Tacoma Narrows Suspension Bridge was a bridge constructed in Washington D.C. in 1940. The structure was a complete loss just after four months of its construction. PRESENTED BY: Muhammad Asad Hayat, Student Department of Civil Engineering, Session (2014-18) COURSE: Civil Engineering Practice COURSE INSTRUCTOR: Engr. Arslan Yaqub, Lecturer, Civil Engineering UNIVERSITY: University of Engineering and Technology, Taxila

1. Tacoma Narrows Suspension Bridge
Presented by:
Muhammad Asad Hayat, 14-CE-148
Civil Engineering Department, UET Taxila

2. Introduction
• Tacoma Narrows Suspension Bridge was
constructed in the Washington D.C., a state in
United States, that connected the Tacoma Narrows
strait of Puget Sound (‘Sound’: narrow channel of
the sea joining two larger bodies of water) between
Tacoma and the Kitsap Peninsula.
• This bridge collapsed just four months after its
construction.
• It was world’s 3rd longest suspension bridge in the
world at the time of its failure.

3. Introduction (Contd…)
• Total cost of bridge was 6.4 million U.S.D (105
million USD in 2012).
• Total structure length was 5939 ft with 5000 ft
suspension span and it consisted of three spans,
one central suspension span or deck and two short
suspension spans one each side. Lengths of spans
were as follows:
1. 1st Short Suspension Span (1100 ft)
2. Central Span (2800 ft)
3. 2nd Short Suspension Span (1100 ft)

4. Introduction (Contd…)
• Center span (Height above water) = 195 ft
• Width of roadway = 26 ft
• Width of sidewalk (On each side) = 5 ft
• Height of East Pier = 247 ft
• Height of West Pier = 198 ft
• Height, Towers above piers = 425 ft

5. Bridge Map

6. Main Cable Deck
Anchorage
Main Towers
1st Short Span 2nd Short Span
Center Span
Bridge at the time of its inauguration ceremony, 1940
E
W N
S

7. Need of Bridge
• This bridge, constructed in two years, was designed
to join Tacoma and less-developed Kitsap Peninsula
which could result in increase in development and
population in both areas.
• Bridge also benefited the US Military (Air Force and
Navy) connecting their air fields and ship yards.
• A road could be constructed to connect these two
places, but it cost more money than to construct a
bridge.
• Bridge was designed to hold 60,000 cars.

8. Historical Background and Funding
• Need of a bridge to connect two cities dates back to
1889 but concreted efforts began in 1920s.
• Initial design of bridge was given by Clark Eldridge
in 1938 who was Washington State Engineer.
• Leon Moisseiff, New York bridge engineer who
served as designer and consultant engineer for the
Golden Gate Bridge - petitioned the PWA (Public
Works Administration) and the RFC (Reconstruction
Finance Corporation) to build the bridge for less.
Moisseiff proposed shallower support which cost
less. Moisseiff’s design won out.

9. Historical Background and Funding
(Contd…)
Initial design of bridge by Eldridge, 23 May 1938
Safe Design ?
Eldridge proposed truss deck for the bridge so that wind could easily pass through it.

10. BUT
WHY?
FAILED!

11. Movements during Construction
• A small movement in deck of bridge was noted
many times by workers even during the construction
of the bridge when wind blew. A small vibration
could be expected in a bridge but vibrations
produced in this bridge were not normal.
• Engineers were aware that bridge was prone to
motion.
• Retrofitting to design, as bridge construction
progressed, by new ideas from engineers was also
done.

12. Prevention of Movements
• Retrofitting of bridge, to avoid the movements and
stiffen the bridge, as mentioned earlier was done by
installing hydraulic jacks and tie down cables.
Tie-down cables

13. Failure
• Analytical studies have shown that this bridge failed
due to winds running at 64 km/h velocity in the
Eastern direction on 7th of November, 1940.
• In fact,
“Tacoma Narrows was made with wind-resisting
plate girders rather than deep stiffening trusses.”
• In design of bridge, aerodynamic forces were not
taken into account due to which winds running at
64km/h speed in one direction caused it to fail.

14. Failure (Contd…)
Twisting of deck of
the bridge due to
wind action

15. Failure (Contd…)
• Bridge was too skinny, fragile and flexible. It had
only two lanes, less mass.
• The wind exerted a lot of pressure, assisted by more
and more severe aerodynamic forces, and made
waves which caused the bridge to twist and collapse
in water.
• During the collapse, the main suspension cables
were thrown violently side to side, twisted, and
tossed many feet into the air.
• On the north cable at midspan, where the support
cable had loosened, it broke more than 350 wires.

16. Failure (Contd…)
• Other wires were severely distorted. The main
cables were a total loss. Their only value was as
scrap metal.
Ultimate Collapse
Courtesy: Wikipedia

17. Failure (Contd…)
Bridge after failure (Center span crashed)

18. Torsional (Twisting) Movements at
the time of failure
The nature and severity of
the torsional movement is
revealed in this image
taken from the Tacoma end
of the suspension span.
When the twisting motion
was at the maximum,
elevation of the footpath at
the right was 28 feet (9m)
higher than the footpath at
left side of the bridge.

19. Torsional (Twisting) Movements at
the time of failure (Contd…)
Aerodynamic forces causing torsional movements

20. Damages to Side Spans
• Side spans, which became unbalanced, were too
severely damaged.
• Sag of approximately 45 feet appeared in both side
spans.

21. What does Physics say about
failure?
• Vertical Oscillations or Torsional Movements, as
mentioned earlier, were noted during construction of
bridge.
• Aerodynamic flutter is regarded as major cause of
failure.
• Resonance effect and vortex shedding also caused
it to fail.

22. Aerodynamic Flutter in Bridge
• Tendency to aerodynamic flutter is present in all
flexible structures i.e. bridges, jet planes,
aeroplanes.
• It is possible only if structural and aerodynamic
forces are running at same frequency.
• Majority of researchers consider aerodynamic
flutter to be the main reason for the failure of
bridge.

23. Understanding Aerodynamic Flutter
• If a father is pushing his kid on swing then swing
would have a natural frequency. If swing would
come back to him after every 4 seconds, then to
maintain the motion of swing, father must push the
swing each time (after every 4 seconds) known as
external force applied with periodic frequency. If he
is not able to do so, he will damp the motion.
• If external periodic frequency matches natural
frequency, we get a resonance frequency. If this
phenomenon remains for some time, it will,
ultimately, cause amplitude to increase due to
proper timing of force i.e. failure of plane wings,
failure of deck of bridge.

24. Understanding Aerodynamic Flutter
(Contd…)
Tendency of a flexible
structure to aerodynamic
flutter causing vibration in
whole body.
Courtesy: Aerodynamic
Flutter analysis of plane
using ANSYS

25. Resonance
• We know that resonance happens when:
“Any system is vibrated at its natural frequency or
resonant frequency.”
• Take example of a rope.
You can think of resonance
as having the natural
frequency of the system
exactly in tune with your
force.

26. Resonance effect on the bridges
• In design of bridge, resonance phenomenon plays
an important role.
• This is susceptibility or tendency of any structure to
respond at an increased amplitude when frequency
of its oscillations meets its natural frequency.
• If it happens, severe vibrations are produced in the
structure causing ultimate failure.
• Many factors are involved in vibrations of bridge i.e.
vibrations due to traffic flow, heavy machinery, wind.
• One such affect was noted in Tacoma Narrows
Bridge e.g. wind.

27. Resonance effect on the bridges
(Contd…)
• If natural frequency of structure is achieved due to
any factor, it will cause more vibrations and more
storage of energy in the system. When this energy
exceeds an object’s load limits, structure will lose its
integrity.
• In case of Tacoma Narrows, when natural frequency
met frequency of wind blowing, it caused
denominator to be very small and therefore caused
increase in amplitude.
windnatural
wind
nsoscillatio
ff
F
A

 Nearly or
completely equal
to zero?

28. Vortex Shedding
• It happens when winds hits a structure, then
vortices of fluid (air) are formed at certain
frequency. When a vortex is formed on one side
of body (Say A), it immediately increases
pressure on side A causing decrease in velocity
in side A and ultimately causing less pressure on
other side (Say B) and more velocity.
• Due to these vortices, a fluctuating force on the
body is induced which causes vibrations in the
structure. If vibrations coincide with the natural
frequency or resonance frequency of structure,
they lead to failure.

29. Vortex Shedding (Contd…)
Imagine wind blowing from
left on the cable cross-
section.
• Small vortex of low pressure
• Move towards low pressure

30. Summary of Physics Analysis
Summarizing the whole analysis:
“A small vibration produced, assisted by the more
and more wind caused the natural frequency of
bridge to meet wind frequency which ultimately
resulted in an increase in the bridge oscillations.
Bridge oscillations increased with time and it
ultimately failed.
Vortex shedding, resonance phenomenon and
aerodynamic flutter were interlinked to each other in
sense of natural frequency of the structure”.

31. Bridge (1950)
• Soon after failure of bridge, it was expected to be
rebuilt with 1-1.5 years.
• Due to WW-II, its construction delayed.
• In 1950, new bridge was completed.
• Second bridge had truss-girders which allowed to
wind to pass through bridge easily.
• New bridge, being wide and thicker, had more
torsional stiffness than previous bridge.
• Wind tunnel test were also performed prior to its
construction on small scale model.

32. Bridge (1950) (Contd…)
1950 Tacoma Narrows Bridge
(1988)
Courtesy: Wikipedia

33. Bridge (2007)
• Due to high traffic flow, another bridge parallel to
1950 bridge was constructed to carry eastbound
traffic.
Westbound Bridge (1950)
Eastbound Bridge (2007)
Twin Bridges of Tacoma

34. Major difference between the two
(1940 & 1950)
• Following are some of differences between 1940 &
1950 suspension bridges:
1940 Bridge 1950 Bridge
Had two lanes Has four lanes
Less stiff More stiff
Less weight More weight
Use of plate girders Use of trusses
Wind-disturbance No wind-disturbance
2.4m deep plate girders 10m deep stiff trusses

35. A Lesson for New Design
• In modern suspension bridges construction, it is now
made mandatory to test the bridge model on small
scale prior to start its construction.
• Science of aerodynamic flutter in bridges was born
after the failure of Tacoma Narrows 1940.
• Effects of resonance on bridges and other flexible
structures were studied thoroughly.
• Vortex shedding and its affects on the bridge were
also taken into account.
• Wide use of open stiffening trusses to let wind pass
through started.

36. A lesson for New Design (Contd…)
• Increase in the weight of bridge because:
Vertical oscillations resistance ∝ Mass of bridge
• Increase in stiffness by using stiffening trusses and
more depth of deck.

37. Any Questions?