EnglishViews: 0 Author: Site Editor Publish Time: 2026-06-06 Origin: Site
U.S. standard A325 and European standard EN 14399—which one should you choose? How should installation torque be calculated? How should on-site inspection be carried out?
1. What Exactly Are A325 and EN 14399?
U.S. Standard A325: A high-strength structural bolt under the ASTM system in the United States. It is the mainstream choice for projects built to U.S. standards in North America, Southeast Asia, the Middle East, and other regions. It is available in Type 1 (medium-carbon steel) and Type 2 (quenched and tempered steel), emphasizing large diameters and high pretension to suit U.S.-standard structural steel design.
European Standard EN 14399: A high-strength structural bolting assembly under the European standards system. It is mandatory for projects built to European standards in the EU, Australia, and some Southeast Asian markets. It includes two main series—HR (large hexagon) and HV (twist-off type). Its core focus is pretension consistency and anti-relaxation performance, making it suitable for Eurocode steel structure design.
2. Bolt Selection Criteria: 3 Core Judgment Standards
2.1 The project design standard is the primary deciding factor
• If the project is designed in accordance with AISC 360, U.S. standard A325 bolts (or the equivalent grade A490) must be used.
• If the project is designed in accordance with EN 1993 (Eurocode 3), EN 14399 bolts must be used.
2.2 Differences in bolt performance parameters
Comparison Item | U.S. Standard A325 | European Standard EN 14399 HR (Large Hexagon) |
Tensile strength | 827–1034 MPa | 800–1000 MPa |
Yield strength | ≥655 MPa | ≥640 MPa |
Applicable diameter range | 1/2"–1-1/2" (metric M12–M36) | M12–M36 |
Pretension requirement | According to ASTM F3125, pretension is 70% of the bolt tensile strength | According to EN 14399-1, pretension is 75% of the bolt tensile strength |
Tolerance standard | UNC/UNF thread tolerance under U.S. standards | ISO 965 thread tolerance under European standards |
Typical applications | factory, bridge and high-rise steel structures | public buildings, industrial plants and bridges |
2.3 On-site construction compatibility
A325: Features a large-hexagon configuration and is installed with torque wrenches. This offers flexible field construction and suits regions accustomed to U.S.-standard installation practices, such as North America and Southeast Asia.
EN 14399: Includes both large-hexagon and twist-off types. The twist-off type controls pretension by shearing off the spline end, providing high installation efficiency and good pretension consistency, and is the mainstream choice in Europe.
3. Installation Torque Values
Torque is the core of bolt installation. Excessive torque can fracture the bolt, while insufficient torque results in inadequate pretension—both directly affect structural safety.
3.1 Core calculation logic
General formula: T = k × D × Fₚ
• T: installation torque (N·m)
• k: torque coefficient (commonly 0.15–0.20 for U.S. standards and 0.12–0.18 for European standards; the manufacturer’s tested value shall prevail)
• D: nominal bolt diameter (mm)
• Fₚ: design pretension force (U.S. standard = 0.7 × tensile capacity; European standard = 0.8 × yield capacity)
3.2 Torque reference table for common specifications
The torque coefficients in the tables below use industry-standard mid-range values (U.S. standard k = 0.16; European standard k = 0.14). In actual construction, the manufacturer’s measured k value and the design documents must prevail.
(1) Torque Table for U.S. Standard A325 Bolts
Bolt Size (U.S. / Metric) | Nominal Diameter D (mm) | Design Pretension Fₚ (kN) | Recommended Installation Torque T (N·m) | Typical Application |
1/2" (M12) | 12.70 | 55 | 112 | Light steel structures; secondary member connections |
5/8" (M16) | 15.88 | 88 | 224 | Conventional plants; secondary beam connections |
3/4" (M20) | 19.05 | 129 | 393 | Main beams, beam-column joints, general steel structures |
7/8" (M22) | 22.23 | 176 | 625 | High-rise steel structures; heavy-duty industrial plants |
1" (M24) | 25.40 | 230 | 939 | Long-span trusses; bridge joints |
1-1/8" (M30) | 28.58 | 290 | 1326 | Heavy bridges; super-tall steel structures |
1-1/4" (M33) | 31.75 | 357 | 1813 | Extra-heavy structures; special engineering works |
1-1/2" (M36) | 38.10 | 510 | 3109 | Large bridges; nuclear power / heavy industrial structures |
(2) Torque Table for EN 14399 HR (Large Hexagon) Bolts
Bolt Size (Metric) | Nominal Diameter D (mm) | Design Pretension Fₚ (kN) | Recommended Installation Torque T (N·m) | Typical Application |
M12 | 12 | 50 | 84 | Light steel structures; secondary members |
M16 | 16 | 90 | 202 | Conventional plants; secondary beam connections |
M20 | 20 | 142 | 398 | Main beams, beam-column joints, general steel structures |
M22 | 22 | 172 | 527 | High-rise steel structures; heavy-duty industrial plants |
M24 | 24 | 203 | 682 | Long-span trusses; bridge joints |
M27 | 27 | 259 | 979 | Heavy bridges; super-tall steel structures |
M30 | 30 | 319 | 1340 | Extra-heavy structures; special engineering works |
M36 | 36 | 460 | 2318 | Large bridges; nuclear power / heavy industrial structures |
4. On-Site Inspection Methods: 3 Practical Approaches to Ensure Qualified Installation
4.1 Torque recheck method (most commonly used).
The re-tightening torque shall not be less than 90% of the design torque and not greater than 110% of the design torque.
Applicable to: U.S. standard A325 bolts and European standard EN 14399 large-hexagon bolts.
4.2 Turn-of-nut method (common to both U.S. and European standards).
The final rotation angle complies with the design requirement, with neither over-tightening nor under-tightening.
Applicable to: Large-diameter bolts and critical load-bearing joints; this is a core method recommended by AISC 360.
4.3 Dedicated inspection for twist-off bolts (EN 14399 HV).
The spline ends of all bolts are 100% sheared off, with no omissions.
Advantages: High installation efficiency, intuitive inspection, and no need for complex tools—making it the preferred option for European-standard projects.
4.4 Ultrasonic inspection (essential for premium projects).
Acceptance criterion: The pretension force reaches 90%–110% of the design requirement.
Applicable to: Primary joints in important bridges, high-rise steel structures, and similar projects; it is the ultimate acceptance method used by owners and supervising engineers.
