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COEFFICIENT OF FRICTION

FRICTION COEFFICIENT FOR ASSEMBLIES WITH PRECONTROLLED BOLTS

(CTICM – CMI sheet 1-2011)

Prestressed bolt connections slip-resistant

Assemblies with prestressed bolts conforming to EN 14399 shall be implemented in accordance with the standard of performance NF EN 1090-2 paragraphs 8 and 12.
With regard to the resistance to the parallel forces to the contact planes of the prestressed bolted connections, the calculation standard NF EN 1993-1-8 (paragraphs 3.4 and 3.9) allows the mobilization of the relative sliding resistance between the parts. For a category B assembly, this slip resistance is required for the service limit state loads, and for a category C assembly, it is required for the ultimate limit state loads.
The assemblies concerned are typically the continuity assemblies by “joint-cover” plates of the beams or lattice bars and, especially when the buildings concerned are in seismic zones, the assemblies of the brace bars on gussets. Stainless steel component assemblies shall not be designed as slip resistant.
Obtaining the expected slip resistance depends very much on a proper implementation of the assembly concerned, which requires first:

  • To have contact surfaces ensuring the expected coefficient of friction,
  • Then, to be able to bring the initial initial docking elements to the contact surfaces,
  • And finally, to achieve a proper tightening of all the bolts to achieve at least the prestressing calculation in each of them.

To verify that these conditions are met, supervision before, during and after bolt tightening is required.
This sheet gives information on the coefficient of friction. The tightening of prestressed bolts will be the subject of an upcoming sheet.
Imposing a “non-slip” criterion between the assembled elements is intended to ensure that the selected limit state ensures that the assembly functions very close to perfect continuity. However, a verification of the resistance of the assembly to the ultimate limit state by considering it of category A as if the bolts were working in shear and in diametric pressure, is also required for a category C assembly as well as for those of category B.

Note 1: For category C assemblies, the occurrence of excessive sliding is not totally excluded following either an exceptional event (shock, explosion, fire, collapse of a foundation, etc.) or an error implementation (improper tightening of bolts and / or contact surfaces that do not comply with the prescribed conditions). Whatever the cause of the slipping event, this should neither lead to a collapse of the assembly nor cause instability of the structure concerned. To make sure of this, we make a verification of the bolts in shear and diametric pressure. In this respect, it is recalled that it is well established that the shear strength of a bolt is independent of the level of prestressing introduced by the clamping.

Note 2: For guidance on the use of prestressed bolts and the assembly category, see Data Sheets 4 and 5 published in IJC # 4-2009 and IJC # 5-2009 under the heading ” When should prestressed bolts be used? “.

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Preparation of contact surfaces

We recall here the main requirements of paragraph 8.4 of the standard NF EN 1090-2 for this type of assembly:

  • The value of the coefficient of friction must in general be determined by tests specified in Annex G.
  • The area of the contact surfaces in prestressed joints shall be specified.
  • The contact surfaces must be free of any contamination, for example oil, dirt or paint.
  • Burrs likely to prevent precise docking of the assembled parts must be eliminated.
  • Uncoated surfaces should be rust free or other non-adherent material.
  • Untreated areas around the perimeter of the tight assembly should not be processed until after the assembly check has been completed.
  • Care must be taken not to damage or polish a rough surface.

Tests for the determination of the coefficient of friction

Appendix G of the NF EN 1090-2 standard describes the tests to be carried out to establish what it defines as the “nominal coefficient of friction” between the contacting surfaces, coated or uncoated, of metal parts assembled by prestressed bolts. , in an assembly resistant to friction by axial stress of the assembled parts.
Bolts specially instrumented and calibrated are used to control with adequate precision the prestresses prevailing therein. Up to three different types of tests can be done in sequence on “cover-up” type specimens of double overlap plates assembled by prestressed bolts.

“Short term” slip test: at least four tests are required, thus providing 8 values of the coefficient of friction.

“Deferred under constant load” slip tests: at least one such test is required. Based on the results of the “short term” tests, a specimen is placed under a constant load equal to 90% of the predictable slip load. The charge is maintained for a period of at least 3 hours. The criterion to be respected is that the slip between 5 minutes and 3 hours under the constant load is not greater than 0.002 mm.

“Extended” creep tests: only if the previous test fails, at least three prolonged creep tests are required. This is to determine the level of the load that will not produce a slip greater than 0.30 mm for the required service life. This type of test is almost always required for assemblies of painted elements.

Fixed values of friction coefficients for certain surfaces

Given the evolution of the criteria and test procedures compared to those of the old standards, and the formulas of painting, it is not advisable to adopt, for the coefficients of friction, values published there are more twenty years old. It is clear that few results are available for modern paints, which correspond to the tests of standard NF EN 1090-2.
In Table 18 of this standard, reproduced below, are provided by 4 surface classes, fixed values of the coefficient of friction that can be used in the calculations.

Classifications that can be considered for friction surfaces

For galvanized surfaces, the following table gives the values proposed by the CTICM, following a study carried out as part of a research funded by the Corem (n ° 92-1).

Source : Galvazinc

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