In steel structures the preloading of bolts is not an assembly detail: it is an execution requirement, and the standard states by which methods it can be achieved.
- ✓ EN 1090-2:2018 (+A1:2024), technical requirements for steel structures
- ✓ Section 8 deals with bolted connections
- ✓ Four preloading methods confirmed by the sources consulted
- ✓ No elongation-based method cited by the sources we checked
EN 1090-2 is the execution standard for steel structures. The full title of part 2 is “Execution of steel structures and aluminium structures — Part 2: Technical requirements for steel structures”, that is «Execution of steel structures and aluminium structures — Part 2: Technical requirements for steel structures». The reference edition is that of 2018, supplemented by amendment A1 of 2024.
If your interest concerns bolted connections, the section to read is section 8, devoted to bolted connections. It is there that the standard addresses the subject of preload: the tensile load to be introduced into the shank of the bolt at tightening, which determines the behaviour of the connection under service load, particularly for slip-resistant shear connections.
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The operational distinction that matters is between bolts that are simply tightened and preloaded bolts. In the second case the value of the preload is a design datum, and the erector must be able to demonstrate that it has been reached: hence the need for defined and verifiable methods, which the standard lists.
The four preloading methods
| Method | Principle |
|---|---|
| Torque method | The preload is obtained by applying a defined tightening torque |
| Combined method | Initial torque followed by a controlled further rotation |
| HRC method | Use of tension control bolts with spline ends that shear off |
| Direct tension indicator (DTI) method | Indicating washers that show when the preload has been reached |
| Normative reference | EN 1090-2:2018 (+A1:2024), section 8, bolted connections |
What the sources consulted say and do not say
The sources we checked — the official contents list of the standard and sector technical documentation — confirm the four preloading methods listed above: torque, combined, HRC and direct tension indicator.
We prefer to tell you just as plainly what those same sources do not report: neither of them cites preloading methods based on measuring the elongation of the bolt, nor ultrasonic methods. We therefore do not claim that EN 1090-2 provides for them. Anyone who has to assess the use of such a technique on a structure subject to the standard must verify its admissibility against the text in force and agree it in the execution documentation.
Ultrasonic measurement of elongation remains in any case an independent testing technique, widely used in other industrial contexts where it is necessary to know the load actually present in a bolt rather than the torque applied.
Recommended instruments
| Instrument | Use |
|---|---|
| Dakota BT1-DL | Portable bolt tension monitor, for use on site |
| Dakota BT2-DL | Benchtop version for testing and characterisation in the workshop |
| Dakota MultiMax | Remote monitoring on four channels |
| MultiUI | Configuration and monitoring of the MultiMax system |
Frequently asked questions
Part 2, titled Execution of steel structures and aluminium structures — Part 2: Technical requirements for steel structures. The reference edition is that of 2018, supplemented by amendment A1 of 2024.
The sources consulted confirm four methods: the torque method, the combined method, the HRC method and the direct tension indicator (DTI) method. The subject is dealt with in section 8 of the standard.
The sources we consulted do not cite elongation-based or ultrasonic preloading methods, and we prefer to say so openly rather than to assume it. Any such use must be verified against the text of the standard in force and agreed in the execution documentation.
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