Advisory desk
NSC 29
Technical Digest 2019
They contain the following requirements:
• Eurocode BS EN 1990 clause A2.4.3.2(2) requires comfort to be verified if
the natural frequency is lower than 2.5 Hz for lateral and torsional modes;
• BS EN 1990 clause A2.4.3.2(1) states that comfort criteria should be
defined in terms of maximum acceptable acceleration and proposes
a horizontal limit for lateral and torsional vibrations of 0.2 ms-2 under
normal use and 0.4 ms-2 for exceptional conditions, but makes these
values nationally determined parameters;
• Clause NA.2.3.10 of the UK National Annex to BS EN 1990 states that
the pedestrian comfort criteria should be as given in NA.2.44 of the
UK National Annex to BS EN 1991-2. However, this clause does not
specify a maximum acceptable acceleration for horizontal movement
under normal use – it (and PD 6688-2) only address synchronous lateral
vibration caused by lateral forces from footfall and does not address
lateral and torsional modes excited by vertical loading.
None of the documents provide limiting horizontal accelerations for
deliberate lateral shaking of the bridge.
A literal reading of all the applicable clauses therefore leads to the
conclusion that a lateral-torsional mode with frequency less than 2.5 Hz
should be verified for horizontal acceleration as BS EN 1990 clause
A2.4.3.2 (2) still applies. However, no acceleration limit is provided as
BS EN 1990 clause A2.4.3.2(1) is modified by the UK NA to BS EN 1991-2
which, itself, does not provide a limit.
Interim recommendations
Work is under way to update the relevant Eurocodes via BSI and CEN.
However, the following interim recommendations are made until such time
as the suite of codes above are made consistent.
i. The design should conform to the requirements of BS EN 1990 clause
A2.4.3.2(2) i.e. a verification of the comfort criteria should be performed
if the fundamental frequency of the deck is less than 5 Hz for vertical
vibrations, and 2.5 Hz for horizontal (lateral) and torsional vibrations.
ii. In the absence of a maximum acceptable acceleration for horizontal
movement under normal use being specified by NA.2.44 of the UK
National Annex to BS EN 1991-2, the recommended value given in
BS EN 1990 clause A2.4.3.2(1) should be used (i.e. 0.2 ms-2), measured at
the level of the deck. The acceleration should be calculated under the
vertical load models of NA.2.44 considering walking paths offset from
the bridge centreline as necessary.
iii. Where the fundamental frequency of the bridge is less than 3 Hz for
horizontal (lateral) and torsional vibrations, consideration should be
given to making provision in the design, in discussion with the client,
for possible installation of dampers to the bridge after its completion.
(This recommendation makes some allowance for uncertainty in the
value of damping and other parameters used in the calculations and
also provides some potential remedy for unacceptable horizontal
accelerations from deliberate shaking should they occur).
iv. Any further limiting criteria for pedestrian comfort, such as under
deliberate shaking, should be determined on a project-by-project basis
and agreed with the client.
v. The potential for unstable lateral responses (synchronous lateral
vibration) should still also be checked using NA.2.44.7 of the UK National
Annex to BS EN 1991-2.
Chris Hendy, Atkins SNC-Lavalin
Chair of SCI’s Steel Bridge Group
Contact: Richard Henderson
Tel: 01344 636555
Email: advisory@steel-sci.com
AD 429
Slip factors for
alkali-zinc silicate paint
This AD note draws attention to the slip factors for alkali-zinc silicate painted
faying surfaces considered in AD 383 which have been updated in the 2018
revision of BS EN 1090-2.
AD 383, which was published in September 2014, discussed the slip factor
for surfaces coated with alkali-zinc silicate paint and the significant influence
of the coating thickness. The AD referred to forthcoming changes to Table
18 of BS EN 1090-2, expected to reflect concerns about the relationship
between the coating thickness and slip factor. In the interim, AD 383
proposed slip factors of 0.3 (if certain recommended practices were followed)
or 0.2 as a conservative value.
BS EN 1090-2 was revised in 2018 and slip factors are presented in Table 17.
For surfaces coated with alkali-zinc silicate paint, the nominal thickness is now
specified as 60 μm, with a dry film thickness between 40 μm and 80 μm.
If the applied coating meets the thickness limits specified in Table 17, a
slip factor of 0.4 may be assumed. AD 383 noted that in practice the coating
thickness can often exceed 80 μm, so coating procedures will need to be
carefully controlled and the dry film thickness measured, to ensure the limits
in Table 17 are satisfied. If such control is not practical, then the conservative
slip factors quoted in AD 383 may be adopted.
Contact: Richard Henderson
Tel: 01344 636555
Email: advisory@steel-sci.com
AD 430
Wind load on unclad frames
The purpose of this note is to correct errors in BRE Special Digest SD5 which
lead to the prediction of significantly higher wind loads on unclad frames
than were calculated using the report which SD5 superseded.
BRE published Special Digest SD5 in July 2004. The document was
produced principally because at the time, the current guidance for
determining wind loads on frames, lattice structures and individual members
was based on the BS code of practice CP3 Chapter V: Part 2 which had
been withdrawn in October 2001. SD5 is based on BS 6399-2 and includes
guidance on determining loads on individual members and lattice structures.
It also includes a section on unclad building frames which is based on and
intended to supersede BRE report BR173, Design guide for wind loads on
unclad framed building structures during construction.
BR173 considers a series of identical parallel frames of overall width W at
spacing S. The parameter S is used to select the appropriate normal force
coefficient CD according to the ratio W/S and the total solidity ratio denoted
φ. In a given direction, φ is presented in BR173 as the sum of the horizontal
and vertical solidity ratios: φ = φv + φh. In the direction perpendicular to the
secondary beams, the horizontal solidity ratio used is the equivalent solidity
ratio which allows for all the secondary beams in a bay denoted φ = φv + φh*
(see item iii in the design example in BR173 para. 4.2.2). In SD5, the total
solidity ratio is erroneously given as φ = φv + φh + φh,s ie the equivalent
horizontal solidity ratio φh,s is added to, instead of substituted for the
horizontal solidity ratio φh. The total solidity ratio in this direction should be
given in SD5 as φ = φv + φh,s .
The spacing of the secondary beams is used in the determination of the
equivalent solidity ratio for secondary beams. In BR173, this parameter
is also denoted S and is likely to be different from the frame spacing but
unfortunately, SD5 does not differentiate between the two parameters.
In SD5, the relevant equation is no. 11: φh,s = (φ1 + φ2)φh where
φ2 = (n – 1)(S/d – 7.5)/25. According to BR173, in the expression for φ2 the
parameter S is the secondary beam spacing not the frame spacing.
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