Bolt Design Basis — AS 4100 Section 9.3

Bolt capacities per AS 4100:2020 are governed by bolt grade, grip condition, and connection geometry.

Single bolt shear capacity (Clause 9.3.2.1):

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 (bolts, Table 3.4)
• f_uf = bolt minimum tensile strength (MPa)
• n_n = number of shear planes through the thread (0 or 1 for single shear)
• n_x = number of shear planes through the shank (1 or 0 for single shear)
• A_c = tensile stress area (through threads)
• A_o = nominal shank area

For bolts with threads excluded from shear plane: phi_V_f = phi x 0.62 x f_uf x A_o (full shank area) For bolts with threads included: phi_V_f = phi x 0.62 x f_uf x A_c (reduced thread area)

Single bolt tension capacity (Clause 9.3.2.2):

phi_N_tf = phi x A_s x f_uf

Where A_s is the tensile stress area (always through threads for tension).

Bolt bearing capacity (Clause 9.3.2.4):

phi_V_b = phi x 3.2 x d_f x t_p x f_up (for standard holes, deformation permitted)

Where:

• d_f = bolt diameter (mm)
• t_p = minimum connected ply thickness (mm)
• f_up = ply tensile strength (MPa)

Bolt Capacities — M20 Grade 8.8 (Typical Australian Structural Bolt)

M20: d_f = 20 mm, A_o = 314 mm^2, A_c (tensile area) = 245 mm^2. Grade 8.8: f_uf = 830 MPa, f_yf = 660 MPa. AS/NZS 3679.1 Grade 300 plate: f_up = 440 MPa.

Worked Example: Beam End Plate — Eccentric Bolt Group

Problem: A 410UB59.7 beam is connected to a column flange by a 10 mm flush end plate with 4 x M20 Grade 8.8 bolts (2 columns x 2 rows). Factored shear V* = 200 kN (beam reaction). Bolt layout: vertical pitch = 90 mm, gauge = 140 mm. Eccentricity from bolt group centroid to weld face: e = 110 mm (end plate thickness + setback). Determine bolt adequacy.

Step 1 — Bolt group geometry: Bolt coordinates from group centroid:

• Top left: (-70, +45)
• Top right: (+70, +45)
• Bottom left: (-70, -45)
• Bottom right: (+70, -45)

Polar moment of inertia: I_p = sum(x_i^2 + y_i^2) = 4 x (70^2 + 45^2) = 4 x (4900 + 2025) = 4 x 6925 = 27,700 mm^2

Step 2 — Actions on bolt group: Direct shear per bolt (vertical): V*_y = 200 / 4 = 50 kN per bolt Eccentric moment: M* = V* x e = 200 x 0.110 = 22.0 kNm = 22,000 kN-mm

Step 3 — Force in most stressed bolt (instantaneous centre method): The force from eccentric moment is proportional to distance from centroid:

For the top-right bolt (x = 70 mm, y = 45 mm): Distance from centroid: r = sqrt(70^2 + 45^2) = 83.2 mm

F_moment = M* x r / I_p = 22,000 x 83.2 / 27,700 = 66.1 kN (perpendicular to radius)

Components: Moment component horizontal: F_x = F_moment x 45/83.2 = 66.1 x 0.541 = 35.8 kN Moment component vertical: F_y = F_moment x 70/83.2 = 66.1 x 0.841 = 55.6 kN

Resultant on most stressed bolt (top right): F_x_total = 35.8 kN (horizontal, from moment only) F_y_total = 50.0 + 55.6 = 105.6 kN (vertical: direct + moment) F_resultant = sqrt(35.8^2 + 105.6^2) = sqrt(1282 + 11,151) = sqrt(12,433) = 111.5 kN

Step 4 — Bolt capacity check: For M20 Grade 8.8, threads included (conservative for typical end plate): phi_V_f = 100.9 kN

Utilization: 111.5 / 100.9 = 1.11 — FAILS for threads included.

Options: (a) Specify threads excluded from shear plane: phi_V_f = 129.4 kN. Utilization = 111.5/129.4 = 0.86. OK. (b) Increase to 6 bolts (3 columns x 2 rows): larger group, reduced per-bolt force. (c) Increase bolt diameter to M24 Grade 8.8: phi_V_f = 145.8 kN (threads included).

Option (a) — Threads excluded: Add note on fabrication drawing: "BOLT GRIP LENGTH TO BE SUCH THAT THREADS ARE EXCLUDED FROM ALL SHEAR PLANES." This requires the threaded portion to be entirely outside the connected plies. For a 10 mm end plate + 16 mm column flange = 26 mm grip, a standard M20 bolt has approximately 38-40 mm of thread. The bolt must be ordered with a reduced thread length or a washer pack to push the thread out of the shear plane.

Option (b) — 6 bolts (3 x 2, pitch 75 mm, gauge 140 mm): Recalculating: I_p increases, F_resultant ~ 72 kN. phi_V_f = 100.9 kN. OK (0.71 utilization). Requires wider end plate (240 mm vs 200 mm) and taller (at minimum 75 x 2 + 2 x 35 = 220 mm).

Preferred design for production simplicity: M20 Grade 8.8, threads excluded, 4-bolt group. Verify thread exclusion with fabricator and include note on drawing.

Ply Bearing Check (Clause 9.3.2.4)

End plate (10 mm, Grade 300, f_up = 440 MPa): phi_V_b = 0.80 x 3.2 x 20 x 10 x 440 / 1000 = 225.3 kN >> 111.5 kN. OK.

Column flange (16 mm, typically Grade 300): phi_V_b = 0.80 x 3.2 x 20 x 16 x 440 / 1000 = 360.5 kN >> 111.5 kN. OK.

Bearing is not critical — bolt shear governs.

Combined Shear and Tension — Moment End Plate

For a moment end plate where the top bolts carry tension from the moment while also resisting shear:

Interaction (Clause 9.3.2.3): (V*/phi_V_f)^2 + (N_tf*/phi_N_tf)^2 <= 1.0

This elliptical interaction is identical in form to CSA S16 and AISC 360.

For a typical moment connection: top bolt N_tf* = 120 kN, V* = 35 kN (from beam reaction distributed to 4 bolts). M20 Grade 8.8, threads excluded:

(35/129.4)^2 + (120/162.7)^2 = 0.073 + 0.544 = 0.617 <= 1.0. OK.

Minimum Edge Distance and Spacing — AS 4100 Clause 9.6

For M20 bolts: minimum edge = 30 mm (sheared), typical = 35 mm; minimum pitch = 50 mm, typical = 60-70 mm.

Frequently Asked Questions

What is the instantaneous centre method, and when must I use it? The instantaneous centre (IC) method accounts for the non-linear load-deformation behaviour of bolts in a group under eccentric load. AS 4100 permits the elastic method (as shown in the worked example) for preliminary design. The IC method, which recognizes that the outer bolts yield first and redistribute load, provides 10-20% higher capacity than the elastic method. Use the ASI Bolted Connection Design Tables for production design — they incorporate the IC method.

Grade 8.8 vs Grade 10.9 — which should I specify? Grade 8.8 is the standard structural bolt in Australia (equivalent to ASTM A325). Grade 10.9 (equivalent to ASTM A490) provides approximately 30% higher capacity but is more brittle, costs 25-40% more, and is not recommended for galvanizing. Specify Grade 8.8 unless bolt count is excessive, then consider Grade 10.9 for selected connections.

Should I use snug-tight or fully tensioned bolts? For bearing-type connections in static building frames, snug-tight (tightened with a standard spanner to the "snug" condition — full effort of a worker on a standard podger spanner) is acceptable per AS 4100 Clause 15.2.5.2. Fully tensioned (turn-of-nut method, Clause 15.2.5.3) is required for: (a) slip-critical connections, (b) fatigue loading, (c) connections with significant load reversal, (d) where pretension is needed to prevent separation under load.

This page is for educational reference. Bolt design per AS 4100:2020 Section 9.3. Verify bolt capacities against current ASI Bolted Connection Design Tables. All structural designs must be independently verified by a licensed Professional Engineer or Structural Engineer. Results are PRELIMINARY — NOT FOR CONSTRUCTION.