According to a summary piece published in Engineering News Record, the Tacoma Narrows Bridge's "basic problem" was its "considerable flexibility, vertically and in torsion." Several reasons contributed to the extreme flexibility, including the deck's small weight. The bridge's designers believed that this would allow the structure to react more quickly to environmental conditions such as waves or wind.
The Narrows bridge was completed in 1940, just months before World War II began. Because of its importance, the U.S. Army took over construction so it could be used for military purposes if needed. The bridge was opened to traffic the next year, after only three years of construction.
Although it was designed to last 50 years, the bridge was already showing signs of stress when it was inspected less than 10 years later. By then, plans were already being considered for a replacement bridge. In fact, work had already started on a new bridge across the Narrows, even though there was no plan to reopen the old one. The first section of the new bridge was opened in 1957, while the original bridge was demolished a few months later.
Initially, there were plans to build another bridge similar to the first one. However, these plans were dropped because they would have cost too much money and taken too long to build.
The Tacoma Narrows Bridge was mostly destroyed by aeroelastic flutter. Wind is permitted to travel through the structure in standard bridge construction by integrating trusses. However, at certain frequencies, wind causes the bridge to deform which leads to more wind... and so on in a vicious cycle known as aeroelastic flutter.
The problem can only be fully controlled by using stiffer materials or repositioning some of the support points for the deck. The designers of this bridge took the easy way out by using lighter materials instead. This decision led to one of the most famous engineering mistakes in US history!
The Tacoma Narrows Bridge was opened to traffic in 1940 and had an official length of 1,710 feet. It crossed the narrow channel between Puget Sound and the city of Tacoma, Washington. Unfortunately, its design used lightweight materials for its suspension system that were unsuited for the wind-induced forces present in that location. Before the bridge was completed, it was discovered that its main span would have been unstable if it had been made from normal steel girders instead of I-beams. The designer's response to this problem was to suggest that the main span be split into three sections that could be lifted up or lowered to reduce the wind load on each section.
In the case of the Tacoma Narrows Bridge, however, it was forced to migrate above and below the structure, resulting in flow separation. This caused the bridge to oscillate severely back and forth, leading to its eventual collapse.
In addition to being extremely fatigued from carrying heavy loads for many years, the main span of the bridge had been weakened by the removal of some of its steel during previous repairs. The combination of problems led to its collapse on January 7, 1975.
The disaster resulted in 38 deaths and brought about change at both a national and state level with regard to bridge safety. It also prompted the American public to demand better protection for our most vital structures.
Currently, all new bridges are designed with robust wind zones so they can survive high winds without collapsing. Old bridges are often replaced with ones that are designed to handle greater loads than those for which they were originally built. In some cases, this means upgrading existing bridges to carry heavier vehicles or pedestrians.
The National Transportation Safety Board (NTSB) reports that between 1999 and 2008, over half of all large-scale structural failures of roadways were due to wind damage.
The Tacoma Narrows Bridge collapsed in 1940 due to aerodynamic instability. The amplitude of the oscillations is determined by the form, natural frequency, and damping of the structure. As the bridge approaches its first mode (most common) frequency, it will begin to vibrate at high amplitudes. This will eventually cause the deck beam to fail.
In conclusion, the collapse of the Tacoma Narrows Bridge was caused by the effect of wave action on a structurally deficient bridge approach. Waves generated by wind over the lake had enough energy to lift the bridge's deck off of its supports!
The Collapse's Engineering The Science of the Tacoma Narrows Bridge Collapse The Tacoma Narrows Bridge was mostly destroyed by aeroelastic flutter. Wind is permitted to travel through the structure in standard bridge design by integrating trusses....
However, due to the unusual shape of the bridge, wind exerted extreme pressure on its surface causing it to vibrate severely. This vibration coupled with the weight of the vehicle crossing the bridge caused the structure to flutter over until it collapsed.
Flutter is a structural instability that occurs when a flexible member such as a rod or wire transmits vibrations from its movement in air to another object, usually a wall or floor. If the object affected by these movements is itself flexible, such as a beam or a membrane, then it will also exhibit structural instability and may collapse under its own weight or be forced over if the load on it is great enough. Fluttering can occur naturally (in clouds for example) or be induced by humans (for example on an aircraft wing). When this happens, the structure is said to be in aerodynamic flutter.
The term "bridge flutter" is also used to describe the same phenomenon that occurs when a bridge collapses due to excessive vibration. The word "flutter" is used here because the motion resembles the movement of a flag waving in the wind.
Geometrically, the Dee Bridge pushed the boundaries of experience in that design class. The most likely reason of the bridge's failure was a torsional buckling instability, to which the bridge girders were predisposed due to compressive stresses induced by the girder's eccentric diagonal tie-rods. The bridge collapsed during a windstorm on November 3, 1872.
Also contributing to the collapse was the fact that there were no lateral supports for the deck beam. Since the bridge had been recently erected, it may be inferred that its designers expected traffic on the road approaching from the west to be light, or at least that they were willing to take the risk. However, as the town grew this assumption proved to be incorrect, and before long the bridge became too heavy for its own structure.
The collapse caused 16 deaths. It was the worst structural disaster in Pennsylvania until the 1980s when the Silver Stadium roof collapsed at Penn State University.
It also brought down the Dee Bridge Rail Road, which ran over the south end of the bridge. This rail line was built by William H. Witherspoon who obtained rights-of-way from various landowners along both sides of the river. In exchange, he paid them cash up front and promised to build roads into their towns. When the railroad reached Pittsburgh, another competitor named Joseph Dougherty offered to extend his line across the Allegheny River for $100,000.