Eurokodnytt nr1 2011
Eurocode news, issue 1 - 2011 - Fresh ideas in the steel construction sector
Fresh ideas in the steel construction sector The introduction of the Eurocode system and the accompanying implementation standard SS-EN 1090-2 offer several choices of method for fitting bolted joints in steel constructions. One of these is to run process tests on the relevant bolted joints, controlling the tightening parameters to optimise the pre-stressing force on the bolted joint.
Within the Swedish steel construction sector, it is clear that gut feel and rules of thumb have often been decisive in fitting and checking bolted joints in steel constructions, even though the execution was governed by BSK 99 and BSK 07. However, BSK was limited in its choice of methods and execution for fitting and checking the pre-stress level in bolted joints. Historically, too, there have been screw fractures in bolted joints because of incorrect and unverified assembly tools and inadequate knowledge of the effect of the coefficient of friction on the pre-stressing force. In many cases, this caused screw failure, incorrect pre-stressing levels or stripping of threads between bolts and nuts, even though good quality bolts and nuts and technical documentation were used in the bolt application.
The switch to the Eurocode system and the introduction of the governing sizing standards for steel structures, SS-EN 1993-1-1 General rules for buildings, and SS-EN 1993-1-8 Design of joints, has triggered a change process and rethink within the steel construction sector around the fitting of new bolted joints. For applications with bolted joints, SS-EN 1090-2 “Technical requirements for steel structures” has opened up more new technical approaches to fitting and checking bolted joints in steel structures. By following SS-EN 1090-2 it is easier to ensure the pre-stressing force that one wishes to achieve in accordance with the execution requirements. One of the fitting methods for bolted joints described in Appendix H to SS-EN 1090-2 is a ‘process test’, where the pre-stressing force is calibrated with the fitting tool to be used. This method optimises the pre-stressing force in the bolted joint.
The product standards applicable to bolts and accompanying nuts and washers for steel constructions have to meet the requirements in the standards SS-EN 15048-1 for non-preloaded assemblies and SS-EN 14399-1 for preloaded assemblies. The mechanical properties for fastening elements in carbon steel and steel alloys are given in the standards SS-EN ISO 898-1 for bolts and screws and SS-EN ISO 898-2 for nuts.
SS-EN 14399 makes it very clear that products from different parts of the standard must not be mixed together. If they are, the product cannot be sold as CE-marked according to the rules. This can constitute a barrier to public procurement. If combinations from different product standards are mixed, the properties of the screw combination are changed and the documented test values accompanying the product are no longer valid.
Choice of method
The choice of method according to EN 1090-2 is of course entirely dependent on the application and the size of assembly to be undertaken. The different options for the choice of tightening methods and product standards according to EN 1090-2 should be seen as possible ways to achieve the right preload force and economical fitting of bolted joints in our steel constructions.
Nordic Fastening Group AB carries out process tests according to EN 1090-2
With its full range of fastening elements for steel constructions, Nordic Fastening Group AB has supplied fastening elements and conducted process tests on a number of large structures, including the construction of the new Swedish national football stadium in Stockholm, the Swedbank Arena. The tests were carried out both in the lab and in the field. To ensure the pre-stressing force in the bolted joint and provide for efficient assembly, Bult & Fästteknik AB was engaged as supplier of both bolted joints and process tests before assembly started.
Author: Björn Malmqvist, Nordic Fastening Group AB