Australian Bolt Pretension — AS 4100 Clause 9.3.8 Minimum Tension Requirements
Complete reference for bolt pretension requirements in Australian steel design per AS 4100:2020 Clause 9.3.8. Minimum bolt pretension values for Grade 8.8 and 10.9 bolts in slip-critical (TF) and tension-bearing (TB) connections. Installation methods including turn-of-nut, tension control (TC) bolts, and torque control.
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When Pretension is Required Per AS 4100
AS 4100:2020 Clause 9.3.8 specifies when bolts must be fully tensioned (pretensioned) rather than installed to snug-tight condition:
| Condition | Pretension Required? | Applicable Category |
|---|---|---|
| Bearing-type connections (static loading) | No | 8.8/S (snug-tight) |
| Slip-critical connections | Yes | 8.8/TF or 10.9/TF |
| Connections subject to load reversal or vibration | Yes | 8.8/TB or 10.9/TB |
| Tension connections with direct tension | Yes | 8.8/TB or 10.9/TB |
| Connections subject to fatigue | Yes | 8.8/TF or 10.9/TF |
| Moment connections in frames | Yes | 8.8/TB or 10.9/TB |
| Column splices in multi-storey frames | Yes | 8.8/TB or 10.9/TB |
| Bracing connections in seismic frames | Yes | 8.8/TF or 10.9/TF |
Per AS 4100 Clause 9.3.8.1, if pretension is required, the bolt installation must achieve a minimum bolt tension (pretension force) equal to the values specified in Clause 9.3.8.3.
Minimum Bolt Pretension Values — AS 4100 Clause 9.3.8.3
The minimum bolt pretension (Pd) is calculated as:
Pd = 0.70 x fuf x At
Where:
- fuf = minimum tensile strength of the bolt
- At = tensile stress area of the bolt
| Bolt Size | At (mm2) | Grade 8.8 Pd (kN) (fuf = 830 MPa) | Grade 10.9 Pd (kN) (fuf = 1040 MPa) |
|---|---|---|---|
| M12 | 84.3 | 49.0 | 61.4 |
| M16 | 157 | 91.2 | 114.3 |
| M20 | 245 | 142.3 | 178.4 |
| M22 | 303 | 176.0 | 220.6 |
| M24 | 353 | 205.1 | 257.0 |
| M27 | 459 | 266.7 | 334.2 |
| M30 | 561 | 325.9 | 408.4 |
| M36 | 817 | 474.7 | 594.8 |
These pretension values represent approximately 70% of the minimum tensile strength of the bolt, calculated using the tensile stress area. This level of preload ensures that the connection remains tight under service loads and provides the clamping force required for slip resistance.
Installation Methods for Pretensioned Bolts
AS 4100 Clause 9.3.8.4 recognises three methods for achieving the required bolt pretension.
Turn-of-Nut Method
The turn-of-nut method is the most widely used pretensioning technique in Australian practice. After bringing the connection to the snug-tight condition (the tightness achieved by a few impacts of an impact wrench or the full effort of a worker using a standard podger bar), additional rotation is applied:
| Bolt Length (L) | Required Rotation from Snug-Tight |
|---|---|
| L <= 4d | 1/3 turn (120 degrees) |
| 4d < L <= 8d | 1/2 turn (180 degrees) |
| 8d < L <= 12d | 2/3 turn (240 degrees) |
Where d is the nominal bolt diameter. The rotation must be applied to the nut while the bolt head is prevented from turning. All plies must be in firm contact at the snug-tight condition before the additional rotation is applied.
The turn-of-nut method is preferred because it does not require specialised tools beyond an impact wrench. However, it requires mark-up and visual inspection — each bolt must be matchmarked before the final turn to verify the rotation was achieved.
Tension Control (TC) Bolt Method
TC bolts (also called twist-off bolts) have a splined tip that shears off when the bolt reaches the required pretension. The bolt is installed using a special TC gun that grips both the bolt head and the spline tip:
- The connection is snugged using an impact wrench or TC gun in low-speed mode
- The TC gun engages the bolt head and spline tip simultaneously
- As the gun applies torque to the nut, the bolt body is held by the spline
- When the pretension reaches the calibrated value, the spline tip shears off
- The missing spline provides immediate visual confirmation of proper installation
TC bolts are widely used in Australian construction because they offer:
- Single-sided installation — the gun only needs access to the nut side
- Visual inspection — no torque wrench or rotation measurement needed
- Consistent pretension — the spline is calibrated to shear at the target tension
- Faster installation — significantly faster than turn-of-nut for large bolt groups
Direct Tension Indicator (DTI) Method
DTI washers have raised bumps that compress as the bolt is tightened. When the gap between the washer face and the bolt head (or nut) reaches a specified value (typically verified with a feeler gauge), the bolt has achieved the required pretension.
The DTI method is less common in Australian practice but is used for:
- Connections where turn-of-nut access is limited
- Retrofit and repair work
- Connections requiring independent verification of tension
Calibrated Wrench Method
The calibrated wrench method uses a torque wrench set to a calculated torque value corresponding to the required pretension. The torque-tension relationship is:
T = k x d x Pd
Where:
- T = applied torque (Nm)
- k = nut factor (typically 0.15 to 0.25 for lubricated bolts)
- d = nominal bolt diameter (m)
- Pd = minimum pretension (N)
This method is the least reliable because the nut factor varies with lubrication, surface finish, thread condition, and plating. Where the calibrated wrench method is used, AS 4100 requires daily calibration against a hydraulic tension calibrator for each bolt diameter, grade, and lot.
Recommended Installation Torque Values
For reference, the following torque values achieve the minimum pretension for Grade 8.8 bolts assuming a nut factor k = 0.20 (typical for lubricated bolts). These values are indicative only — actual torque requirements depend on lubrication and surface conditions.
| Bolt Size | Grade 8.8 Pd (kN) | Torque (k = 0.20) (Nm) |
|---|---|---|
| M12 | 49.0 | 118 |
| M16 | 91.2 | 292 |
| M20 | 142.3 | 569 |
| M22 | 176.0 | 774 |
| M24 | 205.1 | 984 |
| M27 | 266.7 | 1440 |
| M30 | 325.9 | 1955 |
| M36 | 474.7 | 3418 |
Slip-Critical Connection Design
AS 4100 Clause 9.3.3 defines the slip resistance of a pretensioned connection:
phi Vsf = phi x 0.35 x kr x n x Ne x kc
Where:
- phi = 0.80 (capacity factor for slip)
- kr = reduction factor for bolt length (Clause 9.3.2.2)
- n = number of slip planes
- Ne = total clamping force at the faying surface (= number of bolts x Pd)
- kc = reduction factor for hole type:
- Standard holes: kc = 1.0
- Oversized holes: kc = 0.85
- Short slotted holes (slot parallel to load): kc = 0.70
- Short slotted holes (slot perpendicular to load): kc = 0.85
- Long slotted holes (slot perpendicular to load): kc = 0.70
Worked Example: Slip-Critical Connection
Problem: A splice connection uses 8 x M20 Grade 8.8/TF bolts in standard holes with a single slip plane. Determine the slip resistance.
Solution:
- M20 Grade 8.8 pretension: Pd = 142.3 kN
- Total clamping force: Ne = 8 x 142.3 = 1138.4 kN
- Standard holes: kc = 1.0
- Single slip plane: n = 1
- phi Vsf = 0.80 x 0.35 x 1.0 x 1 x 1138.4 x 1.0 = 318.8 kN
The connection has a slip resistance of 318.8 kN. If oversized holes were used instead, the slip resistance would reduce to 0.85 x 318.8 = 270.9 kN.
Faying Surface Preparation
For slip-critical connections, the faying surface condition directly affects the slip factor (0.35 in AS 4100 Clause 9.3.3). The standard slip factor of 0.35 assumes:
- Clean mill scale — surfaces blast-cleaned to near-white metal with intact mill scale
- Bare steel — no paint, galvanising, or coating that reduces friction
- Faying surface preparation — surfaces free of oil, grease, dirt, and loose rust
If the faying surface is painted or galvanised, the slip factor must be verified by testing per AS 4100 Appendix B. Galvanised surfaces typically have a lower slip factor (approximately 0.20-0.25) unless the surface is roughened.
Design Resources
- Australian Steel Design Guide — AS 4100 overview
- Australian Bolt Grades — Grade 8.8 and 10.9 properties
- Australian Bolt Capacity — Shear and tension tables
- Australian Bolt Hole Sizes — AS 4100 hole dimensions
- Australian Weld Sizes — AS 1554 weld requirements
- Bolted Connections Calculator
- Bolt Torque Calculator
- Beam Capacity Calculator
Frequently Asked Questions
When is bolt pretension required per AS 4100? AS 4100 Clause 9.3.8 requires pretension for slip-critical connections (TF category), connections subject to load reversal or vibration, tension connections with direct tension, moment frame connections, column splices in multi-storey frames, connections subject to fatigue, and bracing connections in seismic frames. Simple shear connections in bearing-type construction with static loading can be installed snug-tight (8.8/S category) without pretension.
What is the minimum bolt pretension for M20 Grade 8.8 bolts per AS 4100? The minimum pretension for M20 Grade 8.8 bolts is Pd = 0.70 x fuf x At = 0.70 x 830 x 245 / 1000 = 142.3 kN per AS 4100 Clause 9.3.8.3. This is approximately 70% of the bolt's minimum tensile strength, providing sufficient clamping force for slip-critical connections without overstressing the bolt.
What is the turn-of-nut requirement for Australian structural bolts? For bolts shorter than or equal to 4 diameters, 1/3 turn (120 degrees) from snug-tight. For bolts between 4 and 8 diameters, 1/2 turn (180 degrees). For bolts between 8 and 12 diameters, 2/3 turn (240 degrees). The connection must be brought to snug-tight condition first, with all plies in firm contact, before the final rotation is applied.
How is slip resistance calculated for slip-critical connections per AS 4100? phi Vsf = phi x 0.35 x kr x n x Ne x kc, where phi = 0.80, Ne = total clamping force (sum of all bolt pretensions), n = number of slip planes, and kc = hole type reduction factor (1.0 for standard holes, 0.85 for oversized holes, 0.70 for long slotted holes parallel to load). The slip factor of 0.35 assumes clean mill scale faying surfaces.
What torque is required to achieve the minimum pretension for M20 Grade 8.8 bolts? For M20 Grade 8.8 bolts with minimum pretension 142.3 kN and nut factor k = 0.20: T = k x d x Pd = 0.20 x 0.020 x 142,300 = 569 Nm. This is a reference value — the actual torque depends on the specific nut factor, which varies with lubrication, plating, and thread condition. The nut factor should be verified by calibration testing for each bolt lot.
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.