Bolt Grades — AS 4100 / AS 1252

Australian structural bolts per AS 1252.1 (high-strength) and AS 1252.2 (commercial):

Grade 8.8/S is the default for Australian building structures. A325M is common in mining and heavy industrial.

Bolt Shear Capacity — AS 4100 Clause 9.3.2.1

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

Where n_n = shear planes through threads, n_x = shear planes through shank, A_c = minor diameter area (approx 0.75 times A_o), phi = 0.80.

The 30% reduction for threads in the shear plane is significant — always confirm whether the thread is excluded from the shear plane.

Bolt Tension Capacity — AS 4100 Clause 9.3.2.2

phi times N_tf = phi times A_s times f_uf

Where A_s is the tensile stress area. Note: AS 4100 does NOT apply the 0.75 reduction factor used in AISC 360 and CSA S16 for bolt tension. phi = 0.80.

Ply Bearing Capacity — AS 4100 Clause 9.3.2.4

phi times V_b = phi times 3.2 times d_f times t_p times f_up

For a 10 mm S300 ply (f_u = 440 MPa) with M20 bolts: phi times V_b = 0.80 times 3.2 times 20 times 10 times 440 / 1000 = 225.3 kN per bolt. For edge bolts (e < 2d_f), a reduced bearing capacity applies with a_e as the minimum edge distance.

Minimum Edge Distance — AS 4100 Clause 9.6

Combined Shear and Tension — AS 4100 Clause 9.3.2.3

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

Example: M20 8.8/S bolt with V_f* = 50 kN and N_tf* = 80 kN simultaneously: (50/79.0)^2 + (80/162.7)^2 = 0.400 + 0.242 = 0.642 <= 1.0. OK.

Bolt Group Design with Eccentricity

When a bolt group resists an eccentric shear force, the elastic method (polar moment of inertia) is the standard approach per ASI Design Guide 1.

Direct shear per bolt: V_direct = V* / n. Torsional shear per bolt: V_torsion = V* times e times r_i / (sum r_i^2). Resultant: V_resultant = sqrt(V_direct^2 + V_torsion^2 + 2 times V_direct times V_torsion times cos theta).

For eccentricity > 50 mm (M20 bolts), the torsional component can be 2-3 times the direct shear. The instantaneous centre of rotation method (ultimate strength) may produce more economical results for large eccentricities.

Worked Example 1 — Beam Web Splice

Problem: Design bolted web splice for 410UB54 beam (S300, t_w = 7.7 mm, f_u = 440 MPa). V* = 180 kN. M20 Grade 8.8/S bolts, 22 mm holes.

Step 1 — Layout: Splice plate 250x150x8 mm each side. 2 columns times 3 rows M20 at 70 mm pitch, 80 mm gauge.

Step 2 — Shear: Double shear through threads: V_f per bolt = 2 times 79.0 = 158.0 kN. Group: 6 times 158.0 = 948 kN > 180 kN. OK.

Step 3 — Bearing: phi V_b = 0.80 times 3.2 times 20 times 7.7 times 440 / 1000 = 173.5 kN per bolt. Group: 1041 kN. OK.

Step 4 — Edge distance: e1 = 40 mm >= 1.5 times 22 = 33 mm. OK.

Selected: 2-250x150x8 splice plates S300, 6-M20 Grade 8.8/S bolts.

Worked Example 2 — Moment End Plate (Combined Shear + Tension)

Problem: 310UB40.4 beam to 310UC118 column, flush end plate. V* = 120 kN, M* = 85 kN.m. End plate 200x310x12 mm S300. 4-M24 Grade 8.8/S bolts, 180 mm gauge, lever arm 250 mm.

Step 1 — Bolt Tension: C = T = M* / lever_arm = 85/0.25 = 340 kN (per row, 2 bolts). N_tf* per bolt = 170 kN. N_tf (M24, 8.8/S) = 234.4 kN. OK (73% for tension alone).

Step 2 — Bolt Shear: V* per bolt = 120/4 = 30 kN. V_f (M24 threaded) = 113.8 kN. OK.

Step 3 — Combined: (30/113.8)^2 + (170/234.4)^2 = 0.069 + 0.527 = 0.596 <= 1.0. OK.

Step 4 — End Plate Bending (Yield Line): Required plate thickness t_req = sqrt(4 times 170,000 times 40 / (0.90 times 200 times 300)) = 22.4 mm. 12 mm is inadequate.

Revised with 25 mm end plate: t_provided = 25 mm > 22.4 mm. OK.

Selected: 200x310x25 mm flush end plate S300, 4-M24 Grade 8.8/S bolts. 6 mm FW beam flanges to end plate. Check column flange for prying per AS 4100 Clause 9.3.4.

Bolt Tightening Methods — AS 4100 Clause 15.3

For standard moment end plates with combined shear and tension, tensioned bolts are the minimum requirement.

FAQ

What is the difference between Grade 8.8/S and A325M bolts in Australian practice? Both have f_uf = 830 MPa and identical AS 4100 capacities. 8.8/S is manufactured to AS 1252.1 (Australian standard); A325M to ASTM A325M. 8.8/S is more readily available from Australian suppliers; A325M is common for mining and heavy industrial projects requiring ASTM compliance.

Does AS 4100 require a reduction for long joints? Yes. Clause 9.3.2.5 requires a 0.85 reduction factor for joints where the distance between first and last bolt exceeds 500 mm. For splice joints longer than 15 bolt diameters (300 mm for M20), the reduction applies automatically.

What is the minimum bolt pitch and gauge per AS 4100? Minimum pitch (along force) = 2.5 times d_f (50 mm for M20). Minimum gauge (perpendicular) = 2.5 times d_f. Maximum pitch for corrosion sealing = 12 times t (ply thickness) or 200 mm, whichever is less.

When is prying action considered in Australian bolted connections? Prying must be considered when bolts are in tension and the connected plate deforms, amplifying the bolt force. AS 4100 Clause 9.3.4 requires prying checks unless the plate thickness exceeds the threshold. ASI Design Guide 4 provides comprehensive prying design methodology for end plate connections.

How do I design bolts for a column base plate with moment? Column base plates with moment produce tension in the anchor bolts on one side. Bolt tension = M* / lever_arm - N*/n. Combined shear and tension interaction applies. Anchorage embedment in the concrete foundation must also be checked per AS 3600 — bolt tension capacity alone is insufficient.

Are there any special requirements for galvanised bolts in Australian practice? Galvanised bolts to AS 1214 must have the thread undersized or the nut overtapped to accommodate the zinc coating thickness. The capacity reduction for galvanising is negligible (< 2%) per AS 4100. Specify "hot-dip galvanised after manufacture" — do not use electroplated bolts for structural applications per AS 4100 Clause 15.2.1.

Bolt Hole Types and Clearances — AS 4100 Clause 9.6.2

Australian practice uses three bolt hole types depending on the connection requirements:

Standard round holes (M20 = 22 mm diameter) are used for the vast majority of Australian bolted connections. Oversized holes are permitted only in base plates and connections where slip is acceptable — they must use plate washers or structural washers per AS 4100 Clause 9.6.2.2.

Washer Requirements

For galvanised bolts, hardened washers (HRC 35-45) are required under the turned element (nut or bolt head) to prevent galling of the zinc coating during tightening.

Tension Capacity Comparison: AS 4100 vs AISC 360 vs CSA S16

AS 4100 uses a different approach to bolt tension capacity compared to North American codes:

For an M24 Grade 8.8/S bolt:

• AS 4100: N_tf = 0.80 times 353 times 830 / 1000 = 234.4 kN
• AISC 360: N_tf = 0.75 times (0.75 times 830) times 452 / 1000 = 211.1 kN (using A_b = 452 mm^2, not A_s)
• CSA S16: N_tf = 0.75 times 0.80 times 353 times 830 / 1000 = 175.8 kN (using A_n = 353 mm^2)

AS 4100 produces the highest tension capacity for identical bolts due to the absence of the 0.75 reduction factor and the use of the full tensile stress area. This is deliberate — AS 4100 is calibrated to Australian manufacturing quality control, which demonstrates lower variability in bolt material properties than the international dataset that underpins the 0.75 factor. Australian engineers should NOT apply the AISC 0.75 factor when designing to AS 4100.

Related Pages

• AS 4100 Connection Design — Bolted & Welded Guide
• AS 4100 Moment Connection — Bolted End Plate
• AS 4100 Beam Design — Flexure & LTB
• AS 4100 Column Design — Worked Example
• Australian Bolt Grades — AS 1252 Specification
• AS 4100 Bolt Hole Sizes — Clearance & Tolerances
• AS 4100 Load Combinations — AS 1170 ULS
• AS 4100 Beam Sizes — Australian Sections Chart
• AS 4100 Weld Capacity — Fillet Weld Design
• AS 4100 Base Plate Design — Worked Example
• Bolted Connection Calculator — Free Tool

Educational reference only. Bolt capacity per AS 4100-2020. Verify against current Australian Standards and ASI Design Guides. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent CPEng verification.