Australian Structural Bolt Grades — AS 1252 / AS 1111 Mechanical Properties
Complete reference for structural bolt grades used in Australian steel construction per AS 1252.1 (High-strength steel bolts with controlled tightening) and AS 1111.1 (ISO metric hexagon bolts and screws). Covers Grade 4.6/S, 8.8/S, 8.8/TB, 10.9/S, and 10.9/TB: mechanical properties, marking systems, identification, dimensional standards, and bolt selection for bearing-type and slip-critical connections per AS 4100.
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Bolt Grade Designation System
Australian metric bolt grades use a two-number designation system per ISO 898-1 (incorporated via AS 4291.1):
4.6 means:
- First number (4): minimum tensile strength = 4 x 100 = 400 MPa
- Second number (6): yield strength ratio = 6/10 = 0.6
- Minimum yield strength = 400 x 0.6 = 240 MPa
8.8 means:
- First number (8): minimum tensile strength = 8 x 100 = 800 MPa (actual: 830 MPa for structural bolts)
- Second number (8): yield strength ratio = 8/10 = 0.8
- Minimum yield strength = 800 x 0.8 = 640 MPa (actual: 660 MPa for structural bolts)
10.9 means:
- First number (10): minimum tensile strength = 10 x 100 = 1,000 MPa (actual: 1,040 MPa for structural bolts)
- Second number (9): yield strength ratio = 9/10 = 0.9
- Minimum yield strength = 1,000 x 0.9 = 900 MPa (actual: 940 MPa for structural bolts)
AS 1252 Suffix Designations
| Suffix | Meaning | Application |
|---|---|---|
| /S | Structural bolt, snug-tight or pretensioned | Bearing-type connections (TB category) |
| /TB | Tension-bearing bolt, high-strength, suitable for full pretension | Slip-critical connections (TF category) |
| /TF | Tension-friction bolt, designed for slip-critical use | Slip-critical (equivalent to /TB) |
Grade 8.8/TB is the most common bolt specification in Australian structural steel. Grade 10.9/TB is used where higher strength is needed to reduce the number of bolts in a connection or where bolt size is constrained.
Mechanical Properties — AS 1252 Table 2.1
Grade 4.6/S (Mild Steel Bolts)
| Property | Value | Notes |
|---|---|---|
| Tensile strength f_uf | 400 MPa (min) | Lower bound of the specified range |
| Yield strength f_yf | 240 MPa (min) | 0.2% proof stress |
| Elongation after fracture | 22% (min) | Ductile, suitable for general use |
| Hardness (Brinell) | 114-209 HBW | Soft, easily machined |
| Typical application | Secondary members, handrails, non-structural | Not for structural connections > M20 |
Grade 8.8/S and 8.8/TB (High-Strength Structural Bolts)
| Property | Value | Notes |
|---|---|---|
| Tensile strength f_uf | 830 MPa (min) | 800-1,000 MPa permitted range |
| Yield strength f_yf | 660 MPa (min) | 0.2% proof stress (660 MPa actual) |
| Yield-to-tensile ratio | 0.80 (min) | Good ductility for a high-strength bolt |
| Elongation after fracture | 12% (min) | Sufficient for structural applications |
| Hardness (Rockwell C) | 23-34 HRC | Can be machined, drilled if required |
| Charpy impact | 27 J at 20 C (min) | Adequate for ambient temperature service |
| Pretension (M20) | 102 kN | 70% of proof load |
Grade 8.8 bolts are manufactured from medium-carbon steel, quenched and tempered. The heat treatment produces a tempered martensite microstructure with the required combination of high strength and adequate ductility. Nuts are Grade 8 (ISO 898-2) and washers are hardened steel (AS 1252 washer specification).
Grade 10.9/S and 10.9/TB (Extra High-Strength Bolts)
| Property | Value | Notes |
|---|---|---|
| Tensile strength f_uf | 1,040 MPa (min) | 1,000-1,200 MPa permitted range |
| Yield strength f_yf | 940 MPa (min) | 0.2% proof stress |
| Yield-to-tensile ratio | 0.90 (min) | Higher ratio, less ductility than 8.8 |
| Elongation after fracture | 9% (min) | Lower ductility -- avoid plastic straining |
| Hardness (Rockwell C) | 33-39 HRC | Harder than 8.8 |
| Pretension (M20) | 143 kN | 40% higher pretension than 8.8/TB |
Grade 10.9 bolts require more careful handling and installation than 8.8 bolts. They are more susceptible to hydrogen embrittlement (stress corrosion cracking) if exposed to hydrogen sources during manufacturing or service. For this reason, 10.9 bolts are not recommended for hot-dip galvanized applications unless the galvanizing process is specifically controlled to minimise hydrogen absorption.
Bolt Marking and Identification — AS 1252 Clause 7
Structural bolts must be permanently marked on the head with:
- The manufacturer's identification mark (symbol or letter code registered with Standards Australia)
- The grade designation (8.8 or 10.9)
- The category suffix (S or TB)
Example markings:
- "XYZ 8.8 S" = Grade 8.8 structural bolt, snug-tight, manufactured by XYZ
- "ABC 10.9 TB" = Grade 10.9 tension-bearing bolt, manufacturer ABC
Nuts are marked with the grade number (8 or 10) and the manufacturer's identification. Washers are typically unmarked but are identified by hardness testing if the specification is questioned.
Dimensional Standards for Metric Bolts — AS 1111.1 / AS 1275
Australian structural bolts conform to the ISO metric coarse thread series (M12-M36):
| Nominal Diameter d (mm) | Thread Pitch (mm) | Stress Area A_s (mm^2) | Head Across Flats (mm) | Head Height (mm) |
|---|---|---|---|---|
| M12 | 1.75 | 84.3 | 18 | 7.5 |
| M16 | 2.0 | 157 | 24 | 10 |
| M20 | 2.5 | 245 | 30 | 12.5 |
| M22 | 2.5 | 303 | 32 | 14 |
| M24 | 3.0 | 353 | 36 | 15 |
| M27 | 3.0 | 459 | 41 | 17 |
| M30 | 3.5 | 561 | 46 | 18.7 |
| M36 | 4.0 | 817 | 55 | 22.5 |
Threads are metric coarse series per ISO 724, tolerance class 6g for bolts and 6H for nuts. The thread length is standardised: for bolt lengths up to 125 mm, thread length = 2d + 6 mm; for longer bolts, thread length = 2d + 12 mm.
Grade Selection for Australian Construction
| Application | Recommended Grade | Rationale |
|---|---|---|
| Primary beam-to-column (bearing-type) | 8.8/S or 8.8/TB | Standard for structural bolting |
| Slip-critical connection (TF category) | 8.8/TB | Requires controlled pretension |
| Moment-resisting connection | 8.8/TB or 10.9/TB | Higher grade for reduced bolt count |
| Fatigue-sensitive connection | 8.8/TB | Better fatigue performance than 10.9 |
| Hot-dip galvanized connection | 8.8/TB | Avoid 10.9 in HDG (hydrogen embrittlement) |
| Secondary member (handrail, stair) | 4.6/S | Low-stress, non-structural |
| Base plate holding-down bolts (cast-in) | 4.6/S or 8.8/S | 4.6 for nominal fixity, 8.8 for moment bases |
| Tension-controlled (hanger, bracing in tension) | 8.8/TB | Requires pretension to prevent loosening |
Worked Example: Bolt Grade Comparison for Shear Connection
Problem: A beam-to-column shear connection has a factored design shear of V* = 200 kN. The connection uses 4 bolts in single shear with threads in the shear plane. Compare the required bolt sizes for Grade 4.6/S, 8.8/S, and 10.9/S bolts.
Given:
- Design shear: V* = 200 kN (factored)
- Number of bolts: n_b = 4
- Shear per bolt: V*/n_b = 50 kN
- Threads in shear plane
Solution:
Step 1: Calculate required bolt stress area for each grade
Per AS 4100 Clause 9.3.2.1: phi V_f = phi x 0.62 x f_uf x A_c
For threads in shear plane, use the core area A_c (approximately 0.78 x A_s for the metric coarse thread):
Required core area: A_c_req = V_per_bolt / (phi x 0.62 x f_uf)
| Grade | phi | f_uf (MPa) | A_c_req (mm^2) | Required A_s (mm^2) | Required Bolt Size | Selected Bolt |
|---|---|---|---|---|---|---|
| 4.6/S | 0.80 | 400 | 50,000 / (0.80 x 0.62 x 400) = 252.0 | 323 | M24 (A_s = 353) | M24 |
| 8.8/S | 0.80 | 830 | 50,000 / (0.80 x 0.62 x 830) = 121.5 | 156 | M20 (A_s = 245) | M20 |
| 10.9/S | 0.80 | 1,040 | 50,000 / (0.80 x 0.62 x 1,040) = 96.9 | 124 | M16 (A_s = 157) | M16 |
Step 2: Practical considerations
M16 bolts with 10.9 grade: While technically adequate, M16 is often avoided for primary connections because the small wrench size (24 mm) limits the achievable pretension and the bolts are more susceptible to damage during erection. M20 is the preferred minimum in Australian fabrication for primary structural connections.
M24 bolts with Grade 4.6: The large bolt size requires larger edge distances and wider connected plies, increasing material and fabrication cost.
M20 Grade 8.8: The standard choice. Adequate capacity with reserve, standard fabrication tolerances, good availability.
Result: M20 Grade 8.8 provides the best balance of strength, cost, and fabrication practicality. M16 Grade 10.9 would work technically but is less practical. M24 Grade 4.6 is uneconomical due to the larger plate sizes required.
Frequently Asked Questions
What does 8.8 mean on an Australian bolt head?
The "8.8" marking on a bolt head indicates it is a Grade 8.8 bolt per AS 1252 (ISO 898-1). The first "8" means the minimum tensile strength is 800 MPa (actual: 830 MPa for structural bolts). The second "8" means the yield strength is 80% of the tensile strength (yield ratio 0.8), giving a minimum yield strength of 660 MPa. Grade 8.8 bolts are quenched and tempered medium-carbon steel, and are the standard grade for Australian structural steel bolting.
What is the difference between 8.8/S and 8.8/TB bolts per AS 1252?
The "/S" suffix denotes a structural bolt suitable for snug-tight or pretensioned installation in bearing-type connections. The "/TB" suffix denotes a tension-bearing bolt with additional requirements on ductility Charpy impact toughness) and dimensional tolerances to ensure reliable pretension. While 8.8/S and 8.8/TB have the same mechanical properties (830 MPa tensile, 660 MPa yield), the /TB is tested more stringently for pretension consistency and is required for slip-critical (TF) connections per AS 4100 Clause 9.3.8.
Why is Grade 10.9 not recommended for hot-dip galvanized bolts?
Grade 10.9 bolts are susceptible to hydrogen embrittlement, where atomic hydrogen diffuses into the high-strength steel microstructure and causes brittle intergranular fracture under sustained tensile stress. The hot-dip galvanizing process involves acid pickling (which generates hydrogen) and immersion in molten zinc at 450 degrees C (which can trap hydrogen in the steel). Grade 8.8 bolts are less susceptible because their lower hardness (23-34 HRC vs 33-39 HRC for 10.9) provides greater resistance to hydrogen embrittlement. If 10.9 bolts must be galvanised, mechanical cleaning (grit blasting) instead of acid pickling and a low-hydrogen galvanizing process must be specified.
Can Grade 4.6 bolts be used for structural steel connections in Australia?
Grade 4.6 bolts can be used for structural connections in secondary and non-critical applications where the design loads are low and the bolt size is not constrained. However, for primary structural connections (beam-to-column, splice, bracing, base plate), Grade 4.6 is generally uneconomical because the larger bolt size required increases plate sizes, edge distances, and fabrication costs. Grade 8.8 is the standard for Australian structural steel and is used almost universally for primary structural connections. Grade 4.6 is common for holding-down bolts cast into concrete (anchor bolts), where the concrete pull-out capacity governs over the bolt steel strength.
What markings must appear on Australian structural bolts per AS 1252?
Per AS 1252 Clause 7, each structural bolt must be permanently marked (embossed or indented) on the head with the manufacturer's identification mark and the grade designation. For example, "XYZ 8.8" identifies the manufacturer XYZ and the grade. The category suffix (S or TB) should also appear. Bolts without legible markings do not comply with AS 1252 and must not be used for structural applications. The markings provide traceability to the mill test certificate, which is required for all structural bolts supplied to Australian projects.
Educational reference only. All bolt specifications must be verified against the current edition of AS 1252.1, AS 1111.1, and the project specification. This information does not constitute professional engineering advice.