Bolt Grade Designation System — AS/NZS 1252

AS 4100 recognises three bolt grades for structural steel connections. The grade designation X.Y/S encodes the mechanical properties:

The /S suffix is critical: it designates a structural bolt tested to AS/NZS 1252 with Charpy impact requirements (minimum 27 J at 0 degrees Celsius for sizes up to M24). Commercial bolts to AS 1111 (e.g., Grade 8.8 without /S) are NOT permitted for structural steel connections in Australia.

Grade decoding: The first number is f_uf / 100 (e.g., 8 = 800 MPa class). The second number is 10 x f_yf / f_uf (e.g., 0.8 = f_yf = 0.8 x f_uf). So 8.8 means f_uf = 800 MPa class and f_yf = 0.8 x 830 = 660 MPa.

Bolt Shear Capacity — AS 4100 Clause 9.3.2.1

phi x 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 in shear)
• n_n = number of shear planes with threads excluded (body in shear plane)
• n_x = number of shear planes with threads included (threads in shear plane)
• A_c = nominal shank area = pi x d^2 / 4
• A_o = tensile stress area per AS 1275

For single-shear with threads in plane (n_n = 0, n_x = 1):

Design Shear Capacities — Single Shear, Threads In Plane (kN)

Formula: phi_Vf = 0.80 x 0.62 x f_uf x A_o / 1000

Threads excluded (body in shear plane): Replace A_o with A_c (larger — no thread root reduction). This applies when the bolt grip length is arranged so that the unthreaded shank crosses the shear plane — typically achieved by using longer bolts with washers to position the thread runout away from the shear interface.

Bolt Tension Capacity — AS 4100 Clause 9.3.2.2

phi x N_tf = phi x A_o x f_uf

Where phi = 0.80 for bolts in tension.

Design Tension Capacities (kN)

Note: Tension capacity does NOT depend on whether threads are in the shear plane — the threaded portion always governs bolt tension because of stress concentration at the thread root.

Combined Shear and Tension — Clause 9.3.2.3

When a bolt is simultaneously loaded in shear V* and tension N*, the following interaction must be satisfied:

(V / phi_Vf)^2 + (N / phi_Ntf)^2 <= 1.0**

The quadratic interaction is less conservative than a linear interaction. It accounts for the von Mises yield criterion on the bolt cross-section. For the typical case with V* = 0.5 x phi_Vf and N* = 0.5 x phi_Ntf, the utilisation is sqrt(0.25 + 0.25) = 0.707 < 1.0. OK.

Tightening Methods — AS 4100 Clause 15.2.5

AS 4100 specifies three tightening categories:

Snug-tight is the default for most Australian bearing-type connections. It costs less and is adequate when slip does not affect the structure's ultimate behaviour.

Tensioned bolts are required for:

• Slip-critical connections (friction-grip joints per Clause 9.3.3)
• Connections subject to vibration or load reversal
• Connections where bolt loosening would cause serviceability issues

Minimum Bolt Pretension — AS 4100 Table 15.2.5.1

When pretension is specified, the minimum clamping force is:

The pretension T_b = 0.70 x f_uf x A_o for 8.8/S and 0.70 x f_uf x A_o for 10.9/S, rounded per AS 4100.

Worked Example — Grade Selection for a Portal Frame Knee Connection

Problem: A portal frame knee connection transfers M* = 380 kNm through a bolted end plate. The bolt group has 8 x M24 bolts in tension (top four rows) and 4 x M24 bolts in shear (bottom two rows). Select the appropriate bolt grade.

Step 1 — Tension demand in critical bolts: Assuming elastic distribution, the top bolts carry the maximum tension: N*_max = M* x y_max / (sum y_i^2)

For 4 rows of 2 bolts at 100, 200, 300, 400 mm from the compression flange: sum y_i^2 = 2 x (100^2 + 200^2 + 300^2 + 400^2) = 2 x (10,000 + 40,000 + 90,000 + 160,000) = 2 x 300,000 = 600,000 mm^2

N*_max = 380 x 10^6 x 400 / 600,000 = 253.3 kN per bolt (top row, 2 bolts)

Step 2 — Try Grade 8.8/S: M24 8.8/S tension capacity: phi_Ntf = 234 kN < 253.3 kN. NOT OK.

Step 3 — Try Grade 10.9/S: M24 10.9/S tension capacity: phi_Ntf = 294 kN > 253.3 kN. OK.

Step 4 — Shear check (bottom bolts): V* total = 180 kN / 4 bolts = 45 kN per bolt. M24 10.9/S shear capacity (threads in plane): phi_Vf = 182 kN > 45 kN. OK.

Step 5 — Combined shear-tension (top bolts with small shear component): Assume V*_minor = 10 kN per top bolt. (10 / 182)^2 + (253.3 / 294)^2 = 0.003 + 0.742 = 0.745 < 1.0. OK.

Result: Grade 10.9/S is required. Grade 8.8/S is inadequate for the tension demand in the top bolts. The 30% cost premium for 10.9/S is justified by the connection capacity requirement.

Grade Selection Decision Matrix

Decision rule: Use 8.8/S as the default structural bolt grade. Upgrade to 10.9/S only when 8.8/S capacity is insufficient and increasing bolt count or diameter is not feasible. Never use 10.9/S in seismic-resisting bracing connections without specific testing per AS 4100 Clause 13.2.

Frequently Asked Questions

When should Grade 10.9/S bolts be specified instead of 8.8/S in Australian practice?

Grade 10.9/S bolts (f_uf = 1040 MPa) provide 25% higher shear capacity than 8.8/S (830 MPa) in the same bolt diameter. Specify 10.9/S when: (1) bolt group shear at an existing connection needs upgrading without changing the hole layout; (2) connection size is constrained by available edge distance or gauge limits; or (3) high-strength moment connections require maximum bolt tension capacity. However, 10.9/S bolts have lower ductility than 8.8/S and are more sensitive to hydrogen embrittlement in galvanised applications — always specify mechanically galvanised or zinc-flake coated 10.9/S bolts to mitigate this risk.

What does the /S suffix mean in AS 4100 bolt grades?

The /S suffix (e.g., 8.8/S) designates a structural bolt to AS/NZS 1252. The /S indicates compliance with the full structural bolting standard, including: controlled chemical composition with maximum carbon equivalent, Charpy V-notch impact testing at 0 degrees Celsius (minimum 27 J for sizes up to M24), proof load testing to 93% of specified minimum tensile strength, wedge tensile testing for head integrity, and dimensional verification per AS 1275. Bolts without the /S suffix (commercial Grade 8.8 per AS 1111) are NOT permitted for structural steel connections under AS 4100.

Why is the bolt shear capacity formula different for threads in vs out of the shear plane?

When threads intercept the shear plane, the effective shear area is the tensile stress area A_o (approximately 0.75-0.78 of the nominal shank area for metric coarse threads). When the unthreaded shank crosses the shear plane, the full body area A_c governs. The capacity difference is approximately 25-30%. In Australian practice, most connections have threads in the shear plane by default because standard bolt lengths place the thread runout within the grip. Specifying a longer bolt with extra washers can position the runout outside the shear plane, but this adds cost and is typically only done for critical connections where the 25% capacity gain eliminates a bolt row.

Are Grade 4.6/S bolts still used in modern Australian steel construction?

Yes, in specific applications. Grade 4.6/S bolts (f_uf = 400 MPa) are specified for: (1) secondary member connections (purlins to rafters, girts to columns) where loads are low and the bolt group design is not shear-governed; (2) nominally pinned connections where the bolt acts primarily as a locator pin; (3) holding-down bolts cast into concrete foundations, where the 4.6/S ductility provides better anchor behaviour than higher-strength grades. The lower cost of 4.6/S (approximately 40% less than 8.8/S) makes them economical for high-bolt-count, low-demand applications. However, 8.8/S is the standard for all primary structural connections in Australian practice.

Related Pages

• AS 4100 Bolt Group Capacity — Instantaneous Centre Method
• AS 4100 Bolt Bearing & Tear-Out Checks
• AS 4100 Bolt Spacing & Edge Distance Tables
• AS 4100 Bolt Pretension — Turn-of-Nut Method
• AS 4100 Shear Connection Design Guide
• Bolt Torque Calculator — Free Online Tool
• All Australian Steel Design References

Design Resources

Calculator tools

• Bolt Torque Calculator
• Bolted Connection Calculator
• Splice Connection Calculator
• Steel Column Calculator

Design guides

• AS 4100 Bolted Connection Design Guide
• AS 4100 Beam Design Worked Example
• AS 4100 Base Plate Design — Anchors & Bearing
• EN 1993-1-8 Bolt Grade Equivalents

This page is for educational reference only. Bolt properties per AS 4100:2020 Table 9.2.1 and AS/NZS 1252. Verify bolt grades and capacities against current Australian Standards. All structural designs must be independently verified and certified by a licensed Professional Engineer. Results are PRELIMINARY — NOT FOR CONSTRUCTION.