Column bases
Holding-down bolt projection above concrete
- where
- required projection above concrete (mm)
- grout gap (mm)
- base plate thickness (mm)
- washer thickness (mm)
- nut height (mm)
- full threads beyond the nut
- thread pitch (mm)
Explain this equation
Works out how far a holding-down bolt must stand above the concrete so the nut is fully engaged. Use it when specifying cast-in bolts or checking them before erection.
g is the grout gap under the base plate, t_p the plate thickness and t_w the washer or plate washer. m_n is the nut height. n_t is how many full threads should show beyond the nut and p is the thread pitch. P is the minimum projection.
Everything stacked on the bolt simply adds up: grout space, plate, washer and nut, then a short length of thread to prove the nut is fully on. BS EN 1090-2 wants at least one full thread beyond the nut.
Sense check: an M24 with a 40 mm gap, 25 mm plate, 6 mm washer and 21.5 mm nut, plus two threads, needs about 100 mm.
Watch out: bolts can be cast up to 5 mm low and still be within tolerance, so leave a few millimetres in hand.
Minimum length of holding-down bolt that must stand above the foundation so the nut is fully engaged once the grout gap, base plate and washer are in place.
Enter your values to see the result.
Good to know
BS EN 1090-2 cl. 8.2.2 requires the bolt end to protrude beyond the nut after tightening; at least one full thread, one or two is ideal. SCI guidance gives the top of bolt level as +25 mm to −5 mm of the specified level, so a bolt cast 5 mm low must still give full nut engagement. A typical grout gap is 25–50 mm. Bolt holes in base plates are usually 6 mm over the bolt diameter (NSSS), with loose plate washers.
Common mistake
Forgetting the grout gap or plate washer when specifying cast-in bolts, leaving the nut short of full engagement.
Limits
Geometry only. Embedment, anchorage and bolt capacity are the designer’s responsibility.
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)
- 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)
- SCI P178 Design for construction, column bases (typical 25 mm to 50 mm grout gap with non-shrink cementitious grout; sleeve about 3 × bolt diameter; top of bolt within +25 mm to −5 mm of specified level)
- BCSA National Structural Steelwork Specification, 5th edition (cl. 3.4.5 HD bolt holes 6 mm over bolt diameter; 8.1.6 steel packs; 9.2 foundation and HD bolt check before erection; 9.6 erected components)
- BCSA National Structural Steelwork Specification for Building Construction, 7th edition, 1st revision (2023) – Section 9 accuracy of erected steelwork; Section 7 cambers; 8.6.2 reference temperature 20 °C
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.