Tacoma Narrows Bridge
Collapse
Group 4
Earnán Beary
Gerard Eyres
Francis McCarthy
Louise O’Neill
Background

Construction Started in November 1938

Opened on 11 July 1940

Third Longest Bridge in the World
Background


Deck constructed using girders rather than trusses
In comparison to other suspension bridges it had



Relatively Low Stiffness
Smaller Dead Load
Low damping of the structure resulting in large
oscillations of the bridge deck
What happened and Why?

Nov 7 1940 the oscillations grew




Winds of 42mph
38 oscillations per minute
Amplitude of 3ft
Bridge closed to traffic at 10.00am
What happened and Why?

The Collapse




Centre stay snapped
Bridge began to twist
Torsional Movement caused centre span collapse
Weight of side spans pulled towers 12ft towards
edge of the bridge
Technical factors

Ultimate collapse due to metal fatigue

caused be oscillations and torsional movements
Technical factors

Theories put forward for these movements




Random Turbulence
Periodic Vortex Shedding
Aerodynamic Instability (negative damping)
Random Turbulence



Early theory for the collapse was resonance
However random turbulence in the air would not
have led to steady oscillations
Therefore the resonance theory was discounted

Periodic Vortex Shedding





Theory that blunt objects such as bridge decks could
shed periodic vortices in their wake
Vortex shedding frequency was calculated to be 1Hz
However torsional frequency observed was 0.2Hz
Therefore unlikely vortex shedding caused oscillations
Aerodynamic Instability (negative Damping)






Wind hit the bridge from angle below the horizontal
Aerodynamic lift was caused in the bridge span due to
the wind pressure below the bridge
Caused the bridge to twist in an clockwise direction
As span rotated strain energy built up in the span
This caused the span to rotate counter-clockwise
Oscillations caused the steel girders to fail
Managerial factors

Widely believed that bridge designer not responsible
for collapse.

Dynamic damper on bridge


Damaged from sand entering it when sand-blasting was
occuring on the bridge
Therefore the damper should not have been installed
until all sand-blasting had been completed on the
bridge
Lessons Learnt



A number of lessons were learnt following the collapse
of the Tacoma Narrows Bridge
Following the investigation all
bridges across America are
now tested in wind tunnels
Development new theories of vibration, aerodynamics
and harmonics in relation to bridge design