AD 428A: Lateral and torsional vibration of half-through truss footbridges
This AD note was first issued to provide interim guidance on the design of half-through footbridges. It has now been revised following publication of updated standards and is issued as AD428A.
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. Until the recent publication of NA+A1:2020 to BS EN 1991-2:2003 Incorporating Corrigendum No. 1, Eurocodes and UK National Annexes did not fully address this mode of vibration, so there was a danger that it may have been discounted without proper consideration. This previous 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.
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.
UK design criteria prior to issue of “NA+A1:2020 to BS EN 1991-2:2003 Incorporating Corrigendum No. 1”
The criteria for assessing the dynamic behaviour of footbridges were 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 contained the following requirements:
- Eurocode EN 1990 clause A18.104.22.168(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 A22.214.171.124(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 provided limiting horizontal accelerations for deliberate lateral shaking of the bridge.
A literal reading of all the applicable clauses therefore led to the conclusion that a lateral-torsional mode with frequency less than 2.5 Hz should be verified for horizontal acceleration as EN 1990 clause A126.96.36.199 (2) still applies. However, no acceleration limit was provided as EN 1990 clause A188.8.131.52(1) was modified by the UK NA to BS EN 1991-2 which, itself, did not provide a limit.
Updated provisions in NA+A1:2020 to BS EN 1991-2:2003 Incorporating Corrigendum No. 1
The following requirements have been made in NA+A1:2020 to BS EN 1991-2:2003 Incorporating Corrigendum No. 1 to address the original problems noted above:
- The design should conform to the requirements of BS EN 1990 clause A184.108.40.206(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.
- The maximum acceptable acceleration for horizontal movement under normal use should be taken as the recommended value given in BS EN 1990 clause A220.127.116.11(1)[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.
- 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].
- 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.
- 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