Tacoma Narrows Bridge “The Galloping Gertie”

Why was it built? Commissioned in 1937 to cross the Tacoma Straits of Puget Sound Commissioned in 1937 to cross the Tacoma Straits of Puget Sound Connected Tacoma and Gig Harbor Connected Tacoma and Gig Harbor

People Leon Moisseiff ( ) Leon Moisseiff ( ) –Bridge Designer –Worked on Manhattan and Whitestone Bridges –“Deflection Theory” Stated that the following three opposing forces act on the deck and suspension cables: Stated that the following three opposing forces act on the deck and suspension cables: –one downward force caused by the load of the roadway –one force in one part of the cable, pulling up and to the left –one force in the other part of the cable, pulling up and to the right

Frederick B. Farquharson Civil Engineering Professor at Univ. of Washington Civil Engineering Professor at Univ. of Washington Commissioned to analyze Moisseiff’s Design Commissioned to analyze Moisseiff’s Design

Dynamic Model of the Tacoma Narrows Bridge

Another Suspension Bridge… Total Length: 5,000 ft Total Length: 5,000 ft Center Span: 2,800 ft Center Span: 2,800 ft Tower Height: 420 ft Tower Height: 420 ft Force Loads Force Loads

Comparisons Bridge Center Span (ft) Width (ft) Ratio Geo. Washington :33 Golden Gate :46 Tacoma Narrows :72 Width to Span Ratio

Girder Depth to Span Ratio The Norm 1:90 Golden Gate 1:168 Tacoma Narrows 1:350

3 Suspension Bridges George Washington Bridge Golden Gate Bridge Tacoma Narrows Bridge

Tourist Attraction

Trussed Girder Cross Section Side View

Solid Plate Girder (Tacoma Narrows) Cross Section Side View

Oscillation

Aerodynamic Girder Cross Section

Why Did It Fall? Flexibility Flexibility –Very thin plate girder –Width to span Ratio Aerodynamics Aerodynamics –Plate girder vs wind Perpendicular Wind/Vortices Perpendicular Wind/Vortices

The one that fell and its replacement

3 S’ Social Social –New design used less materials -> lower cost –Connected two sides of Puget sound Scientific Scientific –Suspension Bridge Force loads –Thin & Flexible –Solid Plate Girder Symbolic Symbolic –It fell down –Revolution in bridge engineering –Battle between architecture and engineering

DISASTER STRIKES