Bolting
Tightening torque for preloaded bolts, torque method (BS EN 1090-2)
- where
- reference torque Mr,2 (N·m)
- k-factor from the bolt certificate
- nominal bolt diameter (mm)
- nominal minimum preload (kN)
- first step torque (N·m)
- final torque (N·m)
Explain this equation
Gives torque wrench settings for the two-step torque method of tightening preloaded bolts. Use it when the specification calls for the torque method with K2 class bolts.
k_m is the k-factor printed on the bolt manufacturer’s certificate for that batch; it describes how much torque is needed per unit of clamp force and diameter. d is the bolt diameter and F_p,C the target preload. M_r is the reference torque, M₁ the first pass and M₂ the final pass.
Most of the torque goes into overcoming friction under the nut and in the threads, so torque rises with both bolt size and clamp force, linked by the measured k-factor. The first pass at 0.75 pulls all the plies together evenly; the final pass at 1.10 adds a margin so that most bolts reach at least the target preload despite scatter. With d in mm and F_p,C in kN the result comes out in N·m.
Sense check: an M20 8.8 bolt with k_m = 0.15 needs about 410 N·m reference torque, about 310 N·m first pass and 450 N·m final pass.
Watch out: k_m only holds for the assemblies as delivered. Dry, rusty or re-lubricated bolts can be out by 30% or more.
Torque wrench settings for the two-step torque method of tightening preloaded bolts, from the k-factor on the bolt manufacturer’s certificate.
Enter your values to see the result.
Good to know
BS EN 1090-2 cl. 8.5.3: reference torque Mr,2 = km d Fp,C using km from K2 class assemblies, first step about 0.75 Mr,2 on all bolts in the group, then a second step at 1.10 Mr,2. With d in mm and Fp,C in kN the answer comes out directly in N·m. The torque method needs K2 bolts. For the combined method the first step can use Mr,1 = 0.13 d Fp,C (enter km = 0.13), followed by the specified part turn. Wrenches must be calibrated (±4% to BS EN ISO 6789).
Common mistake
Using a generic torque table instead of the batch km, or tightening in one pass. Torque scatter can be ±30% if bolts are dry, re-used or the lubricant is disturbed.
Limits
Valid only for the assemblies as delivered (same lot, lubrication state). Does not apply to K0 bolts. The execution specification and supplier’s instructions govern.
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)
- SteelConstruction.info: Preloaded bolting (EN 14399 HR and HRC systems, k-class required per tightening method: torque method K2; combined K2 or K1; torque scatter up to ±30%)
- SCI Guidance Note GN 7.05: Preloaded bolts (torque method requires K2; wrench accuracy ±4% to BS EN ISO 6789; at least one thread protruding beyond the nut, EN 1090-2 8.2.2)
- 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)
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.