EN 1993 Bolt Torque Chart — 8.8/10.9 Torque-Tension Table M12-M36

Complete torque-tension reference for European structural bolts per EN 1993-1-8 and EN 1990. Pretension forces, tightening torques, and installation requirements for property classes 8.8 and 10.9, diameters M12 through M36.

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Bolt Property Classes — EN ISO 898-1

Property Class f_ub (MPa) f_yb (MPa) Material Application
4.6 400 240 Low carbon steel Light secondary
5.6 500 300 Low carbon steel General purpose
6.8 600 480 Medium carbon General connections
8.8 800 640 Quenched & tempered Standard structural bolts
10.9 1000 900 Alloy steel Q&T High-strength connections

For structural connections in EN 1993-1-8, 8.8 and 10.9 are the standard choices.

Pretension Forces per EN 1993-1-8

The design pretension force F_p,C for bolted connections is:

F_p,C = 0.7 × f_ub × A_s

Where f_ub is the ultimate tensile strength and A_s is the tensile stress area.

Bolt Size A_s (mm²) F_p,C 8.8 (kN) F_p,C 10.9 (kN)
M12 84.3 47.2 59.0
M16 157 87.9 109.9
M20 245 137.2 171.5
M22 303 169.7 212.1
M24 353 197.7 247.1
M27 459 257.0 321.3
M30 561 314.2 392.7
M36 817 457.5 571.9

Tightening Torque Values

For torque-controlled tightening, the tightening torque T is approximately:

T = k × d × F_p,C

Where k is the nut factor (0.15-0.25 depending on lubrication and surface condition) and d is the nominal bolt diameter in meters.

Recommended Tightening Torques (k = 0.20)

Bolt Size Torque 8.8 (N·m) Torque 10.9 (N·m)
M12 113 142
M16 281 352
M20 549 686
M22 746 933
M24 949 1186
M27 1388 1735
M30 1885 2356
M36 3294 4118

Note: These torque values assume k = 0.20 (as-supplied condition, slightly oiled). For specific surface conditions, apply correction factors:

Condition k Factor Torque Adjustment
As-supplied 0.20 × 1.00
Zinc-coated 0.22 × 1.10
Oiled/greased 0.15 × 0.75
Clean dry 0.25 × 1.25
Hot-dip galvanized 0.25 × 1.25

Direct Tension Method

The alternative to torque-controlled tightening is the combined method per EN 1090-2:

  1. Snug-tight: Tighten to the point where the connected plies are in firm contact
  2. Prescribed rotation: Apply additional rotation per EN 1090-2 Table 18:
    • 90° for general installation
    • 180° for specific preloaded connections

The direct tension method (using a hydraulic tensioner or torque wrench with calibrated friction) is preferred for connections where the pretension force must be accurately controlled, such as slip-resistant connections.

Installation Methods per EN 1090-2

EN 1090-2 requires the following for preloaded bolts:

Method Description Accuracy QC Requirement
Torque control Calibrated torque wrench ±15% Daily calibration check
Combined Snug-tight + prescribed rotation ±10% Match-mark after snug
HRC Torque-controlled tension indicator ±10% Visual gap check
Direct tension Hydraulic tensioner ±5% Oil pressure calibration

Frequently Asked Questions

What is the difference between 8.8 and 10.9 bolts for European structural connections?

8.8 bolts (f_ub = 800 MPa, f_yb = 640 MPa) are the standard for most EN 1993-1-8 connections. 10.9 bolts (f_ub = 1000 MPa, f_yb = 900 MPa) provide 25% higher pretension and are used for high-strength connections, especially slip-resistant joints, where fewer or smaller bolts are desired. Both are quenched and tempered.

What tightening torque should I use for M20 8.8 bolts?

For M20 8.8 bolts with as-supplied condition (k = 0.20), use 549 N·m. For oiled bolts (k = 0.15), reduce to 412 N·m. Per EN 1993-1-8 Clause 3.11, the design pretension is F_p,C = 137.2 kN for M20 8.8, and torque = 0.20 × 0.020 × 137200 = 549 N·m.

Related Pages

Educational reference only. Pretension per EN 1993-1-8:2005 Clause 3.11. Torque values are approximate and depend on friction conditions. Calibrate torque wrenches per EN 1090-2. Results are PRELIMINARY — NOT FOR CONSTRUCTION without independent verification.

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