Australian Bolt Torque Chart — AS 4100 Pretension and Torque-Tension Table

Comprehensive torque-tension reference for structural bolting in Australia per AS 4100:2020 Clause 9.3.8. Covers minimum bolt pretension values for Grade 8.8/TB and 10.9/TB fasteners from M12 to M36, the torque-tension relationship including k-factor variability, installation methods, and verification requirements for slip-critical connections.

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When Pretensioned Bolts Are Required — Clause 9.3.8.1

AS 4100:2020 Clause 9.3.8 mandates pretensioned (fully tensioned) bolts for the following connection categories:

1. Slip-critical connections (TF category) where slip at the faying surface would impair the serviceability or ultimate performance of the structure. Examples include: connections subject to load reversal, connections with oversized or slotted holes, and connections where slip would cause misalignment of machinery or finishes.

2. Connections subject to direct tension where bolt tension fluctuations due to applied loads could cause loosening under cyclic loading. Examples include: hanger connections, column base plates subject to overturning moment, and bracing connections in tension-only systems.

3. Connections where relaxation would be detrimental to structural performance. Examples include: connections in structures subject to vibration (crane runway girders, machinery support structures) and connections where bolt loosening would compromise the load path.

For all other connections, snug-tight installation (tightening by the full effort of a person using a standard podger spanner) is acceptable.

Minimum Bolt Pretension — Clause 9.3.8.2

The minimum bolt pretension (P_t) is specified as a function of the bolt tensile strength and the tensile stress area:

The pretension values are derived from approximately 70% of the minimum tensile strength of the bolt:

P_t = 0.70 x f_uf x A_s

where f_uf = 830 MPa (Grade 8.8) or 1040 MPa (Grade 10.9), and A_s is the tensile stress area from AS 1275.

Pretension Tolerance

The installed pretension must be not less than the minimum value P_t from the table above. AS 4100 does not specify an upper tolerance on pretension, but AS/NZS 1554.1 and the AISC/ASI bolting guidelines recommend that the installed pretension should not exceed 1.15 P_t to avoid bolt yielding during installation and to prevent damage to the connected plies from excessive clamping force.

Torque-Tension Relationship

The relationship between applied installation torque (T) and the resulting bolt pretension (P) is governed by the short-form torque equation:

T = k x d_f x P

where:

• T = installation torque (N.m)
• k = nut factor (dimensionless, typically 0.15 to 0.22)
• d_f = nominal bolt diameter (m)
• P = target bolt pretension (kN)

The k-Factor (Nut Factor)

The nut factor k is not a coefficient of friction but a combined efficiency factor that accounts for:

• Thread friction between the bolt and nut threads
• Bearing friction under the nut face or bolt head
• Geometric effects of the thread pitch and flank angle
• Lubrication condition of the fastener assembly

Typical k-factors for structural bolting in field conditions:

The k-factor should be verified by pre-installation testing for each batch of fasteners using a Skidmore-Wilhelm or equivalent tension calibrator per AS/NZS 1554.1 Clause 5.3. Testing is especially important for galvanized fasteners, where the k-factor is sensitive to the lubricant condition.

Australian Bolt Torque Chart

The following torque values are calculated using k = 0.18 (typical for as-received bolts with light protective oil). These torque values are approximate and should be verified by the structural engineer for the specific project conditions.

Grade 8.8/TB Bolts — Recommended Installation Torque (N.m)

Grade 10.9/TB Bolts — Recommended Installation Torque (N.m)

Conversion to Podger Spanner Effort

For on-site verification when torque wrenches are not available, the following approximate podger spanner lengths produce the target torque with an assumed 25 kg (250 N) installer force:

• To achieve ~300 N.m: 1.2 m spanner length (M20 with k=0.15)
• To achieve ~500 N.m: 2.0 m spanner length (M22 with k=0.18)
• To achieve ~1000 N.m: 2.0 m spanner with 50 kg force (two-person, M30)

Snug-tight condition (the minimum acceptable for non-slip-critical connections) is defined as the tightness achieved by the full effort of a person using a standard podger spanner approximately 450 mm long. This produces a pretension of roughly 15-25% of the minimum bolt tensile strength, which is adequate for bearing-type connections.

Installation Methods Per AS 4100 Clause 9.3.8.3

AS 4100 permits four methods of achieving the required pretension:

1. Torque Control (Turn-of-Nut Method)

The most widely used method in Australian practice. The bolt is first brought to a snug-tight condition, then the nut is rotated through a specified additional turn angle. The required rotation depends on the bolt length and diameter:

The turn-of-nut method is preferred over direct torque measurement because it is less sensitive to k-factor variability. The nut rotation strain-hardens the bolt shank, producing a reliably consistent pretension regardless of thread condition.

2. Calibrated Wrench Method

A torque wrench is calibrated to a target torque value determined from pre-installation verification testing. The calibration must be verified at least once per day per wrench per bolt diameter, and after any drop or impact to the wrench. This method is sensitive to k-factor variation and is generally used only where the bolt surface condition is well controlled.

3. Direct Tension Indicator (DTI) Method

Load-indicating washers with protruding bumps that flatten as the bolt is tensioned. The gap between the DTI washer and the bolt head or nut is measured with a feeler gauge. When the specified gap is achieved, the minimum pretension is confirmed. DTI washers are manufactured to AS/NZS 1554.1 requirements and are colour-coded by bolt size.

4. Tension-Control Bolt Method

Bolts with a splined end that shears off at a calibrated torque corresponding to the required pretension. The bolt is installed using a special shear wrench that grips both the nut and the splined tip, applying counter-torque until the tip fractures. Commonly called "TC bolts" or "twist-off bolts," these are increasingly common in Australian structural fabrication for their speed and reliability.

Verification of Pretension — AS/NZS 1554.1 Clause 5.4

For slip-critical connections (GP and SP categories per AS/NZS 1554.1), the installed pretension must be verified by one of the following methods:

1. Pre-installation verification testing: For each combination of bolt diameter, length, grade, and surface condition, a minimum of three bolts from each production lot must be tested in a tension calibrator to establish the torque-pretension relationship. The calibrated torque is then used for production installation.

2. In-service verification: For critical connections (GP category), at least 10% of installed bolts must be checked using a calibrated torque wrench set to 105% of the target torque. If the wrench clicks without nut rotation, the pretension is adequate. If the nut rotates before the wrench clicks, the bolt must be re-tensioned or replaced.

3. DTI gap measurement: Where DTI washers are used, the gap is verified with the correct feeler gauge for that washer type and bolt diameter.

Worked Example: Torque Specification for Slip-Critical Connection

Problem: A slip-critical connection in a 530UB92.4 beam splice uses M24 Grade 8.8/TB bolts in 26 mm standard holes. The faying surfaces are clean mill scale (slip factor mu = 0.35 per Clause 9.3.8.1). The bolts are as-received condition (light oil, k = 0.18). Determine the target installation torque.

Given:

• Bolt diameter: d_f = 24 mm
• Bolt grade: 8.8/TB
• Minimum pretension from table: P_t = 147 kN
• k-factor (as-received): k = 0.18
• Connection category: TF (slip-critical)
• Faying surface: clean mill scale, mu = 0.35
• Number of faying surfaces: 2 (splice plates both sides)

Solution:

Step 1: Target installation torque

T = k x d_f x P_t = 0.18 x 0.024 x 147,000 = 635 N.m

Step 2: Select installation method

Use turn-of-nut method from snug-tight condition. The bolt grip length (total thickness of plies + washer) is approximately 40 mm. d_f = 24 mm, so grip < 4 d_f (96 mm). Require 1/3 turn (120 degrees) beyond snug-tight.

Step 3: Check slip resistance at serviceability

For M24 Grade 8.8/TB bolt with P_t = 147 kN:

V_sf = mu x n_e x n_b x P_t x k_h

where mu = 0.35, n_e = 2 (two faying surfaces per bolt), n_b = 1 (single bolt check), k_h = 1.0 (standard holes).

Nominal slip resistance per bolt: V_sfn = 0.35 x 2 x 1 x 147 = 102.9 kN

Design slip resistance with phi = 0.70 (slip-critical): phi V_sf = 0.70 x 102.9 = 72.0 kN per bolt.

Step 4: Document the torque specification

The WPS (Welding Procedure Specification -- or in this case, the Bolting Procedure Specification) shall state:

• Bolt type: M24 Grade 8.8/TB, AS 1252
• Installation method: Turn-of-nut, 1/3 turn from snug-tight
• Verification: Calibrated wrench, check 10% of bolts at 670 N.m (1.05 x 635)
• k-factor verification: Pre-installation test, 3 bolts min per lot
• Surface condition: Clean mill scale, free of paint, oil, and dirt in the faying area

Result: Target torque 635 N.m. Turn-of-nut method specified. Slip per bolt 102.9 kN nominal, 72.0 kN design. Complete bolting procedure specification prepared.

Frequently Asked Questions

What torque should be used for M20 Grade 8.8 bolts per AS 4100?

For M20 Grade 8.8/TB bolts in as-received condition (k = 0.18), the recommended installation torque is 367 N.m to achieve the minimum pretension of 102 kN per AS 4100 Clause 9.3.8.2. For lubricated bolts (k = 0.15), the required torque reduces to 306 N.m. These values should be verified by pre-installation testing for the specific bolt batch and surface condition.

What is the difference between snug-tight and pretensioned bolts in AS 4100?

Snug-tight bolts are tightened by the full effort of a person using a standard podger spanner (approximately 450 mm long). They are acceptable for bearing-type connections where slip is not detrimental. Pretensioned bolts are tightened to a specified minimum pretension (approximately 70% of the bolt tensile strength) using calibrated methods. Pretensioned bolts are required for slip-critical connections (TF category), connections subject to direct tension, and connections where bolt relaxation would compromise performance.

Does galvanizing affect the required installation torque?

Yes, significantly. Hot-dip galvanizing increases the k-factor from approximately 0.15-0.18 to 0.20-0.24 for lubricated galvanized bolts, requiring 15-30% more torque to achieve the same pretension. More critically, dry (unlubricated) galvanized bolts can have k-factors as high as 0.35, making torque-based installation unreliable. In Australian practice, galvanized bolts should always be supplied with a lubricant coating, and the k-factor verified by pre-installation testing per AS/NZS 1554.1.

Can I reuse pretensioned bolts per AS 4100?

AS 4100 does not explicitly prohibit the reuse of pretensioned bolts, but AS/NZS 1554.1 recommends that Grade 8.8/TB bolts should not be re-tensioned more than twice. After the initial tensioning, the bolt shank has undergone plastic strain, and the k-factor may have changed due to thread deformation and lubricant depletion. If bolts are to be reused, the k-factor must be re-verified by calibration testing. Grade 10.9/TB bolts are not recommended for reuse due to their lower ductility and greater sensitivity to hydrogen embrittlement after plastic strain.

What is the slip factor for different faying surface conditions?

Per AS 4100 Clause 9.3.8.1 and AS/NZS 1554.1, typical slip factors for common surface conditions are: clean mill scale (mu = 0.35), blast-cleaned with Class 2-1/2 surface (mu = 0.50), blast-cleaned with inorganic zinc silicate paint (mu = 0.40), and hot-dip galvanized with roughened surface (mu = 0.35). The slip factor must be verified by slip testing per Appendix J of AS/NZS 1554.1 for any surface condition not covered by the standard values.

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