It was not the wind. A slight sway made walkers fall into step with it, their footfalls pushed the deck the way it was already moving, and past a critical crowd size that positive feedback loop ran away. The bridge closed two days later. Fitted with 37 viscous dampers, plus tuned mass dampers, it reopened on February 22, 2002.
Claims 19
Each claim as said or shown in the video, with the value it states, its source and the date it was checked.
01
Claim 1: “June tenth, two thousand, London. … a new bridge.” On screen: 10 Jun 2000
Value
the London Millennium Footbridge opened to the public on Saturday 10 June 2000
. D: “a maximum of 2000 people on the deck at any one time”, from video footage; F: “1.3 to 1.5 persons per square metre seen at the peak”. Arithmetic: 80 dots × 25 = 2 000.
Simplified
One dot stands for 25 people. The dots are a unit picture, not a count of positions.
04
Claim 4: “It sways.”
Value
excessive lateral vibration on opening day; lateral acceleration up to 200 to 250 milli-g; video amplitudes about 50 mm (south span) and 75 mm (centre span)
. D: “Unexpected excessive lateral vibrations of the bridge occurred”; “between 200 and 250 milli-g”. N: “about 50 mm at 0.8 Hz and the centre span about 75 mm at 1 Hz, approximately”.
Simplified
The deck is drawn in plan as a schematic strip with one mode shape and a sway exaggerated about 280 times (drawn peak 70 units on a 1 120-unit deck, against 75 mm on 333 m). Disclosed in post.note.
05
Claim 5: On screen: 1 Hz beside the swaying deck; the deck swings at 1 Hz
Value
the lateral modes that were excited lie between 0.5 and 1.0 Hz; lateral footfall forces act at half the walking pace (about 2 Hz)
. D: south span “around 0.8Hz”, centre span “just under 0.5Hz and 1.0Hz”, north span “just over 1.0Hz”; “They occur at half walking frequency” (lateral foot forces), vertical ones “around 2.0Hz”. N: “the lateral sway of a person’s centre of gravity occurs at half the walking pace”. T: “a frequency band of 0.5 to 1.1 Hz”. Cross-check: 2.0 Hz ÷ 2 = 1 Hz.
Simplified
Several modes were excited (0.5 to 1.1 Hz). The figure draws one mode at 1 Hz. Disclosed in post.note.
06
Claim 6: “It isn’t the wind. It’s the walkers.” On screen: WIND, struck out
Value
wind did not drive the sway; an unanticipated pedestrian lateral load did
. N: “it was rapidly concluded that wind buffeting had not contributed significantly”. D: the modal tests showed “the movements were due to an unpredicted external force”, and “the loading effect has been found to be due to the synchronisation of lateral footfall forces within a large crowd of pedestrians”. S: the sway was “not due to the bridge’s innovative design as was first thought”.
Simplified
A hook-sized compression. The crowd is half of a loop; the bridge’s low damping is the other half (row 17). Disclosed in post.note.
07
Claim 7: “A slight sway. To keep their balance, walkers fall into step with it.”
Value
walkers synchronise their steps to the sway, even when it is very small, because it helps their lateral balance
. D: “it is more comfortable for pedestrians to walk in synchronisation with the natural swaying of the bridge, even if the degree of swaying is initially very small” and it “helps them maintain their lateral balance”. S: “many pedestrians fell spontaneously into step with the bridge’s vibrations”; the gait response is “assumed” in the model (their Eq. 2 is “hypothetical but testable”).
Simplified
Whether the walkers lock on before the sway starts or because of it is debated (row 18). The ring (24 sample walkers with a lock threshold each) is a model, not data. Disclosed in post.note.
08
Claim 8: “Their footfalls push the way the deck is already moving.”
Value
the lateral footfall force is applied at the bridge’s resonant frequency and in phase with the deck’s velocity
. D: “footfall forces are applied at the resonant frequency of the bridge, and with a phase such as to increase the motion of the bridge”; “Only the component of force in phase with the velocity puts power into the system.”
Simplified
An average, correlated force: individual footfalls are not all in phase (D: the force–velocity correlation coefficients “vary between 0.34 and 0.73”).
09
Claim 9: On screen: F = k·v, k ~ 300 N·s/m
Value
lateral force per walker is proportional to the deck’s lateral velocity, k about 300 N·s/m over 0.5 to 1.0 Hz
. D: “the lateral force generated by pedestrians is approximately proportional to the response of the bridge”; “the value of k to be approximately 300Ns/m over the frequency range 0.5 to 1.0 Hz”.
Simplified
F is the crowd-averaged correlated force per person, from the Dec 2000 crowd tests.
10
Claim 10: “So the sway grows. More people lock on. A loop.”
Value
the force and the correlation between walkers both grow with the sway: a positive feedback loop
. D: “As the amplitude of the motion increases, the lateral force imparted by individuals increases, as does the degree of correlation between individuals”; its Fig. 11 plots the probability of lock-in against amplitude. S: “each process pumping the other in a positive feedback loop”. F: “This positive feedback loop causes the lateral motion to increase”.
Simplified
The ring’s recruitment order (smallest rate difference first) is a modelling choice in the spirit of S.
11
Claim 11: “Below a critical crowd, the sway stays small. Above it, it runs away.” On screen: {N}c, a flat curve that breaks upward, axis end 2 000
Value
there is a critical number of walkers; below it the effect is not seen, above it the sway grows until walking becomes difficult
. F: “A critical number of people is required to reduce the inherent structural damping of the bridge to zero. Until this critical number is reached there is no evidence of the effect”. S: “no hint of instability until the crowd reaches a critical size, Nc”. D: limiting number N_L = 8π ζ f M / k (uniform crowd, sinusoidal mode). Own calculation, centre-span mode (f 0.48 Hz, M 128–130 t, ζ 0.6–0.8 %, k 300 N·s/m): N_L about 31 to 41 walkers, far below 2 000.
Simplified
No critical number is stated: it depends on each mode’s frequency, mass and damping and on where people walk. The curve and the place of {N}c are illustrative. “Runs away” is bounded: growth stops when people cannot walk normally (D, F).
12
Claim 12: “Two days later, it closes.” On screen: 10 Jun 2000, 12 Jun 2000, 2 d
. D: “On 12 June it was decided to close the bridge in order to fully investigate the cause”. N: “On 12 June the bridge was closed”. 12 − 10 June = 2 days (computed).
13
Claim 13: “Thirty-seven viscous dampers…” On screen: 37DAMPERS, 37 ticks
. D: “A total of 37 viscous dampers are to be installed.” T: “A total of 37 dampers were constructed, of 7 different types”, its Fig. 6 lists 5 + 10 + 2 chevron, 4 vertical-to-ground, 4 + 8 + 4 pier; 17 + 4 + 16 = 37 (computed).
Simplified
Tuned mass dampers were fitted as well (row 19). The 37 ticks sit at a uniform spacing, a schematic of the counts, not the real positions. Disclosed in post.note.
14
Claim 14: “…drain the sway faster than a crowd can feed it.”
Value
the retrofit targets damping above 20 % of critical (was 0.6–0.8 %), designed for 2 walkers per square metre
. D synopsis: “damping levels in excess of 20% of critical”; D: damping measured at “around 0.6 to 0.8% of critical”. F: “a design level of 2 persons per square metre over the whole bridge”, against 1.3 to 1.5 at the opening-day peak. Own calculation, same formula as row 11 with ζ = 20 %: N_L about 1 040, about the 1 150 walkers of 2 per m2 on the 144 m × 4 m centre span.
Simplified
True up to the design crowd (2 per m2), as designed and tested (275 walkers, Dec 2000; 2 000, Jan 2002; see row 16). Not a claim about any possible crowd.
15
Claim 15: “It reopens February twenty-second, two thousand two.” On screen: 22 Feb 2002
. N: “not reopened to the public until 22 February 2002”. T: “officially re-opened to the public on February 22, 2002”.
16
Claim 16: “A crowd steps on. The deck holds.”
Value
a 30 January 2002 test with 2 000 people: peak lateral acceleration 0.25 g undamped, 0.006 g damped; no resonance in any mode; reopened “without problems”
. T: “on January 30, 2002, with a total of 2,000 people”; “Peak measured accelerations reduced from 0.25 g undamped to 0.006 g damped”; “No resonance noted of any mode”; reopened “without problems or difficulties of any type”. 0.25 ÷ 0.006 ≈ 42 (computed).
Simplified
“Holds” means no resonance and a peak of 0.006 g, not zero motion. The full-crowd test was three weeks before the reopening. The account is the damper manufacturer’s, the only source found for the 2 000-person test; the 0.25 g is the opening-day estimate.
17
Claim 17: Caption note: the bridge’s low damping is the other half of the loop
. B abstract: “its role in the initiation of wobbling was debated”; B: simulations suggest “phase lock-in mechanism might not necessarily be the main cause of initial small-amplitude wobbling”, while supporting “that pedestrian lock-in was necessary … to develop large-amplitude wobbling”.
Simplified
The video follows the standard account of D and S (lock-in and sway feed each other).
19
Claim 19: Caption note: tuned mass dampers were fitted too
Value
lateral and vertical tuned mass dampers alongside the viscous dampers
. D: “4 No. pairs of lateral tuned mass dampers to the central span” and “26 pairs of vertical tuned mass dampers will be installed”; F: “we are also installing tuned mass dampers for the vertical modes”.
Simplified
No tuned-mass count is given: Arup’s June 2001 text says 29 pairs of vertical ones and D (November 2001) says 26, and later secondary sources differ again.
Simplifications
As stated in the post’s caption.
Simplified: the bridge is a schematic plan view, with the sway exaggerated and drawn as one 1 Hz mode, the crowd as dots of 25 people, the 37 dampers evenly spaced, and the phase ring and critical-size curve illustrative; the real critical number depends on each mode’s frequency, mass and damping. The crowd is half of the loop and the bridge’s low damping (0.6 to 0.8 percent of critical) the other, and whether walkers lock on before the sway starts or because of it is still debated.
Source S: S. H. Strogatz, D. M. Abrams, A. McRobie, B. Eckhardt, E. Ott, “Crowd synchrony on the Millennium Bridge”, Nature 438, 43–44 (2005). doi:10.1038/438043a
Source B: I. Belykh, R. Jeter, V. Belykh, “Foot force models of crowd dynamics on a wobbly bridge”, arXiv:1610.05366 (2017). https://arxiv.org/abs/1610.05366
June tenth, two thousand, London. Thousands cross a new bridge. It sways.
It isn’t the wind. It’s the walkers.
A slight sway. To keep their balance, walkers fall into step with it. Their footfalls push the way the deck is already moving. So the sway grows. More people lock on. A loop.
Below a critical crowd, the sway stays small. Above it, it runs away.
Two days later, it closes. Thirty-seven viscous dampers drain the sway faster than a crowd can feed it.
It reopens February twenty-second, two thousand two. A crowd steps on. The deck holds.
Timing
Fig. 1
Video 006 as a dial: one turn is 40.9 s. Heavy arcs are the narration. The last word ends 1.0 s before the seam, where the video starts again.