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Assignment3a
Assignment 3a
Tacoma Narrows Bridge Disaster
Introduction
Engineering is one of the many disciplines in which responsibility is an essential standard.
An important responsibility in engineering is to utilize ethics and integrity to ensure mitigation of engineering failures.
Despite this standard, many engineering designs have failed and resulted in disaster. One such example is the Tacoma Narrows bridge failure.
Designed to cross over the Narrows, a part of the Puget Sound passing through Tacoma, WA, the Tacoma Narrows bridge began development in the 1920’s. Providing funds for the project, the
Federal Public Works Administration (PWA) and the Reconstruction Finance Corporation (RFC) made an agreement with the state highway department to begin bridge construction. Projected
costs were estimated to be less than $11 million dollars. Involved in the design and construction were Leon Moisseiff and Clark Eldridge. Theodore L. Condron was involved in reviewing the
design. Initially, Condron was sceptical of the design and its flexibility, but was later convinced by other engineers that it was safe. The construction of the bridge was completed in July
1940 and failed the same year in November. Unfortunately, a dog died while stuck on the bridge during the failure. There were no human casualties. After the failure, a research team was
gathered and determined that the primary cause of the disaster was a lack of aerodynamic consideration in the design process. At the time, aerodynamic forces were not considered essential
in bridge design, so the failure was not blamed on human error (Green and Unruh, 2006).
Engineering Perspective
Larsen (2000) explained that on the day of the bridge failure, winds reached up to 19m/s and forced the structure to twist at a 30 degree difference from its steady state position (p. 243).
In modern analysis of the failure it was found that relating the torsion oscillation frequency and the vortex shedding frequency was highly important to understanding the disaster. In
analyzing models of the bridge, it is suggested that the maximum amplitude limit was exceeded due to vortex torsion which caused the failure (Larsen, 2000). Another source confirms this by
suggesting the vortex-induced model is a reasonable explanation for the failure (Petroski, 2009). Note that there is no known cause of the bridge failure, but modern failure analysis suggests
that the vortex pattern, force, and wind speed relationships are plausible explanations for why the amplitude limit was exceeded. (Larsen, 2000).
Equations
Vortex Frequency:
f = Sr v / d where f = vortex shedding frequency (Hz) Sr = Strouhal number (dimensionless) v = flow velocity (m/s) d = bluff body diameter
Torsion oscillation frequency:
w = √(k/I) where w = frequency (Hz) k = torque constant I = moment of inertia
Probable equation for calculating angle change from steady state position:
∅ = ∑(TL/JG) where ∅ = angle (rads) T = torque (N*m) L = length of segment (m), J = moment of inertia (kg*m) G = elastic modulus (Pa)
Power from vortices:
P = F*v
Aerodynamic Force Free Body Diagram
Lessons Learned
Due to the Tacoma Narrows bridge failing, many essential engineering lessons were learned. The most significant takeaways from the Tacoma Narrows bridge failure were most likely the
consideration of aerodynamics in bridge design, bridge length to width ratios, and bridge aesthetics. First, after the first design failed, engineers made sure to increase overall bridge
stiffness to decrease potential oscillations from wind (Green & Unruh, 2006). Second, It became apparent that the bridge length to width ratio needed improvement to increase stability
(Green & Unruh, 2006). Third, in the initial design, aesthetics were essential design requirements which likely sacrificed structural integrity considerations (Larsen, 2000). In the new
bridge design, engineers made sure the aesthetics were not essential over structural integrity. From a personal perspective, it is appalling that engineers would consider constructing the
first design given the questionable stability characteristics the bridge initially had. It is felt that the responsibility, ethics, and integrity of engineering designs should be taken more
seriously, especially in designs that could be potentially life threatening. Despite the failure of the Narrows bridge, important lessons were learned that fundamentally changed the
engineering design process. Due to Narrows failure, the need to recognize aerodynamic risks in bridges is now an essential part of all bridge designs.
Resources
Larsen, A 2000, 'Aerodynamics of the Tacoma
Narrows Bridge - 60 Years Later', Structural Engineering International, vol. 10, no. 4, pp. 243-248, doi:
10.2749/101686600780481356
Petroski, H 2009, ‘Engineering: Tacoma Narrows Bridges’, American Scientist, vol. 97, no. 2, pp. 103-107.
Green, D & Unruh, W 2006, ‘The failure of the Tacoma Bridge: A physical model’, American Journal of
Physics, vol. 74, no. 706, doi: 10.1119/1.2201854
Tacoma Narrows Bridge Disaster
Introduction
Engineering is one of the many disciplines in which responsibility is an essential standard.
An important responsibility in engineering is to utilize ethics and integrity to ensure mitigation of engineering failures.
Despite this standard, many engineering designs have failed and resulted in disaster. One such example is the Tacoma Narrows bridge failure.
Designed to cross over the Narrows, a part of the Puget Sound passing through Tacoma, WA, the Tacoma Narrows bridge began development in the 1920’s. Providing funds for the project, the
Federal Public Works Administration (PWA) and the Reconstruction Finance Corporation (RFC) made an agreement with the state highway department to begin bridge construction. Projected
costs were estimated to be less than $11 million dollars. Involved in the design and construction were Leon Moisseiff and Clark Eldridge. Theodore L. Condron was involved in reviewing the
design. Initially, Condron was sceptical of the design and its flexibility, but was later convinced by other engineers that it was safe. The construction of the bridge was completed in July
1940 and failed the same year in November. Unfortunately, a dog died while stuck on the bridge during the failure. There were no human casualties. After the failure, a research team was
gathered and determined that the primary cause of the disaster was a lack of aerodynamic consideration in the design process. At the time, aerodynamic forces were not considered essential
in bridge design, so the failure was not blamed on human error (Green and Unruh, 2006).
Engineering Perspective
Larsen (2000) explained that on the day of the bridge failure, winds reached up to 19m/s and forced the structure to twist at a 30 degree difference from its steady state position (p. 243).
In modern analysis of the failure it was found that relating the torsion oscillation frequency and the vortex shedding frequency was highly important to understanding the disaster. In
analyzing models of the bridge, it is suggested that the maximum amplitude limit was exceeded due to vortex torsion which caused the failure (Larsen, 2000). Another source confirms this by
suggesting the vortex-induced model is a reasonable explanation for the failure (Petroski, 2009). Note that there is no known cause of the bridge failure, but modern failure analysis suggests
that the vortex pattern, force, and wind speed relationships are plausible explanations for why the amplitude limit was exceeded. (Larsen, 2000).
Equations
Vortex Frequency:
f = Sr v / d where f = vortex shedding frequency (Hz) Sr = Strouhal number (dimensionless) v = flow velocity (m/s) d = bluff body diameter
Torsion oscillation frequency:
w = √(k/I) where w = frequency (Hz) k = torque constant I = moment of inertia
Probable equation for calculating angle change from steady state position:
∅ = ∑(TL/JG) where ∅ = angle (rads) T = torque (N*m) L = length of segment (m), J = moment of inertia (kg*m) G = elastic modulus (Pa)
Power from vortices:
P = F*v
Aerodynamic Force Free Body Diagram
Lessons Learned
Due to the Tacoma Narrows bridge failing, many essential engineering lessons were learned. The most significant takeaways from the Tacoma Narrows bridge failure were most likely the
consideration of aerodynamics in bridge design, bridge length to width ratios, and bridge aesthetics. First, after the first design failed, engineers made sure to increase overall bridge
stiffness to decrease potential oscillations from wind (Green & Unruh, 2006). Second, It became apparent that the bridge length to width ratio needed improvement to increase stability
(Green & Unruh, 2006). Third, in the initial design, aesthetics were essential design requirements which likely sacrificed structural integrity considerations (Larsen, 2000). In the new
bridge design, engineers made sure the aesthetics were not essential over structural integrity. From a personal perspective, it is appalling that engineers would consider constructing the
first design given the questionable stability characteristics the bridge initially had. It is felt that the responsibility, ethics, and integrity of engineering designs should be taken more
seriously, especially in designs that could be potentially life threatening. Despite the failure of the Narrows bridge, important lessons were learned that fundamentally changed the
engineering design process. Due to Narrows failure, the need to recognize aerodynamic risks in bridges is now an essential part of all bridge designs.
Resources
Larsen, A 2000, 'Aerodynamics of the Tacoma
Narrows Bridge - 60 Years Later', Structural Engineering International, vol. 10, no. 4, pp. 243-248, doi:
10.2749/101686600780481356
Petroski, H 2009, ‘Engineering: Tacoma Narrows Bridges’, American Scientist, vol. 97, no. 2, pp. 103-107.
Green, D & Unruh, W 2006, ‘The failure of the Tacoma Bridge: A physical model’, American Journal of
Physics, vol. 74, no. 706, doi: 10.1119/1.2201854
Engineering is one of the many disciplines in which responsibility is an essential standard. An important responsibility in engineering is to utilize ethics and integrity to ensure mitigation of engineering failures. Despite this standard, many engineering designs have failed and resulted in disaster. One such example is the Tacoma Narrows bridge failure.
Designed to cross over the Narrows, a part of the Puget Sound passing through Tacoma, WA, the Tacoma Narrows bridge began development in the 1920’s. Providing funds for the project, the Federal Public Works Administration (PWA) and the Reconstruction Finance Corporation (RFC) made an agreement with the state highway department to begin bridge construction. Projected costs were estimated to be less than $11 million dollars. Involved in the design and construction were Leon Moisseiff and Clark Eldridge. Theodore L. Condron was involved in reviewing the design. Initially, Condron was sceptical of the design and its flexibility, but was later convinced by other engineers that it was safe. The construction of the bridge was completed in July 1940 and failed the same year in November. Unfortunately, a dog died while stuck on the bridge during the failure. There were no human casualties. After the failure, a research team was gathered and determined that the primary cause of the disaster was a lack of aerodynamic consideration in the design process. At the time, aerodynamic forces were not considered essential in bridge design, so the failure was not blamed on human error (Green and Unruh, 2006).
Larsen (2000) explained that on the day of the bridge failure, winds reached up to 19m/s and forced the structure to twist at a 30 degree difference from its steady state position (p. 243). In modern analysis of the failure it was found that relating the torsion oscillation frequency and the vortex shedding frequency was highly important to understanding the disaster. In analyzing models of the bridge, it is suggested that the maximum amplitude limit was exceeded due to vortex torsion which caused the failure (Larsen, 2000). Another source confirms this by suggesting the vortex-induced model is a reasonable explanation for the failure (Petroski, 2009). Note that there is no known cause of the bridge failure, but modern failure analysis suggests that the vortex pattern, force, and wind speed relationships are plausible explanations for why the amplitude limit was exceeded. (Larsen, 2000).
f = Sr v / d where f = vortex shedding frequency (Hz) Sr = Strouhal number (dimensionless) v = flow velocity (m/s) d = bluff body diameter
w = √(k/I) where w = frequency (Hz) k = torque constant I = moment of inertia
∅ = ∑(TL/JG) where ∅ = angle (rads) T = torque (N*m) L = length of segment (m), J = moment of inertia (kg*m) G = elastic modulus (Pa)
P = F*v
Due to the Tacoma Narrows bridge failing, many essential engineering lessons were learned. The most significant takeaways from the Tacoma Narrows bridge failure were most likely the consideration of aerodynamics in bridge design, bridge length to width ratios, and bridge aesthetics. First, after the first design failed, engineers made sure to increase overall bridge stiffness to decrease potential oscillations from wind (Green & Unruh, 2006). Second, It became apparent that the bridge length to width ratio needed improvement to increase stability (Green & Unruh, 2006). Third, in the initial design, aesthetics were essential design requirements which likely sacrificed structural integrity considerations (Larsen, 2000). In the new bridge design, engineers made sure the aesthetics were not essential over structural integrity. From a personal perspective, it is appalling that engineers would consider constructing the first design given the questionable stability characteristics the bridge initially had. It is felt that the responsibility, ethics, and integrity of engineering designs should be taken more seriously, especially in designs that could be potentially life threatening. Despite the failure of the Narrows bridge, important lessons were learned that fundamentally changed the engineering design process. Due to Narrows failure, the need to recognize aerodynamic risks in bridges is now an essential part of all bridge designs.
Larsen, A 2000, 'Aerodynamics of the Tacoma Narrows Bridge - 60 Years Later', Structural Engineering International, vol. 10, no. 4, pp. 243-248, doi: 10.2749/101686600780481356
Petroski, H 2009, ‘Engineering: Tacoma Narrows Bridges’, American Scientist, vol. 97, no. 2, pp. 103-107.
Green, D & Unruh, W 2006, ‘The failure of the Tacoma Bridge: A physical model’, American Journal of Physics, vol. 74, no. 706, doi: 10.1119/1.2201854