Australian Structural Steel Connection Design — AS 4100 Clause 9

Comprehensive guide to the design of bolted and welded connections in structural steel per AS 4100:2020 Clause 9. Covers connection classification by stiffness and strength, bolt group design (shear, bearing, tension, combined shear and tension), weld group design (fillet and butt welds), block shear, and the design of typical Australian connections including end plates, shear tabs, and base plates.

Quick access: Bolt capacity tables | Bolt spacing requirements | Weld symbols (AS 1101.3) | Shear tab design | Bolted connection calculator

Connection Classification — Clause 9.1

AS 4100:2020 Clause 9.1 requires connections to be classified by both stiffness and strength before commencing design.

By Stiffness

The connection stiffness classification affects the global analysis model. A connection classified as rigid in the analysis must be designed and detailed to achieve the assumed rotational restraint. A connection analysed as simple must be detailed with sufficient rotational capacity to avoid unintended moment transfer.

By Strength

The connection strength should be not less than the forces from the structural analysis. For simple connections, AS 4100 requires the connection to be designed for a minimum of 40 kN or the calculated shear force, whichever is greater. Moment-resisting connections must be designed for the calculated moments from the analysis with appropriate overstrength factors.

Bolted Connection Design — Clause 9.3

Bolt Shear Capacity — Clause 9.3.2.1

The design shear capacity of a single bolt depends on whether the shear plane passes through the threaded or unthreaded portion of the bolt:

For threads included in the shear plane (most common case):

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

where:

• phi = 0.80 (capacity factor for bolts in shear)
• f_uf = minimum tensile strength of the bolt (830 MPa for Grade 8.8, 1040 MPa for Grade 10.9)
• n_n = number of shear planes through the unthreaded shank
• n_x = number of shear planes through the threaded portion
• A_c = core area (minor diameter area) of the bolt
• A_o = nominal plain shank area of the bolt

For M20 Grade 8.8 bolt, single shear, threads in shear plane: A_o = pi x 20^2 / 4 = 314 mm^2 A_c approx. 245 mm^2 phi V_f = 0.80 x 0.62 x 830 x 245 x 10^(-3) = 100.8 kN

Bolt Tension Capacity — Clause 9.3.2.2

phi N_tf = phi x f_uf x A_s

where A_s is the tensile stress area per AS 1275.

For M20 Grade 8.8: A_s = 245 mm^2, phi N_tf = 0.80 x 830 x 245 x 10^(-3) = 162.7 kN.

Combined Shear and Tension — Clause 9.3.2.3

When a bolt is subjected to simultaneous shear and tension:

(V* / phi V_f)^2 + (N_tf* / phi N_tf)^2 <= 1.0

This circular interaction formula is used for bolts in connections such as rigid end plates subjected to combined moment and shear, and hanger connections where the bolt carries both the vertical reaction and horizontal thrust.

Ply Bearing and Tearout — Clause 9.3.2.4

phi V_b = phi x 3.2 x d_f x t_p x f_up

(Refer to the dedicated bearing and tearout reference page for comprehensive treatment.)

Bolt Pretension and Slip Resistance — Clause 9.3.8

For slip-critical connections (TF category), the slip resistance per bolt is:

phi V_sf = phi x mu x n_e x P_t x k_h

where:

• phi = 0.70 (slip-critical)
• mu = slip factor (0.35 clean mill scale, 0.50 blast-cleaned)
• n_e = number of effective faying surfaces
• P_t = minimum bolt pretension
• k_h = hole factor (1.00 for standard holes, 0.85 for oversized/slotted holes)

Weld Design — Clause 9.7

Fillet Weld Capacity — Clause 9.7.3

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

phi V_w = phi x 0.6 x f_uw x t_t x k_r

where:

• phi = 0.80 (structural purpose, SP category), 0.60 (general purpose, GP category)
• f_uw = nominal tensile strength of the weld metal (typically 480 MPa for E48XX electrodes matching Grade 300)
• t_t = design throat thickness = t_w / sqrt(2) for equal-leg fillet
• k_r = reduction factor for lap length

For E48XX electrode (f_uw = 480 MPa).

Butt Weld Capacity — Clause 9.7.2

Full-penetration butt welds in tension, where the weld is reinforced and ground flush, may be designed for the full strength of the connected plate. No weld capacity check is required -- the plate strength governs.

Partial-penetration butt welds are designed using the throat thickness of the prepared groove, and the capacity is calculated using the same formula as for fillet welds but with the prepared throat dimension instead of the 45-degree throat.

Block Shear — Clause 9.1.4

Block shear is a limit state combining tensile fracture on one plane and shear yielding (or fracture) on a perpendicular plane at the end of a tension member or at a bolt group. The design block shear capacity:

For the case with shear yielding and tension fracture:

phi R_bs = phi x (0.6 x f_y x A_vg + f_u x A_nt) <= phi x (0.6 x f_u x A_vn + f_u x A_nt)

Worked Example: End Plate Connection Design

Problem: A 410UB59.7 Grade 300 beam is connected to a 250UC89.5 column flange using a flush end plate connection. The beam end reaction (factored) is V* = 180 kN (shear only, simple connection). Design the end plate, bolts, and welds. Use M20 Grade 8.8 bolts, 10 mm Grade 300 end plate, and 8 mm fillet welds (E48XX electrode, SP category).

Solution:

Step 1: Number of bolts from shear capacity

Design shear per bolt: phi V_f = 0.80 x 0.62 x 830 x 245 x 10^(-3) = 100.8 kN (threads in shear plane, single shear).

Number of bolts required: n_b >= 180 / 100.8 = 1.79. Use 4 bolts (two columns of two rows). Conservative design, provides redundancy.

Step 2: Bearing check on end plate (10 mm Grade 300)

phi V_b = 0.90 x 3.2 x 20 x 10 x 440 x 10^(-3) = 253.4 kN per bolt. Bearing is not critical (far exceeds bolt shear).

Step 3: Bolt spacing and edge distances

Minimum pitch: 2.5 x 20 = 50 mm. Use 70 mm. Minimum gauge: 50 mm. Use 80 mm (fits within column flange width of 256 mm). End distance: 1.5 x 22 = 33 mm. Use 40 mm.

Step 4: End plate dimensions

Width: 180 mm (approximately beam flange width + 2 x edge distance for fillet weld access). Depth: 40 + 70 + 40 = 150 mm below beam soffit. OK for typical end plate.

Step 5: Weld design — beam web to end plate

Shear per unit length on web fillet weld (load shared over web depth = 410 - 2 x 12.8 = 384.4 mm):

v_w* = V* / (2 x d_w) = 180,000 / (2 x 384.4) = 234 N/mm = 0.234 kN/mm = 234 kN/m.

8 mm fillet weld capacity (SP): phi V_w = 1.087 kN/m x (8/6) ... wait, from the table above for 8 mm: 1.087 kN/m = 1087 N/mm.

Actually, recalculating: for 8 mm fillet, t_t = 8/sqrt(2) = 5.66 mm. phi V_w = 0.80 x 0.6 x 480 x 5.66 = 1304 N/mm = 1.304 kN/mm = 1304 kN/m for SP category.

Demand: 234 kN/m << 1304 kN/m. 8 mm fillet weld is adequate with large reserve.

Step 6: Block shear check on beam web

Block shear path: vertical shear through bolt holes (A_vg includes web area between top of end plate to beam soffit), tension fracture across gauge distance between bolt columns. The check is typically not critical for standard end plate connections where the end plate is thicker than the beam web.

Result: 4 M20 Grade 8.8 bolts in 10 mm Grade 300 end plate with 8 mm fillet welds. Connection capacity governed by bolt shear (403 kN total > 180 kN demand). Design is adequate.

Frequently Asked Questions

What is the difference between a rigid and a simple connection per AS 4100?

A rigid connection transfers significant bending moment between connected members with negligible rotation (< 0.005 rad at service load). Examples include full-strength welded moment connections and extended end plates with bolts in the tension zone. A simple (pinned) connection transfers shear and axial force but negligible moment, with significant rotational capacity (> 0.020 rad at service load). Examples include shear tabs, fin plates, and web side plates. The connection classification affects the global analysis model and must be consistent with the detailing.

When is block shear the governing limit state in a bolted connection?

Block shear governs when the bolt group is close to the end of a member or when the gauge distance (transverse bolt spacing) is small relative to the bolt diameter. It is most critical in tension member end connections where the bolt group is in a coped (notched) beam web, in angle legs with narrow connected legs, and in gusset plates with closely spaced bolts. Block shear typically controls over bearing when a_nt (net area in tension) is small relative to A_vg (gross area in shear).

What is the capacity factor phi for different connection limit states per AS 4100?

Per AS 4100 Table 3.4: phi = 0.80 for bolts in shear and tension; phi = 0.90 for ply bearing and tearout; phi = 0.80 for fillet welds (SP category, structural purpose); phi = 0.70 for slip-critical connections; phi = 0.90 for block shear (as a plate yielding/fracture limit state); phi = 0.80 for butt welds (SP category). The different phi factors reflect different levels of uncertainty in the capacity prediction and the consequences of failure for each limit state.

How are bolt groups designed for eccentric loading per AS 4100?

For bolt groups subjected to shear with eccentricity, the elastic method (vector analysis) calculates the bolt force as the vector sum of the direct shear (V*/n, distributed equally) and the torsional component from the moment (V* x e, distributed proportionally to the distance from the bolt group centroid). AS 4100 also permits the instantaneous centre of rotation method, which accounts for the non-linear load-deformation behaviour of bolts and provides a less conservative capacity for eccentrically loaded bolt groups. The elastic method is used for routine design due to its simplicity; the IC method is used for optimisation.

Are standard AS 4100 connections adequate for seismic design?

For structures in seismic regions, connections must satisfy AS 4100 Clause 9 requirements and the additional ductility and capacity design provisions of AS 1170.4 (Earthquake actions in Australia) and NZS 1170.5 where applicable. Moment-resisting connections in seismic frames must be designed for the overstrength capacity of the connected beam (phi_o x phi M_s) rather than the analysis forces. Simple connections must have sufficient rotational ductility to accommodate the inter-storey drift without fracture. The bolt and weld capacity factors are not modified for seismic, but the design actions include the seismic load combinations per AS 1170.0.

Educational reference only. All design values must be verified against the current edition of AS 4100:2020 and the project specification. This information does not constitute professional engineering advice. Always consult a qualified structural engineer for design decisions.