CROSS Safety Report
Incorrectly installed tension control bolts could have led to serious consequences
Overview
A reporter finds incorrectly installed system HRC preload bolt assemblies, better known as tension control bolts, during a site inspection of a new highway bridge over a railway. The incorrectly fitted bolts could have had major structural implications, and led to other safety risks, if the installation error had not been spotted.
Key Learning Outcomes
For the contractor’s site team:
- Connections can often be the weak link in structures and attention to detail is required
- A procedure should be followed when critical fixings, such as tension control bolts, are used
- When using tension control bolts, plies must be brought together as the first part of making a joint
- If you are uncertain about technical information provided by the fixing manufacturer, seek clarification from their technical support team
- Quality control and competent supervision on site help to ensure the correct fixings are installed in accordance with the manufacturer’s requirements
- Critical connections must be checked after or during installation, as prescribed in the inspection and test plan
Full Report
Find out more about the Full Report
The Full Report below has been submitted to CROSS and describes the reporter’s experience. The text has been edited for clarity and to ensure anonymity and confidentiality by removing any identifiable details. If you would like to know more about our secure reporting process or submit a report yourself, please visit the reporting to CROSS-UK page.
During an inspection of the site assembly of large steel plate girders for a highway bridge over a railway line, as shown in Figure 1 below, a reporter observed a bolting operative incorrectly installing 24mm diameter system HRC preload bolt assemblies, better known as tension control bolts, at a joint.
The reporter explains that a joint with tension control bolts should be made in two stages. Firstly, drawing the joint plies together using a bedding torque (often with a simple spanner or torque gun) and, secondly, tightening the bolts to their final preload. In the case of tension control bolts, this is done using a special wrench that reacts against a sacrificial spline on the end of the bolt which shears off when a certain torque is achieved. Each stage involves tightening in sequence from the most rigid to least rigid part of the connection, usually from the centre outwards.
In the case observed by the reporter, an operative was tightening in one stage using a specialist wrench and working in no particular order. After completion of the joint, a bolt that was supposedly fully tightened was observed to be loose, as shown in Figure 2, and could be easily unscrewed by hand.
The reporter considers there were two potentially significant consequences:
- The connection clearly contained bolts without the designed preload. This may have had major structural implications if it had not been spotted
- The bridge was over a mainline railway. If the bolt had worked loose, it could have fallen onto the railway with potential risk to the railway
The reporter says that the bolt installer must not have been following the correct bolt tightening procedure and that the site team, it appears, was not inspecting and signing off at each stage. Site bolting of preloaded connections should be adequately controlled through all stages. The reporter adds that responsible persons on contracts with large bolt arrays, such as this one, should consider periodic verification of bolt preload using torque wrenches, lift off jacks, or other appropriate equipment.
The reporter concludes that tension control bolts are simple to install and are the choice of many specifiers. Their simplicity of installation may, however, lull installers into cutting corners as appears could have happened in this case.
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Feedback
I have had some experience with tightening large groups of bolts and, when doing so with a calibrated torque wrench, found that a bolt will lose some tension when adjacent bolts are tightened. This is logical as the tension load in the bolt is balanced by compression in the immediate area of the bolt between the two plates being bolted together. The resulting compressive strain between the plates is small relative to the tension strain in the bolt since the load is spread over a much wider area than the bolt shank. Nevertheless, this compressive strain will be marginally increased when adjacent bolts are tightened leading to a loss in tension in the first bolt tightened. It is therefore necessary to carry out a re-tightening sequence.
Tightening bolt procedures, that rely on achieving adequate bolt strain without reliance on the somewhat variable relationship between applied torque and strain, are preferred. Such methods are the part-turn method and load-indicating washers. However, these methods are unsuitable for re-tightening. I am uncertain of a satisfactory retightening method but would suggest:
a) Initially tightening the bolts by the part turn method measuring the torque required to achieve the part turn and finding an average value of the torque for the bolt group.
b) Re-tighten the bolts in the same sequence by applying the calculated torque with a calibrated torque wrench.
c) Repeat b) above until all bolts are tightened to the same torque within a reasonable tolerance.
The views of others on bolt retightening procedures would be welcome.
This is unambiguously an installation issue on site and could occur, not just with tension control bolts, but with any type of preloaded fastener.
Instruction regarding the installation of preloaded bolting assemblies in connections is detailed in BS EN 1090-2 clause 8 and all installation methods require bolting assemblies to be tightened in at least two stages. In addition, installation shall be carried out progressively from the most rigid part of the joint to the least rigid part.
Comprehensive site-based training for the correct installation of tension control bolts is available.
If any bolting assemblies are used as slave bolts to initially bring steel plies together, they must be marked as such. Once all the bolts in the connection are fully tightened, the “marked” slave bolt assemblies must be removed, discarded and replaced with new bolting assemblies.
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The reporter raises an important subject and makes a number of very appropriate suggestions to help prevent similar occurrences. The tightening of high strength friction grip bolts (HSFG), the forerunner to tension control bolts, received much attention in decades past but has perhaps now been off the horizon for some time. Many procedures are involved but they all start with plies being in contact which necessitates a sequential process of tightening, starting from the middle and progressing out. Not much strain is required to create the tension in a HSFG, and that small amount cannot be used in just pulling plies into contact.
Tension control bolts are no different, as the reporter says, plies must be brought together as the first part of making a joint. The manufacturer’s requirements will show this and must be strictly observed. BS EN 1090-2:2018 Execution of steel structures and aluminium structures - Technical requirements for steel structures provides guidance upon the use of tension control and other preloaded bolts.
plies must be brought together as the first part of making a joint
If a tension control bolt is inadequately tightened, as it appears it was in the case observed by the reporter, its shear strength will not be adversely affected. However, the attributes of slip resistance (in bolt clearance holes), fatigue resistance, and resistance to loosening will all be lost, the latter being especially important in structures supporting moving loads or subject to vibration as with bridge works.
Maintaining the lubrication condition is paramount
While these bolts are looked upon by some as tension controlled bolts, they should be looked upon as torque controlled bolts. The tension in the bolt is critically dependent on the coefficient of friction achieved between the threads on the nut and bolt, which can be significantly affected by changes in lubrication conditions during transport, storage and use - a lesson learned on a number of projects. That said, there are many advantages to using tension control bolts but they must not be seen as a ‘simple’ solution. They require similar checks and balances as applied to their, more labour intensive to install, competitors and predecessors.
Follow an installation procedure
Fixings are critical and often the weak point in any system. Contractors should have a procedure to follow when critical fixings, such as tension control bolts, are used. Technical information should be sought from the manufacturer’s technical support team as required, to assist in developing appropriate procedures.
Competent installation by trained operatives, and effective supervision by competent supervisors, should prevent occurrences such as those reported. A robust inspection and test plan (ITP) should be in place to ensure that structural components, particularly fixings, are suitably and sufficiently designed and installed.
CROSS-UK report 1185 - Wrong length blind bolts lead to unsafe bridge structure, published in 2023, considered inspection and test plans for bolted connections on a bridge.
Note this report was updated on the 04/07/2023