Bolting
Nominal minimum preload Fp,C for preloaded bolts (BS EN 1090-2)
- where
- nominal minimum preload (kN)
- nominal ultimate tensile strength of the bolt (N/mm²)
- tensile stress area (mm²)
Explain this equation
Gives the minimum clamping force a preloaded structural bolt must reach once tightened. Use it to set torque values and to check slip-resistant connections.
f_ub is the nominal ultimate tensile strength of the bolt grade: 800 N/mm² for 8.8 and 1000 N/mm² for 10.9. A_s is the tensile stress area of the thread. F_p,C is the resulting preload.
The bolt is tightened to 70% of the force that would break it across the threads, leaving a margin below yield while clamping the plies hard together. The 0.7 is the factor set in BS EN 1090-2 and BS EN 1993-1-8. Dividing by 1000 converts newtons to kilonewtons.
Sense check: an M20 8.8 bolt has a preload of about 137 kN, roughly 14 tonnes of clamp. An M24 10.9 is about 247 kN.
Watch out: this applies only to preloaded assemblies to BS EN 14399. Ordinary non-preloaded bolts are only tightened snug.
The preload a preloaded (HR, HV or HRC) bolt must reach when tightened, used to set torque values and in slip-resistant connection design.
Enter your values to see the result.
Good to know
BS EN 1090-2 cl. 8.5.1 and BS EN 1993-1-8 take Fp,C = 0.7 fub As unless the specification says otherwise. Bolt assemblies must be to BS EN 14399 and used with the tightening method and k-class stated in the execution specification. Typical values: M20 8.8 137 kN; M24 8.8 198 kN; M20 10.9 172 kN; M24 10.9 247 kN.
Common mistake
Using the nominal diameter area instead of As, or applying preload values to non-preloaded BS EN 15048 bolts, which are only tightened to snug-tight.
Limits
Nominal value only. Achieved preload depends on the tightening method, lubrication and calibration.
Sources
- BS EN 1090-2:2018+A1:2024 Execution of steel structures and aluminium structures – Technical requirements for steel structures (8.2.2 bolt protrusion; 8.5 preloaded bolts: Fp,C = 0.7 fub As; torque method Mr,2 = km d Fp,C, first step about 0.75 Mr,i, second step 1.10 Mr,i; combined method Mr,1 = 0.13 d Fp,C simplification; k-classes K0, K1, K2; Annex B tolerances)
- Bolt & Nut Council / TNO report 236: Voorgespannen boutverbindingen (reproduces EN 1090-2 cl. 8.5.2–8.5.4: Mr,2 = km d Fp,C for K2; 0.75 Mr,i first step; 1.10 Mr,i second step, equivalent to 1 + 1.65 Vk with Vk = 0.06; K2 km range 0.10–0.23 from EN 14399)
- BS EN 14399-1 and -3/-4 High-strength structural bolting assemblies for preloading (systems HR and HV, property classes 8.8 and 10.9, k-class declared on the certificate)
- BS EN ISO 898-1:2013 Mechanical properties of fasteners made of carbon steel and alloy steel – Bolts, screws and studs (stress area As = π/4 × ((d2 + d3)/2)², d3 = d1 − H/6; fub 800 N/mm² for 8.8, 1000 N/mm² for 10.9)
Results are calculations from the published methods and figures shown, and depend on the data you enter. Always work to the engineer’s drawings and bending schedules, BS 8666, BS EN 1992, BS EN 1090-2, BS 5531/the BCSA National Structural Steelwork Specification and manufacturers’ data. Lifting must be planned under LOLER by a competent person.