Bolt Specification & Capacity Reference
This page contains complete reference data for structural bolts to BS EN ISO 4014 (part threaded) and BS EN ISO 4017 (fully threaded), covering grades 4.6, 8.8, and 10.9, sizes M12 to M36. Data includes bolt dimensions, tensile stress areas, and single shear and tension capacities per BS EN 1993-1-8 with UK NA partial factors (γM2 = 1.25). All values are reference data for non-preloaded bolts in clearance holes; actual design must follow current structural codes.
Bolt Dimensions (M12–M36)
| Bolt Size | Shank Diameter (mm) | Hole Size (mm) | Head Diameter (mm) | Head Height (mm) | Tensile Stress Area (mm²) |
|---|
Bolt Grades & Material Properties
| Grade | Material | Yield Strength (MPa) | Tensile Strength (MPa) | Common Applications |
|---|---|---|---|---|
| 4.6 | Carbon steel | 240 | 400 | General purpose, non-critical connections |
| 8.8 | Carbon steel | 640 | 800 | General structural work, most connections |
| 10.9 | Carbon steel, hardened & tempered | 900 | 1000 | High-strength connections, preloaded |
Single Shear Capacity (kN) — Non-preloaded
| Bolt Size | Grade 4.6 | Grade 8.8 | Grade 10.9 |
|---|
Double Shear Capacity (kN) — Non-preloaded
| Bolt Size | Grade 4.6 | Grade 8.8 | Grade 10.9 |
|---|
Tension Capacity (kN) — Non-preloaded
| Bolt Size | Grade 4.6 | Grade 8.8 | Grade 10.9 |
|---|
Combined Shear and Tension
Bolts subject to combined shear and tension must satisfy the interaction formula per BS EN 1993-1-8 Table 3.4:
Fv,Ed / Fv,Rd + Ft,Ed / (1.4 × Ft,Rd) ≤ 1.0
The 1.4 factor on tension means bolts in combined loading have some reserve capacity. At full shear utilisation, you can still carry zero tension. At full tension utilisation, you can still carry approximately 72% of the shear resistance. This interaction must be checked for bolts in end plate connections and similar joints where both shear and tension are present simultaneously.
Standards reference
- BS EN ISO 4014 — Hexagon head bolts, part threaded, product grade C
- BS EN ISO 4017 — Hexagon head screws, fully threaded, product grade C
- BS EN 14399 — High strength structural bolts with preloaded tensioning (HSFG)
- BS EN 1993-1-8 — Design of steel structures, connection rules and joints
All capacity values are reference data only and must be verified against current design standards and engineer calculations before use in any structural application.
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Frequently asked questions
What is the difference between grade 4.6, 8.8 and 10.9 bolts?
These grades describe the bolt's strength and ductility. The first number (×100) is the ultimate tensile strength in MPa; the second number is the ratio of yield strength to ultimate strength. Grade 4.6 has an ultimate strength of 400 MPa and yield of 240 MPa, making it softer and more ductile. Grade 8.8 (the most common in UK building steelwork) has 800 MPa ultimate and 640 MPa yield, giving good strength with reasonable toughness. Grade 10.9 has the highest strength (1000 MPa ultimate) but less ductility, making it suitable for preloaded connections and heavily stressed joints. Grade 10.9 single shear capacity is actually lower than grade 8.8 in some configurations because of design conservatism in Eurocode 3 for very high strength steels.
What size structural bolt do I need for a standard beam-to-column connection?
This depends on the connection load, moment, and shear resistance required, which vary with beam depth, steel grade, and connection type. As a rough guide, M20 grade 8.8 bolts (tension resistance 141 kN, single shear 94 kN per bolt) are typical for end plate connections in ordinary building frames, often used in pairs or groups. M16 might be adequate for lighter frames or secondary connections. M24 would be used for heavier loads or splice plates. Always design to BS EN 1993-1-8 using actual design loads.
What is the difference between BS EN ISO 4014 and BS EN ISO 4017?
BS EN ISO 4014 specifies bolts that are part threaded (unthreaded shank, thread starts partway along the bolt). BS EN ISO 4017 specifies fully threaded bolts, where the thread runs the full length. For structural connections, part-threaded bolts (4014) are preferred because the unthreaded shank can pass through the clearance hole without friction, reducing bending stress and giving cleaner load transfer. Fully threaded bolts (4017) are used when you need threads all the way to the bolt head, or for holding down bolts cast into concrete where the embedded length must be fully threaded for anchorage.
What is HSFG and when should I use preloaded bolts?
HSFG stands for High Strength Friction Grip. These are bolts (usually grade 8.8 or 10.9) tightened to a controlled preload in accordance with BS EN 14399, so that load transfer is by friction rather than shear on the bolt. This method can be advantageous in connections subject to vibration, cyclic loading, or slip-critical design where bolt shear deformation must be minimised. Preloaded bolts require calibrated wrench control and are more expensive to install than ordinary bolts. They are not common in typical building frames unless specified by the engineer for a particular load case.
What hole diameter should I use for M20 structural bolts?
For M20 bolts to BS EN ISO 4014/4017 in clearance holes, the standard hole diameter is 22 mm (2 mm clearance). This is the size shown in BS EN 1993-1-8 and standard connection tables. Oversized holes (typically 26 mm) are sometimes used for tolerance on assembly, but increase clearance and reduce bolt stiffness, so should only be used if explicitly allowed by the engineer. Slotted holes are occasionally used for thermal movement or ease of assembly, but reduce bolt capacity and must be accounted for in design.