Achieving the correct preload in high-strength structural bolting is a critical requirement under EN 1090-2 and Eurocode 3 standards. Correct EN 14399-9 DTI installation is essential for guaranteeing that the assembly reaches the required 70% of the nominal bolt tensile strength preload. Using Direct Tension Indicators (DTIs) eliminates the uncertainties associated with torque-based methods, providing a reliable, verifiable mechanical measure of tension. This guide outlines the comprehensive four-step installation procedure and rigorous inspection standards required for compliance. When safety and certification depend on the fasteners you choose, trust the FULLERKREG DTI washer collection for unmatched precision and traceability. For specialized guidance or project-specific quotes, please consult our engineering team at sales@fullerkr.com.
Theory of Preloaded Bolting Assemblies
- Fatigue resistance: Preloading significantly increases the fatigue life of the joint by minimizing stress fluctuations in the bolt.
- Vibration resilience: A properly preloaded assembly resists self-loosening under severe vibration.
- Load reversal mitigation: Preloaded joints maintain structural integrity even when external forces reverse direction.
- Dimensional stability: The high clamping force prevents slip between connected parts. Shear transfer is achieved entirely by frictional resistance between the plates, not by bearing against the bolt shank. EN 1993-1-8 strictly requires the preload to equal 70% of the nominal ultimate tensile strength of the bolt.
EN 1090-2 Permitted Preloading Methods
| Method | Principle | Lubrication Sensitivity |
|---|---|---|
| DTI Method | Measures direct mechanical compression | Low (K0 class sufficient) |
| Combined Method | Initial torque + calculated part-turn | High (Requires K1 class) |
| Torque Control Method | Tightening to a specific torque value | Very High (Requires K2 class) |
| HRC/TC Bolt Method | Spline shears off at predetermined tension | Moderate (K0 with HRD nut) |
| Part-Turn Method | Snug tight + prescribed rotation angle | Low (K0 class sufficient) |
Recommended 4-Step DTI Assembly Installation Procedure
Step 1: Align and Prepare
Align the structural members securely. Use drift pins to line up the holes; never use the structural bolts as drift pins, as this can damage the threads and ruin the tightening characteristics.
Step 2: Snug the Connection
Bring the plies into firm contact by snugging the bolts to approximately 50% of the final preload. Work systematically from the most rigid part of the connection outward to the free edges. This step can be performed using an impact wrench or a standard spanner.
Step 3: Final Tensioning
Tighten the bolts to the specified average gap, following the exact same systematic pattern used during snugging. Leave the drift pins in place during this phase. Maintaining the pattern is especially critical for nut-fitted DTIs to ensure even compression.
Step 4: Replace Drift Pins
Remove the remaining drift pins, install the final bolting assemblies with their DTIs, and tension them to the specified gap.
Systematic Tightening Patterns
- Single or double row connections: Always start at the end where the plates are in tightest contact and work toward the free ends.
- Large web splices: Start from the center of the splice array and work outward in all directions.
- Consistency: The identical tightening pattern must be used for both the snug pass and the final tensioning pass.
Four DTI Assembly Configurations
| Configuration # | DTI Position | Rotated Element | Feeler Gauge | Hardened Washer Required |
|---|---|---|---|---|
| 1 | Under bolt head | Nut | 0.40mm | No |
| 2 | Under nut | Bolt head | 0.40mm | No |
| 3 | Under nut | Nut | 0.25mm | Yes, under nut |
| 4 | Under bolt head | Bolt head | 0.25mm | Yes, under bolt head |
Hardened Washer Requirements
According to standards, 10.9 grade bolts generally require hardened washers (EN 14399-5 or EN 14399-6) under both the head and the nut. For 8.8 grade bolts, a hardened washer is typically only required under the turned element. Special plate washers must be used for oversize or slotted holes. Note: While some independent studies suggest direct nut-on-DTI tightening can be more consistent, EN 1090-2 specifies intermediate washers unless project specifications explicitly permit their omission.
Inspection of Assembled DTIs
Feeler Gauge Thickness by Configuration
| Condition | Feeler Gauge Thickness (mm) |
|---|---|
| Under bolt head / nut rotated | 0.40 |
| Under nut / bolt head rotated | 0.40 |
| Under nut / nut rotated | 0.25 |
| Under bolt head / bolt head rotated | 0.25 |
Feeler Gauge Refusal Requirements
| Total Protrusions on DTI | Minimum Required Refusals |
|---|---|
| 4 | 3 |
| 5 | 3 |
| 6 | 4 |
| 7 | 4 |
| 8 | 5 |
| 9 | 5 |
Full Compression Tolerance
EN 1090-2 Annex J allows up to 10% of the DTIs in any given bolt group to show full compression (all gaps closed). The UK National Structural Steelwork Specification (NSSS) 5th Edition confirms that full compression is not a cause for rejection provided the manufacturer's certificate guarantees the bolts will not yield or fracture under such conditions.
On-Site Testing Requirements
- Test a minimum of 8 sample assemblies per diameter and grade.
- Testing must be conducted using a calibrated load cell (e.g., Skidmore-Wilhelm).
- Tighten the assembly until the DTI reaches the specified gap.
- Verify that the induced tension (Fb,I) meets the requirement: Fp,C ≤ Fb,I ≤ 1.2 × Fp,C.
Minimum Preload Values (Fp,C in kN)
| Bolt Grade | M12 | M16 | M20 | M22 | M24 | M27 | M30 | M36 |
|---|---|---|---|---|---|---|---|---|
| 8.8 | 47 | 88 | 137 | 170 | 198 | 257 | 314 | 458 |
| 10.9 | 59 | 110 | 172 | 212 | 247 | 321 | 393 | 572 |
Lubrication Requirements and K-Classes
K-Class Definition
| K-Class | Definition & Suitability |
|---|---|
| K0 | No specific requirements except to achieve preload; highly suitable for the DTI Method. |
| K1 | Individual k-factor must be 0.10 - 0.16; required for the Combined Method. |
| K2 | Mean k-factor 0.10 - 0.23 with Vk ≤ 0.10; strictly required for the Torque Control Method. |
K-Class by Tightening Method
| Method | Required K-Class |
|---|---|
| DTI Method | K0 |
| Part-Turn Method | K0 |
| Combined Method | K1 |
| Torque Control Method | K2 |
| HRC/TC Bolt Method | K0 (with HRD nut); otherwise K2 |
The primary advantage of the DTI method is that it relies purely on mechanical deformation rather than friction, requiring only K0 lubrication—the least restrictive and most reliable class for variable field conditions.
FAQ
-
What is the correct 4-step procedure for installing EN 14399-9 DTI washers?
The procedure is: 1) Align with drift pins, 2) Snug to ~50% preload, 3) Final tension to the specified gap, 4) Replace drift pins and tension remaining bolts. -
How many feeler gauge refusals are required for a 5-protrusion DTI?
For a 5-protrusion DTI, a minimum of 3 feeler gauge refusals is required to verify correct tension. -
What feeler gauge thickness do I use for EN 14399-9 DTI inspection?
Use 0.40mm when the DTI is under the stationary element, and 0.25mm when it is under the turned element. -
Do 10.9 grade bolts require hardened washers on both sides?
Yes, per EN 1090-2, 10.9 grade bolts generally require hardened washers (EN 14399-5 or -6) under both the head and the nut. -
What is K0 lubrication class, and why is it sufficient for DTI installation?
K0 has no specific friction requirements because the DTI method measures direct mechanical tension, making it immune to the frictional variations that plague torque-based methods. -
Where can I buy EN 14399-9 DTI washers with full installation support?
You can purchase compliant indicators and receive expert engineering support through the FULLERKREG DTI washer collection. Contact our team at sales@fullerkr.com for assistance.
Conclusion
Proper DTI installation and inspection according to EN 14399-9 and EN 1090-2 are critical for the safety and longevity of preloaded structural connections. By following the 4-step procedure and relying on K0 mechanical indication, field crews can ensure perfect tension every time. Equip your project with the industry's most reliable tension indicators.
Email us at: sales@fullerkr.com