EN 14399 — High-Strength Structural Bolting Assemblies

EN 14399 is the harmonized European standard for high-strength structural bolting assemblies for preloading. It defines the complete assembly — bolt, nut, and washer(s) — as a system. The standard is published in 10 parts:

HR vs HV Bolt Systems

HR System (EN 14399-3)

The HR system uses hexagon head bolts and nuts where the bolt thread is in the shear plane. HR bolts are available in property classes 8.8 and 10.9. The preload is achieved through controlled tightening with calibrated equipment.

Key features:

• Thread in shear plane permitted (full-thread or partial-thread)
• Standard hexagon head per EN ISO 4014/4017
• Nuts per EN 14399-3 (style 1, property class 10)
• Available in M12-M36 diameters

HV System (EN 14399-4)

The HV system uses hexagon fitting bolt assemblies with larger across-flats dimensions, designed so the thread is NOT in the shear plane. HV bolts are supplied only in property class 10.9 and are tightened by the combined method (snug-tight + prescribed rotation).

Key features:

• Thread excluded from shear plane
• Larger across-flats dimensions
• Nuts per EN 14399-4 (style 1, property class 10)
• Preload achieved via combined tightening method (EN 1090-2)

k-Class System — Nut Factor Classification

EN 14399 defines k-classes (K0, K1, K2) to describe the scatter of the nut factor k (ratio of tightening torque to induced preload):

T = k × d × F_p,C

For construction in EN 1090-2, k-class K2 is the default (as-supplied condition). k-class K1 is required for slip-resistant connections where preload reliability is critical.

EN 14399 Preload Forces — F_p,C

The design preload force per EN 1993-1-8 Clause 3.11(1):

F_p,C = 0.7 × f_ub × A_s

Where f_ub is the ultimate tensile strength of the bolt material and A_s is the tensile stress area per EN ISO 898-1.

Tightening Torques per EN 14399 — k-Class K2

For k-class K2 (general structural, as-supplied with k = 0.14):

Note: These torque values assume k = 0.14 (the mid-range of k-class K2). In practice, k can vary between 0.08 and 0.20 within K2. On-site calibration per EN 1090-2 Clause 8.4 is mandatory.

Tightening Torques — Surface Condition Corrections

The nut factor k depends on lubrication and surface condition. Apply corrections to the K2 reference values:

Direct Tension Indicators (DTI) — EN 14399-9

Direct tension indicators (also called load-indicating washers) provide visual confirmation that the required preload has been achieved. Per EN 14399-9, DTIs are used with HR or HV systems with standard bolts, nuts, and a hardened washer on top of the DTI.

DTI method per EN 1090-2:

1. Assemble connection with DTI under bolt head or nut
2. Snug-tighten to bring plies into firm contact
3. Further tighten until DTI protrusions compress to the specified gap (typically 0.4 mm feeler gauge cannot enter)
4. Visual verification — no additional measurement equipment required

DTIs eliminate the uncertainty of torque-based tightening by measuring actual bolt tension directly.

Installation and QC per EN 1090-2

EN 1090-2 mandates the following for preloaded bolting assemblies:

Design Verification — Slip Resistance per EN 1993-1-8

For Category C (slip-resistant at SLS) connections per EN 1993-1-8 Clause 3.9:

F_s,Rd = k_s × n × μ × F_p,C / γ_M3

Where:

• k_s = hole factor (1.0 for normal holes, 0.85 for oversize, 0.70 for slotted)
• n = number of friction interfaces
• μ = slip factor (typically 0.30 for unpainted surfaces, 0.40 for grit-blasted)
• F_p,C = preload force
• γ_M3 = 1.25 (partial safety factor)

Frequently Asked Questions

What is the difference between EN 14399 HR and HV bolt assemblies?

HR assemblies (EN 14399-3) are standard hexagon bolt systems available in 8.8 and 10.9 with the thread permitted in the shear plane. HV assemblies (EN 14399-4) have larger across-flats dimensions, are 10.9 only, and are designed so the thread is excluded from the shear plane. HR is generally torque-controlled; HV uses the combined snug-tight plus prescribed rotation method per EN 1090-2.

Which k-class should I specify for structural connections?

For general structural connections per EN 1993-1-8, k-class K2 (k = 0.08-0.20, σ ≤ 0.06) is the default specification. For slip-resistant connections where preload reliability is critical (e.g., fatigue-loaded joints, Category C connections), specify k-class K1 (k = 0.10-0.16, σ ≤ 0.04) with controlled lubrication. k-class K0 is reserved for laboratory applications and special calibrations.

How are EN 14399 preload values verified on site?

On-site verification follows EN 1090-2 Clause 12.5.3: systematic visual inspection of all bolts for full tightening, plus random torque audit of at least 5% of bolts (minimum 2 per connection). The audit torque is 1.10 × reference tightening torque. If any bolt in the sample fails, audit 100% of bolts in the connection. For DTI assemblies (EN 14399-9), a 0.4 mm feeler gauge must not enter more than 10% of the DTI circumference.

What torque should I use for M24 HV 10.9 bolts to concrete-encased column splices?

For M24 HV 10.9 bolts with k-class K2 (as-supplied, k = 0.14), the reference torque is 830 N·m. For hot-dip galvanized bolts (k = 0.18), increase to 1,067 N·m. Always perform a pre-installation verification test per EN 1090-2 on a representative sample of the assembly delivered to site, as the actual k factor for the batch may differ from nominal values.

Related Pages

• EN 1993 Bolt Pretension → — Pretension per EN 1993-1-8
• EN 1993 Bolt Spacing → — Edge/end distances per Table 3.3
• EN 1993 Bolt Bearing → — Bearing resistance per Clause 3.6
• EN 1993 Bolt Group Capacity → — Eccentric bolt groups
• All European References →

Educational reference only. Preload values per EN 14399 and EN 1993-1-8:2005 Clause 3.11. Torque values are approximate and depend on the actual k factor of the delivered assembly. Perform pre-installation verification per EN 1090-2 Clause 8.5.4. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent verification by a qualified structural engineer.

Design Resources

Calculator tools

• Bolt Torque Calculator
• Bolted Connection Calculator
• Splice Connection Calculator
• Slip-Resistant Connection Calculator

Design guides

• EN 1993-1-8 Bolted Connection Worked Example
• Bolted Connection Worked Example
• Bolted Connection Checklist
• Bolt Torque Worked Example
• Steel Connection Calculator Guide

Reference pages

• EU Bolt Pretension
• EU Shear Tab Connection
• UK Moment Connection

Disclaimer: This content is for educational purposes only. Results must be verified by a licensed professional engineer. Steel Calculator provides preliminary design tools — NOT a substitute for professional engineering judgment.