Listening to the Mark Patterson show on Radio Foyle earlier and hearing plans of a proposed Marathon section across the Peace Bridge, my first thought was to Tweet a caution quoting the warning on the Albert (Chelsea) Bridge in London.

Soldiers from Chelsea Barracks when crossing the Bridge had a sign dating from 1973 warning marching ranks to break step while crossing. Bearing in mind that the runners will include men women and children, the thought crossed by mind as to what effect 2,000 marathon runners pounding across Derry’s Peace Bridge might have?

Without sounding too much of a Jonah - and ignorant of the construction logistics of the Peace Bridge, I believe the following, based on notes from Wikipedia, is worth repeating:

As a comparison, the Millennium Bridge, officially known as the London Millennium Footbridge, is a

steel suspension bridge for pedestrians crossing the Thames in London. Construction of the bridge began in 1998 and opened on 10 June 2000.

Londoners nicknamed the bridge the "Wobbly Bridge" after participants in a charity walk to open the bridge felt an unexpected and, for some, uncomfortable swaying motion.

The bridge's movements were caused by a 'positive feedback' phenomenon, known as synchronous lateral excitation. The natural sway motion of people walking caused small sideways
oscillations in the bridge, which in turn caused people on the bridge to sway in step, increasing the amplitude of the bridge oscillations and continually reinforcing the effect. Unexpected lateral vibration caused the bridge to be closed on 12 June 2000 for modifications. After two days of limited access the bridge was closed for almost two years while modifications were made to eliminate the wobble entirely.

The wobble was attributed to an under-researched phenomenon whereby pedestrians crossing a bridge that has a lateral sway have an unconscious tendency to match their footsteps to the sway, thereby exacerbating the sway. The tendency of a suspension bridge to sway when troops march over it in step was well known, which is why troops are required to break step when crossing such a bridge. On the day of opening the bridge was crossed by 90,000 people, with up to 2,000 on the bridge at any one time.

Any bridge with lateral frequency modes of less than 1.3 Hz, and sufficiently low mass, could witness the same phenomenon with sufficient pedestrian loading. The greater the number of people, the greater the amplitude of the vibrations.

Other bridges which have seen similar problems include:
Birmingham NEC Link bridge: Groves Suspension Bridge, Chester and Auckland Harbour Road Bridge. mgs31may13wds424