European research project "SIROCO"

Juli 2014 until June 2017 Execution and reliability of slip-resistant connections for steel structures using CS and SS

Contact person at the Institute for Metal and Lightweight Structures: Nariman Afzali M.Sc.

Bolted preloaded slip-resistant connections are traditionally used in steel structures when slip and/or deformation have to be highly restricted and in cases where the structure is subjected to variable loading which may lead to fatigue damage. Main application fields are e.g. bridges, cranes, radio masts and tubular towers as well as truss girders for wind turbines.
The slip resistance of these connections is mainly influenced by the friction of the contact surfaces, the level of preload in the bolts and imperfections of the structural plates clamped together. The procedures to determine the slip resistance as described in EN 1993-1-8 "Design of Joints" and EN 1090-2 "Execution of steel structures and aluminium structures - Part 2 Technical requirements for steel structures" are not very clear for use in practice and urgently need improvement and clarification. The aim of SIROCO was to clarify these open questions.
Furthermore, alternative modern types of fasteners in slip-resistant connections are lock bolts, H360 bolts and injection bolts. Direct tension indicators (DTI) are promising means to assure the correct preload during tightening. For this reason, these alternative bolts and methods were considered in SIROCO as well. To determine the characteristics of the injection resin for injection bolts, similar procedures need to be followed as for determining the slip factor. It is questionable whether this is adequate for non-preloaded and preloaded injection bolts.
The friction of surfaces in slip-resistant connections is described by the slip factor µ which is given for carbon steel for some surface treatments in the execution standard for steel structures, EN 1090-2. For surface treatments which are not explicitly cited in EN 1090-2, e.g. hot-dip galvanized carbon steel plates and modern coating systems, a slip factor test is required, which is standardized in Annex G of EN 1090-2.
These existing rules are valid for carbon steel only and so comprehensive investigations were conducted on the preloading behaviour and slip resistance behaviour of preloaded bolted connections made of austenitic, duplex, lean duplex and super duplex stainless steel.
Herewith, the main technical objectives of the project were:

  • to improve the cost effectiveness of slip-resistant connections made of carbon steel by considering innovative bolts and preloading methods as well as innovative coating systems and fill important gaps in understanding regarding galvanized steel connections and
  • to generate new information on stainless steel connections with regard to preloading of stainless steel bolts and to the execution of slip-resistant connections made of stainless steel.
  • The overall study included the exploitation of design solutions targeting
  • an improved test procedure for measuring the slip factor,
  • innovative preloading methods and coating systems for carbon and stainless steel surfaces to increase the surface roughness,
  • development of design and execution rules for preloading of stainless steel bolts and slip-resistant connections made of stainless steel.

All reports listed below can be downloaded in PDF format.

Table of reports

Work package Title Deliverables and final report
WP 1 Test Procedure Slip Factor  
Task 1.1 Recommendation of applicable methods for measuring the preload in bolts Download
Task 1.2 Specification of the test speed for the test procedure Download
Task 1.3 New criteria for the determination of the slip load Fsi Download
Task 1.4 Criteria for the creep and extended creep tests Download
Task 1.5 Preloading procedures to achieve a reduced preloading level in the elastic range of the bolt material with sufficient reliability Download
WP 2 Long Term Effects (CS)  
Task 2.1 Slip factors for the preload level of Fp,C for bolt classes 10.9 and 8.8 Download
Task 2.2 Specification of the test speed for the test procedure Download
Task 2.3 New criteria for the determination of the slip load Download
WP 3 Alternative Bolts and Preloading Methods (CS)  
Task 3.1 Usability of lock bolts and H360® in slip-resistant joints including re-tightening of these alternative fasteners and estimation of the loss of preload Download
Task 3.2 One technical report of test series on non-preloaded and preloaded injection bolts, set ups of analytical models and guidelines for design and installation of injection bolts Download
Task 3.3 Loss of preloading in DTI assemblies and criteria for using DTIs in slip-resistant connections Download
WP 4 Alternative Surface Treatments and Coatings (CS)  
Task 4.1 Slip factors for innovative surface preparations of carbon steel and definition of an optimized thickness for the low dry film primer Download
Task 4.2 Slip factors for different preparations of hot-dip galvanized steel surfaces and corrosion behavior of various coating systems Download
Task 4.3 Slip factors depending on application on storage parameters Download
Task 4.4 Corrosion behavior of ethyl-silicate-zinc coatings Download
WP 5 Preloading of SS bolts  
Task 5.1 Report on available material data Download
Task 5.2 Report on creep and stress relaxation behavior of stainless steel plates Download
Task 5.3 Report on creep and stress relaxation behavior of stainless steel bars and bolts Download
Task 5.4 Preloading behavior and preloading levels for stainless steel bolt assemblies including relaxation with detailed specifications for recommended preloading methods Download
Task 5.5 Design specifications for preloading bolted connections made of stainless steel Download
Task 5.6 Report on numerical model calibration Download
WP 6 Slip-Resistant Connections of SS  
Task 6.0 Topography between contact surfaces in slip-resistant bolted joints Download
Task 6.1 Surface roughness data of stainless steel plates Download
Task 6.2 Slip factors for typical stainless steel surface finishes and new types of coatings for stainless steel Download
Task 6.3 Report on the parametric study Download
Task 6.4 Design recommendations and design examples Download
WP 7 Guidelines and Exploitation Activities  
Task 7.1 Guidelines on the design and execution of slip-resistant connections made of CS and amendments to Eurocode 3 and EN 1090-2 with background documentation Download
Task 7.2 Guidelines on the design and execution of slip-resistant connections made of SS and amendments to Eurocode 3 and EN 1090-2 with background documentation Download
Final Report   Download

In the following the goals and objectives of each work package are described in more detail.

Work Package 1

The main objectives of this WP were as following:

  • to achieve an improved test procedure for the determination of slip factors and
  • to close the lack of undefined or unclear defined rules given in the test procedure of Annex G of EN 1090-2.

Work Package 2 - Long term effects (CS)

In EN 1993-1-8 for preloaded connections the preload is related to the ultimate tensile stress of the bolt material. In EN 1090-2 rules are given how to perform the tightening of the bolts dependent on the chosen tightening method in order to reach the intended preload in the bolts of slip-resistant connections. In these rules attention is paid to the fact that after performing the tightening the preload in the bolts will decrease due to deformations in the threads of the bolts and nuts. This effect is compensated by a certain "overload" when using the torque method and when using the turn of the nut method or the combined method, this compensation is not necessary. The effect of creep caused by compressing the applied surface treatment such as galvanising or paint is of course investigated in the procedure of determining the slip-factor.
However, when the preload will be related to the equivalent yield stress level of the bolt material, only the torque-based tightening methods can be used. Then the reliability of the slip-resistance of the connection relies not only on the reliability of the friction coefficient but also on the reliability of the level of preload in the bolts.
The work programme aims at obtaining sufficient theoretical and experimental evidence to improve and extend the existing rules in EN 1993-1-8 and in EN 1090-2.

Work Package 3 - Alternative bolts and preloading methods in slip resistant connections (CS)

The main objectives of this WP were the investigation whether alternative bolts or preloading methods than HV or HR bolts can be used in slip-resistant connections with sufficient reliability.
The most promising alternatives are:

  • Lock Bolts, which are already widely used in mechanical engineering so far,
  • H360® system of Alcoa Fastening Systems,
  • Injection Bolts, further development and optimization of connections with injection bolts to achieve slip and creep resistant bolted connections considering various influencing parameters and
  • Direct Tension Indicators, so called DTIs. The use of DTIs is an alternative preloading method.

The main focus in this WP was on the loss of preload using the alternative bolts resp. method due to the fact that for slip-resistant connections it must be sure that the preload will be constant over the life time.

Work Package 4 - Alternative surface treatments and coatings (CS)

The main objectives of the WP were the investigation of the influence of various surface parameters on the slip factor of carbon steel and hot-dip galvanized steel applications.

  • Carbon Steel applications
  1. Influence of surface preparation and blasting material (chill casting, slag, steel casting)
  2. Influence of coating material type or coating system
  3. Influence of application conditions, conditioning period and stability in storage
  • Hot-dip galvanized steel applications
  1. Influence of surface preparation and post treatment
  2. Determination of level of preload loss
  • Investigation of the corrosion protection of carbon steel applications:
  1. Stress application by salt spray test according to ISO 9227 (NSS)
  2. Stress application by continuous condensation according to ISO 6270-1
  3. Determination of adhesive strength by cross-cut (ISO 2409) or pull-off test (ISO 4624)
  4. Visual evaluation after the end of the stress application with regard to corrosion protection properties
  5. Measurement of corrosion and delamination around a scribe

Work Package 5 - Preloading of stainless steel bolts (SS)

The main objective of this work package was to provide

  • preloading levels and
  • preloading methods

For stainless steel connections taking into account the effect of material relaxation in bolt assemblies. For this reason, the relaxation of bolts and plates as isolated elements will be studied as well as the relaxation behaviour of the whole assembly to clearly separate the base material behaviour from other effects resulting from the assemblies (such as friction of the threads and so on).
The characteristics of creep of selected stainless steel grades at room temperature will be determined using a combination of tensile tests and creep tests. The test results will be used to define an appropriate mathematical model for the creep deformation of stainless steel materials for numerical simulation preloaded bolt connections.

Work Package 6 - Slip-resistant connections made of stainless steel (SS)

The main objectives of this WP were as following:

  • to provide design parameters (slip factors) of preloaded joints subjected to shear load for slip-resistant connections
  • under consideration of various surface preparations and/or coatings of the clamped plates.

Work Package 7 - Guidelines and exploitation activities

This WP was enabled effective dissemination of major project deliverables by:

  • Preparing guidelines on the design and execution of slip-resistant connections separately for carbon and stainless steel connections;
  • Preparing amendments to EN 1090-2 regarding the slip-test procedure and enhanced slip factors considering modern surface treatments and alternative coatings for connections made of carbon and stainless steel;
  • Preparing amendments to EN 1993-1-8 for better use of slip-resistant connections considering different load levels guaranteeing sufficient reliability;
  • Preparing amendments to EN 1993-1-8 for connections with injection bolts and to EN 1090-2 for testing and execution of connections with non-preloaded and preloaded injection bolts;
  • Preparing amendments to EN 1090-2 for the application of lock bolts and direct tension indicators;
  • Preparing amendments to EN 1993-1-4 for including design rules for preloading of stainless steel bolts.