tacoma bridge

Research title: Tacoma Narrow Bridge

An-Najah National UniversityFaculty 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

Agenda Introduction Problem definition Main contribution The hypothesisMethodologyResult and discussionConclusion and recommendation

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.

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 .

Main Contribution This research contributes the following :

Understand what was the real reason about the Tacoma bridge collapse.The solution of this problem .

The hypothesis : the causes of the collapse of the following

theories are :1 .Random turbulence

2 .Periodic vortex shedding 3 .Aerodynamic instability (negative damping)

Methodology:

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 .

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 .

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 .

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

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

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 .

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.

Solution of the problem

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.

New Tacoma bridge today

: CONCLUSION1 .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 could have been avoided, primarily during the design phase .

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 .