Technical / Advisory Desk

later, when the NOTE was inserted directly after that subclause: given in EN 1090-1 allows any design method (even BS 5950)
to be used, subject to agreement with the client. So prudent
NOTE If an alternative design rule is substituted for an designers will obtain explicit client agreement as a matter of
application rule, the resulting design cannot be claimed to course. Designers of ‘kit’ structures (predesigned buildings,
be wholly in accordance with EN 1990 although the design towers, temporary bridging and the like) are in a more difficult
will remain in accordance with the Principles of EN 1990. position. As would we all, were the system to decide that this is a
When EN 1990 is used in respect of a property listed in loophole and needs closing.
an Annex Z of a product standard or an ETAG, the use of
an alternative design rule may not be acceptable for CE Conclusions
Marking. The distinction between Principles and Application Rules is
much less important than it first appears, but this is because of
NOTEs are informative, not normative, but this one is quite the way the European Design Standards and Product Directives
threatening: if you want the CE Mark, follow the official interact. To qualify for a CE Mark it may be necessary to conform
Application Rules. with Eurocode Application Rules, not just Principles. Remember
So now a distinction is drawn between a design which is CE Marking became mandatory in July! Supplementary design
‘wholly in accordance with EN 1990’ and a design which is merely rules (from NCCI or of the designer’s own devising) may only be
in accordance with its Principles. permissible if they do not conflict with anything normative in the
Where does this leave us? If we must have a CE Mark, must we Standard. That is the official vision, even if it seems disconnected
obey the Application Rules? Arguably this means that in practice from reality.
the distinction between Principles and Application Rules ceases Even if the CE Marking aspect is put to one side, it does seem
to be worth notice. We can, of course, stop fretting over any fair to suggest that if the Principles / Application Rules distinction
questionable designations but that’s cold comfort. is to be retained in the next generation of Eurocodes it needs to
Advice from BCSA is that client agreement can provide an be expressed in a much more consistent manner than hitherto.
escape route. One of the four different methods for CE Marking Alternatively, let it fade away unmourned.

AD 383
Use of alkali-zinc silicate paint in
slip-resistant bolted connections
The Advisory Desk has been made aware of from German recommendations. However, on also be undertaken to demonstrate whether
some concerns about the use of alkali-zinc re-checking these recommendations it was allowance should be made for loss of preload with
silicate paint in slip resistant connections, such found that the slip factor of 0.4 relates to an time, due to paint creep under the bolt. It should
that the structural reliability of the slip resistant upper thickness limit of 40 microns. The original also be required that the painting inspectors are
connection might be compromised. There is German recommendations also noted that when a trained in the necessary painting workmanship
as yet no published data about the frictional maximum thickness of 60 microns is used, the slip measures required to maintain the quality of the
resistance of surfaces when the coating is factor is reduced, which correlates with findings of coating on the contact surfaces, consistent with
applied in normal production conditions (rather the tests mentioned previously. Hence, Table 18 of the quality in those tests.
than closely controlled test conditions) but EN 1090-2 will be modified, reducing the thickness If additional slip testing on site conditions is
instances of significantly lower slip factors (than to a maximum of 40 microns not to be specified, then, until research can prove
recommended) have been experienced in practice. Until Table 18 is modified, the Advisory otherwise, it is recommended that designers
According to Table 18 of EN 1090-2, an alkali- Desk therefore recommends that, for structural assume a reduced slip factor of 0.30, and that the
zinc silicate paint with a thickness of between reliability, the execution specification should execution specification includes the following
50 and 80 microns may be classed as a ‘Class B’ include additional inspection requirements additional requirements: (a) the paint thickness
surface, with a slip factor (friction coefficient) and acceptance limits, depending on the factor actually applied to the contact surfaces shall
of 0.40. It appears that while this value may be that the designer has assumed. The following is be checked, to ensure that the mean coating
appropriate when the thickness is at the lower suggested: thickness does not exceed 60 microns; (b) the
end of the thickness range and that the curing of When a slip factor of 0.4 has been assumed (for paint manufacturer’s curing recommendations are
the paint has been fully in accordance with the alkali-zinc silicate coated contact surfaces), the rigorously followed.
manufacturer’s recommendations, the slip factor constructor shall be required to carry out slip tests If it is preferred to not impose any additional
is very sensitive to the quality of the application. in accordance with EN 1090-2 Annex G to confirm workmanship requirements for the coating on the
In practice, application does often result in that adequate friction will be achieved as a result contact surfaces, then the designer should assume
thicknesses exceeding 80 microns locally over of the painting and curing procedures that will be a slip factor not greater than 0.2.
the contact surface and in less than ideal curing used on the structure. (Note that such conditions
conditions. Tests modelling practical application are not the same as ‘laboratory conditions.) The
conditions have shown slip factors as low as 0.20. coating thickness for the test should be 25%
The issue of the slip factor to be used with greater than the average thickness achieved on
alkali-zinc silicate paint has been raised with the the contact surfaces under normal production Contact: David Iles
CEN committee responsible for EN 1090-2 and conditions for the structure (rather than the Tel: 01344 636525
it was advised that the slip factor of 0.4 comes thickness stated in G.3 of EN 1090-2). Tests should Email: advisory@steel-sci.com

NSC
28 September 14