisting), the base plate is designed for combined axial load and bending moment. The anchor bolts are placed outside the column flanges to maximise the lever arm, and the critical failure modes are:

1. Base plate bending (yield line analysis) — the plate is checked as a cantilever from the column face to the bolt line
2. Concrete bearing (AS 3600) — the grout/concrete bearing stress is limited to 0.85 x f'_c per AS 3600 bearing provisions
3. Anchor bolt tension (AS 4100 Clause 9.3.2.2) — bolts in the tension zone carry the uplift force from the moment couple
4. Anchor bolt shear — transferred through the grout bed to the concrete footing, with shear lugs added for high shear demands (> 50 kN)

6. Worked Example — Web Side Plate Connection

Design Brief

Connect a simply supported 250UB31.4 secondary beam (Grade 300PLUS) to the web of a 310UC96.8 primary column (Grade 300PLUS). Design shear force V* = 85 kN. 8.8/S M20 bolts to AS/NZS 1252. Single web side plate.

Step 1 — Select Plate Geometry

From ASI Design Guide 5, for a 250UB31.4:

• Plate: 180 x 90 x 10 mm Grade 300
• Bolts to beam web: 3 x M20 8.8/S at 70 mm pitch, 30 mm edge distance
• Bolt gauge to column web: 70 mm (2 columns x 2 rows = 4 x M20 8.8/S)

Step 2 — Check Bolt Group to Beam Web

Eccentricity e = 45 mm (half plate thickness + half column web thickness + gap):

• Design for bolt group with eccentric shear using the instantaneous centre of rotation method, or conservatively, check all bolts for:
  • Direct shear: V*_vertical per bolt = 85/3 = 28.3 kN
  • Eccentric moment: M* = 85 x 0.045 = 3.825 kN.m
  • Maximum resultant per bolt (end bolts): R* = sqrt((28.3 + 3.825 x 0.035/0.0163)^2 + (3.825 x 0.070/0.0163)^2) ... or use ASI DG 5 tabulated capacity.

Using the ASI DG 5 capacity table, a 3-bolt web side plate with the above geometry on a 250UB31.4 has a design shear capacity V_design = 128 kN > 85 kN — OK.

Step 3 — Check Bolt Shear (Single Shear Plane per Bolt Group)

The bolts to the column web are in a 2x2 layout. Design shear per bolt:

• V*_v per bolt = 85/4 = 21.3 kN (vertical)
• Horizontal component from eccentric moment: 3.825 x 0.035 / (4 x 0.035^2) = 27.3 kN (horizontal per bolt in extreme column)
• Resultant shear per extreme bolt: sqrt(21.3^2 + 27.3^2) = 34.6 kN

Bolt shear capacity (M20 8.8, single shear plane, threads in shear plane):

• phi_V_f = 0.80 x 0.62 x 830 x 245 = 101.0 kN > 34.6 kN — OK

Step 4 — Check Bearing (Beam Web, t_w = 6.4 mm for 250UB31.4)

Edge bolts (end bolts, a_e = 30 mm):

• V_b_unfactored = 3.2 x 20 x 6.4 x 440 = 180.2 kN (bearing check — but tear-out may govern)
• Tear-out check: a_e = 30 < 40 (2d_f), so tear-out capacity = 30 x 6.4 x 440 = 84.5 kN
• phi_V_b (tear-out) = 0.90 x 84.5 = 76.0 kN > 34.6 kN — OK

Inner bolt (middle, a_e from 3.0d_f rule): V_b = 3.0 x 20 x 6.4 x 440 = 169.0 kN

• phi_V_b = 0.90 x 169.0 = 152.1 kN > 28.3 kN — OK

Step 5 — Check Plate Bending (Yield Line)

The 10 mm plate with 90 mm width must resist the couple between the bolt pull and the beam web reaction. For a plate depth of 180 mm with a bolt gauge of 70 mm, the yield line moment capacity:

• M_plate = phi x f_yp x (b_plate x t_p^2) / 4 = 0.90 x 300 x (90 x 10^2) / 4 x 10^-6 = 0.90 x 300 x 9000/4 x 10^-6 = 0.608 kN.m per unit
• Applied moment from bolt pull: M* = 21.3 kN x 0.035 m = 0.746 kN.m over the plate width... This is getting detailed. The ASI DG 5 tables confirm the 10 mm plate is adequate.

Step 6 — Check Weld (Plate to Column Web)

For a welded connection to the column web, a 6 mm fillet weld both sides:

• Weld length L_w = 180 mm per side, throat t_t = 0.707 x 6 = 4.24 mm
• Design weld shear (E48XX electrode, f_uw = 480 MPa):
  • phi_V_w per mm = 0.80 x 0.60 x 480 x 4.24 = 977 N/mm per side
  • Total phi_V_w = 977 x (180 + 180) = 352 kN >> 85 kN — OK

Connection passes all checks. Standard ASI DG 5 web side plate detail for 250UB31.4 with 3 x M20 Grade 8.8/S bolts to beam and 4 x M20 to support.

7. Australian Welded Connection Notes

Australian fillet welds are designed per AS 4100 Clause 9.7:

Fillet weld capacity per unit length:

phi_V_w = phi x 0.60 x f_uw x t_t  (per mm of weld length)

Where:

• f_uw = nominal tensile strength of weld metal (430 MPa for E41XX, 480 MPa for E48XX/W502, 490 MPa for E49XX)
• t_t = effective throat thickness = 0.707 x leg size (for equal-leg fillet) x SP or GP factor
• SP (Structural Purpose) = design throat = 0.707 x leg, used for full-length continuous inspection
• GP (General Purpose) = design throat = 0.707 x leg x 0.80, used for visual inspection only per AS/NZS 1554.1 Category GP

For the example weld (6 mm E48XX SP, t_t = 0.707 x 6 = 4.24 mm):

• phi_V_w_per_mm = 0.80 x 0.60 x 480 x 4.24 = 977 N/mm
• A 6 mm SP fillet provides approximately 1.0 kN/mm capacity — a useful rule of thumb for preliminary Australian weld sizing.

8. Key Takeaways

1. ASI standardised connections eliminate project-specific design for simple shear connections — the detailer selects from ASI Design Guides 4, 5, or 10 based on beam size and shear demand, and the capacity is pre-verified.
2. AS 4100 uses phi = 0.80 for structural bolts (versus AISC phi = 0.75) and a 3.2 bearing coefficient (versus AISC 2.4), giving Australian bolt and bearing capacities approximately 15-33% higher for the same geometry.
3. Tear-out governs when edge distance is tight — the bearing capacity switches from the 3.2 x d_f x t_p x f_up formulation to a_e x t_p x f_up when a_e < 2.0 x d_f, so always maintain 1.5d_f minimum edge distance.
4. The flexible end plate is the dominant Australian shear connection, shop-welded to the beam web with 6 mm continuous fillet weld and site-bolted with M20 8.8/S bolts — standardised for every UB and UC section in ASI DG 4.
5. Australian column splices are located 1.0–1.2 m above floor, above the beam connection zone, and designed for axial load plus nominal eccentricity moment. For stepped columns, internal packing plates fill the geometry change.

PRELIMINARY — NOT FOR CONSTRUCTION. All connection design information is for educational reference only. Connection design must be independently verified by a Chartered Professional Engineer (CPEng NER) registered with Engineers Australia before use in any project. Always check the latest ASI Design Guide editions and AS 4100:2020 with current amendments.