Modular Steel (3/10): How to perform the quality check of a safe Bolted Connection?

Have you been following this blog series about 10 key points of siFramo Modular Steel System? The previous one detailed the unique assembly element of this system, Self-Forming Screw.

We will now focus on:

1. Quality control of these siFramo bolted connections and why is it so important.
2. What we recommend as a best practice to ensure the correct tightening of bolted connections in steel structures?
3. Can a safe connection be spotted by the naked eye or does it require torque-control?

Role of torque control in the quality check of a safe bolted connection

U.S. military Global Hawk aircraft drone crashed in Afghanistan on the 20th August 2011. Investigators concluded the cause of the accident was an improperly torqued fastener. Luckily, no casualties were involved, but it costed millions to learn this lesson.

Any standard bolt that may look tightened might in fact not be. In that case the connection would ultimately get lose by vibration in any form. The siFramo FLS screw however provides an unprecedented level of safety because when the naked eye can see that it has been used, it proves already that it will do 90% of the job. This is because it cannot come out on its own nor by any vibration. It is in this regard neither dependent on a washer, a nut or even a counter nut but entirely on its own vacuum sealed thread connection.
To assure full compliance with Eurocode 3 (EN 1993) for "Design of steel structures", which Sikla uses as basis for calculation to determine load capacities, it's important to follow specified tightening torques and perform torque checks on regular basis.

FLS Screw Torque

siFramo Self-Forming Screw tightening torque specifications

How to torque check a bolted joint?

Torque measurements can be done via static or dynamic methodologies. The first one states that the bolted connection torque control is performed after the tightening, often with a torque wrench. The dynamic method refers to torque measurements performed during the tightening, using a power tool, such as an impact driver. At Sikla UK, we recognize the importance of both methods in our own prefabrication process. We use cordless digitally controlled impact wrenches to achieve consistent results and our technicians perform regular quality audits using calibrated torque wrenches.

Impact Wrench and  Torque WrenchImpact wrench and Calibrated torque wrench

Some of the benefits of using cordless digital impact wrenches which we recognize are:

  • Efficient assembly process
  • High precision
  • Transparent torque audit reports


So why use Sikla?

As part of our service, we supply frames prefabricated in our certified quality-controlled environment. All CE marked systems are subject to the certified factory production control according to EN 1090 and may therefore be used to EXC 2 for load-bearing structures.

Bolted connection assembly and torque control_detailPrefabrication process

Prefabricated steel framesPrefabricated frames ready for dispatch

Alternatively, we can provide advice on selection of the right tools to help you work with precision and efficiency.

Coming up next - don't miss it:


Join the next Sikla UK siFramo webinar focused in an introduction to the system, project and applications. Register now!

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Marek Vargapal

Written by Marek Vargapal

Business Development Engineer (Energy from Waste & Power Generation)