Tacoma Narrows Bridge

1. Opened to traffic: July 1, 1940 Collapsed: November 7, 1940 Tacoma Narrows Bridge Lewis Cluck - lxc130030 Advanced Requirements Engineering – SE 6361.Z01 Oct. 26, 2012

2. Design of Collapsed Bridge Modified Tacoma Narrows design used 8-foot deep girders and increased span to width ratio allowing for a slimmer and more elegant look. Also cheaper: $7 million

3. Problems Noticed The structure experienced considerable vertical oscillations while it was still under construction, several strategies were used, in vain, to reduce the motion of the bridge. They included:[2] • attachment of tie-down cables to the plate girders, which were anchored to 50-ton concrete blocks on the shore. This measure proved ineffective, as the cables snapped shortly after installation. • addition of a pair of inclined cable stays that connected the main cables to the bridge deck at mid-span. These remained in place until the collapse, but were also ineffective at reducing the oscillations. • finally, the structure was equipped with hydraulic buffers installed between the towers and the floor system of the deck to damp longitudinal motion of the main span. The effectiveness of the hydraulic dampers was nullified, however, because the seals of the units were damaged when the bridge was sand-blasted before being painted.

4. Investigating the Issue Professor Frederick Burt Farquharson from the at the University of Washington hired to perform wind-tunnel tests and recommend solutions in order to reduce the oscillations of the bridge.  • 1:200 scale model of bridge • 1:20 scale model of deck section

5. Aeroelastic Flutter • Fluttering is a physical phenomenon in which several degrees of freedom of a structure become coupled in an unstable oscillation driven by the wind. • This movement inserts energy to the bridge during each cycle so that it neutralizes the natural damping of the structure; the composed system (bridge-fluid) therefore behaves as if it had an effective negative damping (or positive feedback), leading to an exponentially growing response. • The wind speed that causes the beginning of the fluttering phenomenon (when the effective damping becomes zero) is known as the flutter velocity. Fluttering occurs even in low-velocity winds with steady flow. Hence, bridge design must ensure that flutter velocity will be higher than the maximum mean wind speed present at the site.

6. Suggested Fixes Five days before the collapse, the test studies are concluded.

7. Aeroelastic Flutter (cont) In other words, the oscillations increase in amplitude with each cycle because the wind pumps in more energy than the flexing of the structure can dissipate, and finally drives the bridge toward failure due to excessive deflection and stress.

8. Torsional Motion

9. Changes to Bridge Design • Dampening Mechanisms

10. Changes to Bridge Design • Stiffening Trusses with lateral bracing

11. Changes to Bridge Design • Differences in roadway support

12. Changes to Bridge Design • Wind grates function as vents to alleviate oscillations caused by wind

13. Changes in Bridge Design • Tower height increased to reduce cable sag ratio for 1:10 to 1:12 • Old tower: 425 ft • New tower: 467 ft

14. Wind Tunnel Testing • In designing this structure, bridge engineers first used wind tunnel tests to determine the behavior and stability of a physical model of a proposed bridge. • The research and design provided significant information to suspension bridge engineers nationwide and had an important effect on all suspension bridge designs that followed.

15. Changes in Design Practice The United States government requires that all bridges built with federal funds must first have their preliminary design subjected to wind tunnel analysis using a 3-dimensional model.

16. Span Ratio Comparison

17. Tacoma Narrows Bridges Today

18. Current Tacoma Narrows Bridges

19. 2007 Construction

20. References • ^Washington State Department of Transportation (2005). “1940 Narrows Bridge – The Machine". Retrieved 2012-11-20. • Rita Robison. "Tacoma Narrows Bridge Collapse." In When Technology Fails, edited by Neil Schlager, pp. 18–190. Detroit: Gale Research, 1994.