AS 4100 Connection Design — Bolted & Welded Steel Connection Guide

Complete reference for AS 4100:2020 Clause 9 bolted and welded steel connection design. Covers bolt shear and tension capacity (Clause 9.3), fillet and groove weld design (Clause 9.7), bolt group analysis using elastic and instantaneous centre of rotation methods, weld group analysis, block shear (Clause 9.2.3), bearing and tear-out at bolt holes, and prying action in tension connections. Includes a worked bolt group example with combined shear and moment.

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AS 4100 Connection Design — Overview

Steel connection design under AS 4100:2020 follows limit state principles, with Clause 9 governing connections and Clause 15.3 governing bolt hole detailing. Design strength is determined using the factored resistance approach with capacity factors (phi) specified in Table 3.4. Connection design in Australian practice follows the detailed methodology in the AISC (Australian Institute of Steel Construction) Design Capacity Tables (DCT) and the SMAC (Safery of Steel Structures) structural design aids.

The governing capacity factors for connection design per AS 4100 Table 3.4 are:

AS 4100 phi = 0.80 for bolts is 7% less conservative than AISC 360 phi = 0.75, meaning Australian-designed bolts carry 6.7% more factored load per bolt all else equal. However, Australian bolts are typically Grade 8.8 (ISO 898-1), not SAE J429 Grade 5 or A325, so direct comparison requires matching strength grades.

Bolt Shear Capacity (AS 4100 Clause 9.3.2)

The design shear capacity of a single bolt in a connection is:

phi-Vfn = phi × 0.62 × fuf × (kr × kd × ks) × Ac * or Ao

where:
  phi   = 0.80 (capacity factor for bolts)
  fuf   = minimum tensile strength of bolt steel (MPa)
  Ac    = cross-sectional area at threads (tension area) (mm²)
  Ao    = plain shank area (mm²) — used when threads excluded from shear plane
  kr    = reduction factor for multiple bolt lengths (Cl. 15.3.2)
  kd    = reduction factor for oversize holes (Cl. 15.3.3)
  ks    = reduction factor for slotted holes (Cl. 15.3.4)

The 0.62 factor converts tensile strength to shear strength and accounts for the combined stress state in the bolt. For typical connections where threads are intercepted by the shear plane, Ac is the reduced area. This is comparable to the AISC "threads included" case and the CSA S16 phin = 0.70 factor.

Bolt Shear Values — Grade 8.8

Grade 8.8 bolts (fuf = 830 MPa) are the most common structural bolts in Australia. ISO metric coarse thread series applies.

Bolt Tension Capacity (AS 4100 Clause 9.3.3)

The design tension capacity of a single bolt is:

phi-Ntf = phi × 0.80 × fuf × Ac

where:
  phi   = 0.80
  0.80  = factor converting Fu to bolt tension limit
  fuf   = minimum tensile strength (MPa)
  Ac    = tensile stress area at threads (mm²)

For M20 Grade 8.8: phi-Ntf = 0.80 × 0.80 × 830 × 245 = 130.1 kN per bolt.

Combined Shear and Tension (AS 4100 Clause 9.3.4)

For bolts subject to combined shear and tension:

(Vf* / phi-Vfn)² + (Ntf* / phi-Ntf)² ≤ 1.0

This is a circular interaction curve, more generous than the AISC linear interaction. At Vf* = 0.5 × phi-Vfn, the allowable tension is Ntf* = 0.866 × phi-Ntf — about 15% more than the AISC linear rule allows.

Bolt Group Analysis — Elastic Method

For eccentric bolt groups, AS 4100 permits both the elastic (vector) method and the instantaneous centre of rotation (IC) method. The elastic method treats the bolt group as a rigid body and superposes direct shear and torsional components:

Vi = V / n                    (direct shear, equal per bolt)
Mi = M × ri / J                (torsional shear at bolt i)
Vri = √(Vi² + Mi² + 2×Vi×Mi×cos-theta-i)

Where J = sum(ri²) is the polar moment of inertia of the bolt group. The maximum Vri must not exceed phi-Vfn. This method is conservative for ductile Grade 8.8 bolts — actual capacity is typically 10-30% higher than the elastic method predicts.

Instantaneous Centre of Rotation Method

The IC method accounts for the deformation capacity of individual bolts:

• Each bolt carries load proportional to its deformation (compatible with the rotation about the IC)
• Bolt deformation capacity follows the Fisher-Sloane exponential decay model
• The IC location is found iteratively such that equilibrium is satisfied

AS 4100 Clause 9.3.2.1 permits the IC method, and it is detailed in the AISC bolted connection design guide. For bolt groups with significant eccentricity (e > 300 mm for typical groups), the IC method can provide 25-40% more capacity than the elastic method.

Weld Group Analysis (AS 4100 Clause 9.7.3.10)

AS 4100 specifies two methods for fillet weld design: the directional method (force-based) and the simple method (conservative). Both methods use the weld throat thickness as the design section.

Fillet Weld Capacity — Directional Method

The directional method resolves weld forces into longitudinal (parallel to weld axis) and transverse (normal to weld axis) components:

Longitudinal shear:   phi-vw = phi × 0.60 × fuw × tt
Transverse tension:   phi-vw = phi × 0.75 × fuw × tt (increased for transverse loading)

where:
  phi  = 0.80 (capacity factor for welds)
  fuw  = nominal tensile strength of weld metal (MPa)
  tt   = weld throat thickness = 0.707 × s for equal leg fillet (mm)
  s    = weld leg length (mm)

The transverse strength enhancement factor of 1.25 (0.75 / 0.60) accounts for the triaxial confinement effect in transversely loaded fillet welds — the same principle as the AISC 360 directional strength increase.

Fillet Weld Capacity — Simple Method

The simple method uses a single uniform design capacity per unit length regardless of orientation:

phi-vw = phi × 0.60 × fuw × tt × kr

where kr = 1.0 for weld run lengths up to 170 × tt

The simple method is 20% more conservative than the directional method for transverse welds but eliminates the need for force resolution. It is preferred for preliminary design and for connections where the load direction on individual weld segments is not clearly defined.

Weld Group — Torsional Method

For eccentric weld groups, the elastic method superposes direct shear and torsional components resolved at the weld centroid:

fv = V / Lw                        (direct shear stress)
ft = M × r / Jw                     (torsional stress at point i)
fr = √(fv² + ft² + 2×fv×ft×cos-theta-i)

Where Jw = sum(r² × delta-li) is the weld group polar moment of inertia. The design throat thickness tt-req = fr / (phi × 0.60 × fuw). This method is standard in Australian practice and is the basis for weld design tables in the AISC Design Capacity Tables.

Block Shear (AS 4100 Clause 9.2.3)

Block shear is a limit state where a block of material tears out through bolt holes along a critical path combining shear planes and a tension plane:

phi-Vb = phi × min(0.60 × fui × Ant + 0.60 × fyi × Agv,
                    0.60 × fui × Anv + fui × Ant)

where:
  phi  = 0.75 for block shear
  fui  = minimum tensile strength of connected ply (MPa)
  fyi  = yield strength of connected ply (MPa)
  Ant  = net area in tension (mm²)
  Anv  = net area in shear (mm²)
  Agv  = gross area in shear (mm²)

The two equations represent the two possible failure modes: (a) tension rupture + shear yield, and (b) shear rupture + tension rupture. AS 4100 phi = 0.75 for block shear matches AISC phi = 0.75 (AISC uses phi-BS = 0.75 for both LRFD and ASD combined).

Bearing and Tear-Out (AS 4100 Clause 9.3.2.4)

Bearing capacity at bolt holes is the product of bearing stress, bolt diameter, and ply thickness:

phi-Vb = phi × 3.2 × fup × d × tp

where:
  phi  = 0.80
  fup  = tensile strength of connected ply (MPa)
  d    = bolt diameter (mm)
  tp   = ply thickness (mm)

The 3.2 factor corresponds to a bearing stress limit of 3.2 × fup, approximately equal to 3 × fup at serviceability with edge distance effects. For bolts near the end of the member, tear-out capacity is governed by:

phi-Vb = phi × fup × tp × a_min

where:
  a_min = minimum of edge distance ae or bolt spacing/2 (mm)

For standard bolt holes with 1.5d end distance and 2.5d spacing, bearing governs over tear-out for most Australian sections.

Prying Action (AS 4100 Clause 9.3.3.1)

Prying action is the amplification of bolt tension caused by flexural deformation of the connected parts (plate or angle). When a tension bolt is located on a plate that can deform, the resulting lever action increases the force in the bolt above the applied direct tension.

The AS 4100 T-stub model calculates:

Ntf* = applied tension per bolt including prying
phi-Ntf = phi × 0.80 × fuf × Ac  (must be ≥ Ntf*)

The prying ratio Q = Ntf* / Nt-applied depends on:

• Plate thickness tp (thicker plates reduce prying)
• Bolt gauge g (distance between bolt lines)
• Flange overhang b (beyond bolt centreline)
• T-stub length p (per bolt)
• Yield strength fyi of the connected element

When Q > 1.0, the bolt force exceeds the applied tension. The design must either thicken the plate (reducing deformation) or increase the bolt size to accommodate the amplified force. Australian practice typically uses 16-25 mm end plates for moment connections to limit prying effects.

Worked Example — Bolt Group in Combined Shear and Moment

Problem: Design a bolted bracket connection for a 200UC52 column supporting a 300PFC beam stub. The connection carries a factored shear of Vf* = 200 kN and a factored moment of Mf* = 40 kN-m at the bolt group centroid.

Design Parameters:

• Bolts: M20 Grade 8.8, threads included in shear plane
• Connected plies: 12 mm bracket plate, Grade 300 (fyp = 300 MPa, fup = 440 MPa)
• Bolt spacing: 70 mm vertically, 70 mm horizontally
• Edge distance: 35 mm (1.75 × d, minimum 1.5d OK)
• Bolt group: 2 columns × 4 rows = 8 bolts

Step 1 — Bolt Shear Capacity: For M20 Grade 8.8 (fuf = 830 MPa): Ac = 245 mm² phi-Vfn = 0.80 × 0.62 × 830 × 245 / 1000 = 100.7 kN per bolt

Step 2 — Elastic Bolt Group Analysis: Vertical bolt spacing: sv = 70 mm (4 rows → 3 gaps) Bolt group centroid: at centre row

Moment of inertia of bolt group: Ixx = 4 × (105² + 35² + 35² + 105²) = 4 × (11025 + 1225 + 1225 + 11025) = 4 × 24500 = 98,000 mm² Iyy = 8 × 35² = 8 × 1225 = 9,800 mm² J = Ixx + Iyy = 107,800 mm²

Distance to farthest bolt: r = √(105² + 35²) = √(11025 + 1225) = √12250 = 110.7 mm

Step 3 — Force Components on Farthest Bolt: Direct shear per bolt: V/n = 200 / 8 = 25.0 kN (downward) Torsional shear: M × r / J = 40 × 10⁶ × 110.7 / 107,800 = 41,080 N = 41.1 kN

Angle between direct shear and torsional shear (from geometry): theta = atan(105/35) = 71.6° — but torsional shear acts perpendicular to r

Resolving at farthest bolt: Direct shear (vertical): 25.0 kN ↓ Torsional shear (tangent to r): 41.1 kN — at 71.6° from vertical

Resultant: Vr = √(25.0² + 41.1² + 2 × 25.0 × 41.1 × cos 71.6°) Vr = √(625 + 1689 + 2 × 25.0 × 41.1 × 0.316) Vr = √(625 + 1689 + 649) = √2963 = 54.4 kN

Step 4 — Check: Vr = 54.4 kN ≤ phi-Vfn = 100.7 kN → OK (54% utilisation)

Step 5 — Bearing Check: Bearing on 12 mm Grade 300 bracket plate: phi-Vb = 0.80 × 3.2 × 440 × 20 × 12 / 1000 = 270.3 kN per bolt OK — bearing does not govern.

Step 6 — Edge Distance Check: End distance = 35 mm Tear-out: phi-Vb = 0.80 × 440 × 12 × 35 / 1000 = 147.8 kN per bolt OK — tear-out does not govern.

Result: 8-M20 Grade 8.8 bolts in 4 × 2 configuration is adequate. Connection utilisation is 54% in shear, 54% in tear-out. Could reduce to 6 bolts (3 rows × 2 columns) for 72% utilisation.

Connection Design in Australia — Practical References

Australian steel connection design is supported by:

• AISC Design Capacity Tables (DCT) — tabulated bolt group capacities for standard configurations
• AISC Connection Design Guide — detailed design procedures for common connection types
• SMAC Design Aids — structural steel connection capacity tables
• InfraBuild Section Book — section properties for all Australian UB, UC, PFC, and hollow sections
• AS 4100 Handbook (SA HB 105) — commentary and design examples published by Standards Australia

For quick connection design, the Steel Calculator Connection Design Tool provides automated bolt and weld group analysis with AS 4100 capacity factors.

Educational reference only. Verify against AS 4100 and relevant standards. Results are PRELIMINARY — NOT FOR CONSTRUCTION.