Installation methods
A bolt does its job through preload. There are four honest ways to get there, and they are not interchangeable.
1 · Torque control
A calibrated wrench to a figure from a table that matches the grade, size and lubrication. Simplest and least accurate: ±25–30% preload scatter from friction alone. Fine for general fastening; not sufficient for slip-critical or gasketed joints on its own.
2 · Turn-of-nut
Snug-tight first (the full effort of a worker on a spud wrench, or a few impacts), mark nut and bolt, then a specified further rotation — typically ⅓ to ⅔ turn depending on length. Preload comes from bolt stretch, not friction, so it is far more consistent. The standard method for A325M/A490M under RCSC and for HV assemblies under EN 1090.
3 · Direct tension indicators
A washer with raised bumps that flatten at the design preload; a feeler gauge confirms the gap. Visual verification after the fact — valuable where inspection is separated from installation. Bumps must face the correct part (usually the bolt head), and DTIs are single-use.
4 · Hydraulic tensioning
The stud is stretched hydraulically and the nut run down by hand, then the load released onto the nut. No torsion, no friction, ±10% or better, all bolts on a flange simultaneously. The method for large studs, pressure flanges and wind-turbine and tower foundations. Needs extra thread projection — about 1 × d — for the tensioner puller.
Anchor bolts in concrete
- Template, not string lines. Bolts fixed in a steel or plywood template at the top and, for long bolts, tied at the bottom, so the pour cannot move them. Tolerance for column base anchor bolts is commonly ±3 mm on position and ±6 mm on projection — check the project specification.
- Projection. Base plate + grout + washer + nut + 2–3 threads, plus tensioner allowance if used. Too short cannot be fixed after the pour; too long is trimmed.
- Protect the thread. Cap or tape threads during the pour and until the column is set; concrete laitance in the thread is a common cause of nuts that will not run on.
- Do not bend to fit. A misplaced Grade 55 or 105 bolt is not corrected by heating and bending on site. Notify the engineer.
Flange studs
Star (criss-cross) sequence in at least three passes: 30%, 60%, 100% of target, then a final circular pass at 100% to even out relaxation. Lubricate nut faces and threads with the compound the torque figure assumed. Re-check after the first thermal cycle on hot service.