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