Australian Bolt Specification Guide — AS 1252 and AS 4100

Definitive reference for structural bolting in Australia per AS/NZS 1252 and AS 4100:2020. Covers the three permitted bolt grades (4.6/S, 8.8/S, 10.9/S) with complete mechanical properties, metric coarse thread specifications, bolt and nut marking requirements, design capacities for shear and tension, tightening methods, and selection guidance. All values in metric units.

Related pages: Bolt Grade Selector | Bolt Torque Chart | Bolt Group Capacity | Bolt Bearing & Tearout | Bolted Connection Calculator

AS/NZS 1252 Bolt Grade System — Complete Specification

AS/NZS 1252:2016 specifies the requirements for high-strength steel bolts, nuts, and washers for structural engineering. AS 4100:2020 Clause 9.2.1 recognises three bolt grades for structural connections:

Grade Designation Decoding

The bolt grade number follows the ISO 898-1 system adopted by AS/NZS 1252:

8.8/S decodes as:

• First digit '8' = f_uf / 100 = 800 MPa class (actual: 830 MPa minimum)
• Second digit '0.8' = f_yf / f_uf = 0.8 (actual: 660 / 830 = 0.795, slightly conservative rounding)
• '/S' suffix = structural, Charpy tested

So 8.8/S means: tensile strength 800 MPa class, yield ratio 0.8, structural grade with Charpy testing.

10.9/S decodes as:

• First digit '10' = f_uf / 100 = 1,000 MPa class (actual: 1,040 MPa minimum)
• Second digit '0.9' = f_yf / f_uf = 0.9 (actual: 940 / 1040 = 0.904)
• '/S' = structural grade

Critical distinction: Grade 8.8 without /S is a commercial bolt to AS 1111 — it has the same mechanical properties BUT does not have the Charpy impact requirement, the full traceability, or the mandatory test certification. Grade 8.8 (non-S) bolts MUST NOT be substituted for Grade 8.8/S bolts in structural connections.

Complete Shear Capacity Table — All Grades, All Sizes

Design shear capacity per AS 4100 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 for bolts in shear.

Single Shear, Threads in Shear Plane (n_x = 1, n_n = 0)

This is the most common condition — the threaded portion of the bolt intersects the shear plane. Design shear capacity = phi x 0.62 x f_uf x A_o:

Single Shear, Threads Excluded (Body in Shear Plane, n_n = 1, n_x = 0)

Design shear capacity = phi x 0.62 x f_uf x A_c (where A_c = pi x d^2 / 4, larger than A_o):

Complete Tension Capacity Table — All Grades, All Sizes

Design tension capacity per AS 4100 Clause 9.3.2.2: phi N_tf = phi x A_o x f_uf

Where phi = 0.80 for bolts in tension (same phi as shear).

Metric Coarse Thread — Complete Dimensional Data per AS 1275

Australian structural bolts use ISO metric coarse pitch threads. The thread dimensions are governed by AS 1275 (equivalent to ISO 724):

The tensile stress area A_o is the standard metric calculation: A_o = (pi/4) x [(d2 + d3) / 2]^2

Bolt and Nut Marking Requirements

AS/NZS 1252 mandates permanent identification markings on the bolt head and nut face:

Bolt Head Markings

• Manufacturer's identification symbol (unique to each manufacturer)
• Grade designation: '4.6/S', '8.8/S', or '10.9/S'
• Markings must be raised or indented (forged/stamped during manufacture)
• Paint or ink markings are NOT acceptable for structural bolts (they can be removed during handling or corrode)

Nut Markings

• Grade designation: '8/S' for use with 8.8/S bolts, '10/S' for use with 10.9/S bolts
• Manufacturer's identification symbol (optional but recommended)
• The nut height must be at least 0.8 x d (e.g., M20 nut minimum height = 16 mm)

Verification in the Fabrication Shop

The bolt markings should be verified upon receipt of each batch. The test certificate should be matched to the heat number and batch identification. Any bolt without legible head markings must be rejected — do not rely on colour coding, packaging labels, or delivery docket descriptions.

Grade Selection Guide — When to Use Each Grade

Grade 4.6/S — Light and Secondary Applications

Use for:

• Purlin and girt connections (cleats, sag rods)
• Handrail and walkway connections
• Light bracing (tension-only rods, nominally loaded)
• Stair stringer connections
• Signage and architectural metalwork
• Connections where the bolt capacity is not the governing limit state

Do NOT use for:

• Primary beam-to-column connections
• Splice connections
• Connections subject to fatigue
• Slip-critical connections
• Connections requiring pretensioning

Grade 8.8/S — General Structural Workhorse

Use for:

• Primary beam-to-column connections
• Column splices (bearing-type)
• Brace-to-gusset connections
• Base plate anchor bolts (when zinc plated or galvanized)
• Moment end plate connections
• Standard structural connections in all building types

This is the default structural bolt grade for Australian steel fabrication. Over 90% of structural bolts specified in Australia are Grade 8.8/S.

Grade 10.9/S — High-Demand Applications

Use for:

• Heavy moment-resisting connections where bolt count must be minimised
• Connections with restricted geometry (tight bolt groups)
• Fatigue-critical connections (higher preload reduces stress range in the bolt)
• Slip-critical connections where fewer bolts reduce fabrication cost
• Retrofit and strengthening projects where existing steel limits bolt placement

Precautions: 10.9/S bolts are more sensitive to hydrogen embrittlement, particularly after electroplating or pickling. Baking (200°C for 4 hours within 4 hours of plating) is mandatory for electroplated 10.9/S bolts. Galvanized 10.9/S bolts require careful control of the pickling process to avoid hydrogen uptake.

Comparison: AS 1252 vs International Bolt Standards

Worked Example: Bolt Grade and Size Selection

Problem: A beam-to-column shear connection transfers a factored shear force V* = 210 kN through a web side plate with 4 bolts in single shear. The bolts are M20 Grade 8.8/S with threads in the shear plane. Check bolt adequacy and consider whether 10.9/S bolts could reduce the bolt count.

Step 1: Check 4x M20 Grade 8.8/S

From the shear capacity table, single shear capacity (threads in plane) for M20 Grade 8.8/S = 100.9 kN.

Total shear capacity of 4 bolts: phi V_f = 4 x 100.9 = 403.6 kN > 210 kN — OK.

Utilisation = 210 / 403.6 = 0.52 — significant reserve. Could the bolt count be reduced?

Step 2: Check 2x M20 Grade 8.8/S

phi V_f = 2 x 100.9 = 201.8 kN < 210 kN — FAIL (utilisation = 1.04). Two bolts are inadequate.

Step 3: Try M20 Grade 10.9/S with 2 bolts

Single shear M20 Grade 10.9/S = 126.5 kN.

phi V_f = 2 x 126.5 = 253.0 kN > 210 kN — OK.

Utilisation = 210 / 253.0 = 0.83.

Step 4: Check M24 Grade 8.8/S as alternative to 10.9/S

Single shear M24 Grade 8.8/S = 145.3 kN.

phi V_f = 2 x 145.3 = 290.6 kN > 210 kN — OK.

Decision: Three options:

1. 4x M20 Grade 8.8/S (utilisation 0.52) — simple, standard, additional bolts add redundancy
2. 2x M20 Grade 10.9/S (utilisation 0.83) — saves 2 bolts but requires special procurement
3. 2x M24 Grade 8.8/S (utilisation 0.72) — larger holes needed (26 mm vs 22 mm)

For typical Australian practice, Option 1 (4x M20 8.8/S) is the standard choice — the two additional bolts add negligible cost compared to requiring Grade 10.9/S procurement and the larger M24 holes. If the connection geometry is constrained (tight column flange limiting bolt spacing), Option 2 or 3 would be selected.

Frequently Asked Questions

Can Grade 8.8/S bolts be hot-dip galvanized?

Yes, Grade 8.8/S bolts are routinely hot-dip galvanized in Australian practice. However, two precautions are essential: (1) The bolts must be supplied slightly undersized or the nuts must be tapped oversize (typically +0.4 mm on pitch diameter) to accommodate the zinc coating thickness, otherwise the bolt will not engage the nut. Galvanized nuts for Grade 8.8/S bolts are marked '8/S G' to indicate the overtapping. (2) 10.9/S bolts are more susceptible to hydrogen embrittlement during the pickling stage of the galvanizing process and require special process controls. Mechanical galvanizing (peen plating) is sometimes preferred for 10.9/S bolts in diameters up to M24 as an alternative to hot-dip galvanizing.

What bolt length should I specify for a structural connection?

The bolt grip length (total thickness of connected plies) determines the required bolt length. Add the nut height (0.8d) plus 2-3 thread pitches for thread protrusion beyond the nut. For M20 (pitch 2.5 mm), the nut height is approximately 16 mm, and thread protrusion of 2 x 2.5 = 5 mm, giving a minimum bolt length = grip + 21 mm. Round up to the nearest standard length (5 mm increments for bolts up to 100 mm, 10 mm increments thereafter). Always specify that the thread run-out must be positioned so that threads are excluded from the shear plane if the bolt capacity is calculated on the shank area (A_c). This typically requires a bolt 5-10 mm longer than the grip plus nut alone.

Are stainless steel bolts permitted for structural connections in Australia?

Yes, stainless steel bolts to AS/NZS 4673 are permitted for structural connections, but with significant caveats. AS 4100 does not provide design rules for stainless steel fasteners — the designer must use AS/NZS 1554.6 (structural stainless steel welding) and AS/NZS 4673. Stainless bolts (typically Grade A4-70 or A4-80 per ISO 3506) have lower strength than Grade 8.8/S carbon steel bolts. A4-70 has f_uf = 700 MPa and f_yf = 450 MPa, approximately 84% of the strength of 8.8/S. The major application is in highly corrosive environments (marine, chemical processing) where the life-cycle cost of carbon steel bolts with coating maintenance exceeds the higher initial cost of stainless. Stainless bolts must NEVER be substituted 1:1 for carbon steel bolts of the same size without recalculation of all limit states.

What is the minimum edge distance for bolted connections per AS 4100?

Per AS 4100 Clause 9.6.3, the minimum edge distance from the centre of a bolt hole to the edge of a plate is 1.5 x d_f for a sheared or hand-flame-cut edge, and 1.25 x d_f for a rolled, machine-flame-cut, sawn, or planed edge. For M20 bolts: minimum edge distance = 1.5 x 20 = 30 mm (sheared edge) or 1.25 x 20 = 25 mm (sawn edge). In practice, Australian fabricators standardise on 35 mm edge distance for M20 bolts and 40 mm for M24 bolts to provide a margin above the code minimum. The minimum bolt spacing (centre-to-centre) is 2.5 x d_f per Clause 9.6.2, giving 50 mm for M20 bolts. Australian standard gauges for 75 mm wide PFC channels use a 45 mm gauge with 30 mm edge distance from the back of the channel.

Educational reference only. All design values must be verified against the current edition of AS 4100:2020, AS/NZS 1252:2016, and the project specification. This information does not constitute professional engineering advice. Always consult a qualified structural engineer for design decisions.

Disclaimer: This content is for educational purposes only. Results must be verified by a licensed professional engineer. Steel Calculator provides preliminary design tools — NOT a substitute for professional engineering judgment.