presentation about Tacoma bridge

1. An-Najah National University
Faculty of Engineering
This presentation is prepared as a Partial Fulfilment of the
Requirements of “Principles of Scientific Research and Technical
Writing” Course-64300
Prepared by : Ibtehal Alawnih
Ghada Mukhemir
Supervisor : Dr. Aysar Yasin

2. Agenda
Introduction
Problem definition
Main contribution
The hypothesis
Methodology
Result and discussion
Conclusion and recommendation

3. Introduction:
The 1940 Tacoma Narrows Bridge was the first
incarnation of the Tacoma Narrows Bridge, a
suspension bridge in the U.S. state of
Washington .
It opened to traffic on July 1, 1940, and
dramatically collapsed into Puget Sound on
November 7 of the same year. the bridge was
the third longest suspension bridge in the world
in terms of main span length.

5. Problem Definition
From the time the bridge was built, it began to move
vertically in windy conditions, The motion was
observed even when the bridge opened to the public.
Several measures aimed at stopping the motion were
ineffective, and the bridge's main span finally
collapsed under 40-mile-per-hour (64 km/h) wind .

6. Main Contribution
This research contributes the following:
Understand what was the real reason about the
Tacoma bridge collapse.
The solution of this problem.

7. The hypothesis :
the causes of the collapse of the following theories
are:
1. Random turbulence
2. Periodic vortex shedding
3. Aerodynamic instability (negative damping)

8. Methodology:

9. Results and discussion
The fundamental weakness of the Tacoma Narrows
Bridge was its extreme flexibility, both vertically and in
torsion. This weakness was due to the shallowness of
the stiffening girders and the narrowness of the
roadway, relative to its span length.

10. Random Turbulence
An early theory was that the wind pressure simply
excited the natural frequencies of the bridge. This
condition is called "resonance." The problem with
this theory is that resonance is a very precise
phenomenon, requiring the driving force frequency to
be at, or near, one of the system's natural
frequencies in order to produce large oscillations.

11. Vortex Shedding
Theodore von Karman, a famous aeronautical
engineer, was convinced that vortex shedding drove
the bridge oscillations. A diagram of vortex shedding
around a spherical body is shown in the next slides .
A problem with this theory is that the natural vortex
shedding frequency was calculated to be 1 Hz. But,
The tensional mode frequency, that the bridge
collapse on it 0.2 Hz.

13. The alternating force disappears when the motion
disappears. This phenomenon is also modeled as
free vibration with negative damping.

14. Assume that the wind direction was not perfectly
horizontal, perhaps striking the bridge span from
below, as shown .

15. Specifically, the windward edge rotates upward while
the leeward edge rotates downward. Aerodynamic lift
is generated because the pressure below the span is
greater than the pressure above.

16. The wind's lift force now effectively places a counter-
clockwise moment on the span.
The span's angular momentum will not allow it to
simply return to its initial rest position.

17. Solution of the problem

18. The force of the wind hitting the bridge was reduced
by using open trusses, rather than the solid
stiffening girders used in the first bridge, resulting
in less force on the bridge from the wind because
the wind acted on a smaller area. The bridge was
stronger because it had a wider roadbed and had
larger stiffening struts. Furthermore, wind tunnel
testing was performed to verify the design of the
new bridge prior to its construction.

19. New Tacoma bridge today

20. :CONCLUSION
1. When it comes to new innovations and design for
structures, it is important to remember public safety
are the advances in design in line with the advances
with the materials used.
2. Collapse of the Tacoma Bridge brought the
engineering world to that the aerodynamic phenomena
in suspension bridges were neither adequately
understood in the profession nor addressed in the
design.
3. Moiseiff proposed a new design that was very
slender, and lighter. The collapse of the Tacoma
Narrows Bridge showed the importance in damping,
vertical rigidity, and tensional resistance in suspension
bridges the collapse of the Tacoma Narrows Bridge

21. Recommendations:
The following is a list of methods that could have been
used to prevent the collapse of the bridge:
1. Using open stiffening trusses, which would have allowed
the wind free passage through the bridge.
2. Increase the width to span ration.
3. Increase the weight of the bridge.
4. Dampening the bridge.
5. Using an unturned dynamic damper to limit the motions
of the bridge.
6. Increase the stiffness and depth of the trusses or
girders.
7. Streamlining the deck of the bridge.