Fin plates
The use of S355 fin plates
Increasing interest in the use of S355 for fin plates prompted questions about the stiffness of such
connections – are they still nominally pinned? David Brown of the SCI presents the results of the
project comparing the behaviour of fin plate connections with both S275 and S355 fin plates.
Existing guidance
Rules for the design and detailing of fin plates were originally presented in
the BS 5950 version of the Green Book1. At the time, fin plates were all from
S275 material. Standardised connection details were presented, with design
rules for each of the components. In support of the introduction of this type
of connection to the UK, a series of physical tests were completed by Moore
and Owens2.
With any nominally pinned connection, ductility is required. One critical
detailing rule to achieve ductile behaviour was therefore that either the
supported beam web, or the fin plate, could be no thicker than d/2 in S275
material or 0.42d in S355 material. This rule was arranged that for Class 8.8
bolts, the bolt shear resistance (perceived as a relatively brittle failure mode)
was no less than the bearing resistance – which was perceived as a ductile
behaviour.
Thus for an M20 Class 8.8 bolt, according to BS 5950, the shear resistance is
92 kN
The bearing capacity for a bolt (assuming the end distance was not critical)
is given by:
Pbs = kbsdtppbs
where:
kbs = 1.0 for bolts in standard clearance holes
d is the bolt diameter
tp is the thickness of the plate
pbs = 460 N/mm2 in S275 and 550 N/mm2 in S355 (from Table 32 of BS 5950)
Thus for a 20 mm bolt in 10 mm thick S275 material, the bearing capacity is
given by:
Pbs = 1.0 × 20 × 10 × 460 × 10-3 = 92 kN
If the material was S355, to ensure the shear resistance of the bolt is not
critical, then
tp <
The advent of the Eurocodes
When the Eurocodes were introduced in 2005, two important changes had
an impact on the rules for the design of fin plate connections. Firstly, the
Eurocode demanded that the connections be formally classified – in the
case of a fin plate to demonstrate that the connection was nominally pinned
and secondly, the bearing resistance according to the Eurocode increased
substantially.
Bearing resistance to BS EN 1993-1-8
According to BS EN 1993-1-8, the bearing resistance is given by:
Fb,Rd =
If end and edge distance do not limit, then k1 = 2.5 and αb = 1.0. In 10 mm
thick S275 material, with fu = 410 N/mm2 the bearing resistance for an M20
bolt becomes 164 kN, much higher than the BS 5950 value of 92 kN, and
much higher than the bolt shear resistance, which according to the Eurocode
is 94 kN for a Class 8.8 M20 bolt. Thus the previous rule to ensure ductility,
that the bearing resistance should be less than the shear resistance, was
impossible to meet in practice.
Connection classification to BS EN 1993-1-8
The Eurocode provided rules for the numerical calculation of connection
stiffness, and a stiffness limit for nominally pinned connections. The rules are
12 NSC
92 × 103
1.0 × 20 × 550
= 8.36 mm or 0.42d
k1bfudt
M2
Technical Digest 2018
unfortunately only appropriate for end plate connections. Clause 5.2.2.1(2)
also allows a joint to be classified on the basis of experimental evidence or
evidence of previous satisfactory performance.
The Green Book to the Eurocode
In 2014, SCI and BCSA published the Eurocode Green Book3. The view taken
was that there was both test evidence and significant previous experience
to demonstrate that the standardised connections performed satisfactorily
in practice, but that was conditional on the previous proven rules being
followed. The Eurocode Green Book was at pains to point out that only the
standardised connections were known to be satisfactory, and that varying
the details might invalidate the proven behaviour. An important part of the
limited scope was that the previous rules regarding fin plate or beam web
thickness must be observed.
Changes to modern practice and the need for research
In recent years, the use of S355 has become more widespread, such that
S355 is now the normal grade for rolled sections in the UK. In parallel, the use
of S355 plate is becoming more common, and some steelwork contractors
wished to use S355 fin plates. The limiting thickness of 8 mm was considered
by many to be simply too thin – and so the need to assess the performance
of fin plate connections with S355 plate was identified. The objective of the
research was simply to compare moment-rotation and stiffness performance
of fin plate connections. If connections with S355 fin plates were markedly
stiffer than those with S275 plates, the classification as nominally pinned
would be threatened.
Research programme
Firstly, an extensive desk study was undertaken to identify tests of fin plate
connections. Physical test results are essential if the Finite Element (FE) model
is to be calibrated – in other words to demonstrate that the FE model is a
good model of the real behaviour. The test results must be comprehensive,
as the measured properties of the components are needed, not just the
nominal values. In addition, the results must be sufficiently detailed to
allow a comparison of the moment-rotation behaviour. After reviewing the
available test results, the original research by Moore and Owens2 was the most
comprehensive containing the necessary data.
For the connection chosen to calibrate the FE model, the comparison
between the FE and the test results is shown in Figure 1.
Figure 1: Comparison of FE and test results