Tacoma Narrows Bridge “The Galloping Gertie”. Why was it built? Commissioned in 1937 to cross...

Tacoma Narrows BridgeTacoma Narrows Bridge

““The Galloping Gertie”The Galloping Gertie”

Why was it built?Why was it built?

• Commissioned Commissioned in 1937 to in 1937 to cross the cross the Tacoma Tacoma Straits of Straits of Puget SoundPuget Sound

• Connected Connected Tacoma and Tacoma and Gig HarborGig Harbor

PeoplePeople

• Leon Moisseiff ( 1872- 1943) Leon Moisseiff ( 1872- 1943) – Bridge Designer Bridge Designer – Worked on Manhattan and Whitestone BridgesWorked on Manhattan and Whitestone Bridges– ““Deflection Theory”Deflection Theory”

•Stated that the following three opposing forces Stated that the following three opposing forces act on the deck and suspension cables:act on the deck and suspension cables:

– one downward force caused by the load of the roadwayone downward force caused by the load of the roadway– one force in one part of the cable, pulling up and to the one force in one part of the cable, pulling up and to the

leftleft– one force in the other part of the cable, pulling up and to one force in the other part of the cable, pulling up and to

the rightthe right

Frederick B. Farquharson Frederick B. Farquharson

• Civil Engineering Professor at Univ. of WashingtonCivil Engineering Professor at Univ. of Washington

• Commissioned to analyze Moisseiff’s DesignCommissioned to analyze Moisseiff’s Design

Dynamic Model of the Tacoma Narrows BridgeDynamic Model of the Tacoma Narrows Bridge

Another Suspension Another Suspension Bridge…Bridge…

• Total Length: 5,000 ftTotal Length: 5,000 ft

• Center Span: 2,800 ftCenter Span: 2,800 ft

• Tower Height: 420 ftTower Height: 420 ft

• Force LoadsForce Loads

ComparisonsComparisons

BridgeBridgeCenter Span Center Span

(ft)(ft) Width (ft)Width (ft) RatioRatio

Geo. WashingtonGeo. Washington 35003500 106106 1:331:33

Golden GateGolden Gate 42004200 9090 1:461:46

Tacoma NarrowsTacoma Narrows 28002800 3939 1:721:72

Width to Span Ratio

Girder Depth to Span RatioGirder Depth to Span Ratio

The NormThe Norm    1:901:90

Golden GateGolden Gate    1:1681:168

Tacoma NarrowsTacoma Narrows    1:3501:350

3 Suspension Bridges3 Suspension Bridges

George Washington

Bridge

Golden Gate Bridge Tacoma Narrows Bridge

Tourist AttractionTourist Attraction

Trussed GirderTrussed Girder

Cross Section

Side View

Solid Plate Girder (Tacoma Solid Plate Girder (Tacoma Narrows)Narrows)

Cross Section

Side View

OscillationOscillation

AerodynamicAerodynamic GirderGirder

Cross Section

Why Did It Fall?Why Did It Fall?

• FlexibilityFlexibility– Very thin plate girderVery thin plate girder– Width to span RatioWidth to span Ratio

• AerodynamicsAerodynamics– Plate girder vs windPlate girder vs wind

•Perpendicular Wind/VorticesPerpendicular Wind/Vortices

The one that fell and its replacementThe one that fell and its replacement

3 S’3 S’

• SocialSocial– New design used less materials -> lower costNew design used less materials -> lower cost– Connected two sides of Puget soundConnected two sides of Puget sound

• ScientificScientific– Suspension Bridge Force loadsSuspension Bridge Force loads– Thin & FlexibleThin & Flexible– Solid Plate GirderSolid Plate Girder

• SymbolicSymbolic– It fell downIt fell down– Revolution in bridge engineeringRevolution in bridge engineering– Battle between architecture and engineeringBattle between architecture and engineering

DISASTER STRIKESDISASTER STRIKES