Advisory Desk
AD 428:
Draft guidance: lateral and torsional vibration of
half-through truss footbridges
Purpose of this guidance
This note alerts designers to
the potential susceptibility of
narrow half-through footbridges
to excitation by pedestrians in a
lateral-torsional mode. Eurocodes
and UK National Annexes do not
currently fully address this mode
of vibration, so there is a danger
it may be discounted without
proper consideration. This gap in
the standards has led to the need
to retrofit dampers and/or provide
additional stiffening to some
recently constructed footbridges
where excitation occurred due
to pedestrians walking eccentric
to the deck centreline and, more
significantly, from deliberate
shaking of the deck.
NSC 27
March 19
Affected mode of vibration
Half-through footbridges, without
plan bracing to the top chord, often
have as their lowest natural mode of
vibration a lateral-torsional mode.
A typical example is shown in
Figure 1. The mode occurs because
the open bridge cross-section has a
low torsional stiffness with a shear
centre below the deck level about
which axis the rotation occurs.
Current UK design criteria and
their interpretation
The criteria for assessing the
dynamic behaviour of footbridges
are outlined in the following
Eurocodes (BS EN) and BSI Published
Documents (PD):
• BS EN 1990:2002+A1:2005 as
modified by UK National Annex
• BS EN 1991-2:2003 as modified by
UK National Annex
• PD 6688-2: 2011
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 lateraltorsional
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-byproject
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
Figure 1 – Lateral and torsional mode of vibration
/Half-through_bridges#Half-through_footbridges
/Design_codes_and_standards#National_Annexes
/Design_of_steel_footbridges
/Design_for_half-through_construction
/Floor_vibrations#Damping
link