The Tacoma-Narrows Bridge (Tacoma Bridge) belongs to the category of hanging bridge structures. Located in Washington State, United States of America. It is laid through the Tacoma-Nerrose Strait, which, in turn, is part of Puget Saud Bay.
History of creation
Originally built according to the design of Leon-Solomon Moiseev, a native of Russia. He is known as a structural engineer, bridge builder, an active participant in public life. The Takomsky bridge was opened for movement in July 1940. Already at its construction, the builders drew attention to the vibrations and swaying of the bridge road with increasing wind. This was due to the insufficiently high stiffness beam. In everyday life, the bridge became known as the “Golopaya Gerti”.
Bridge characteristics
At the time of the construction of the Takomsky bridge, it was a remarkable building. It was a hanging (cable-stayed) three-span structure. Its total length was 1810 meters. And the length of the central suspended span is 854 meters. The bridge was about 12 meters wide. The main bearing cables in diameter were 438 millimeters. The stiffness beam reached a height of 2.44 meters, which was recognized in the subsequent miscalculation. The construction of the bridge was held by steel pylons, standing on concrete supports (bulls).
Wreck
On November 7, 1940, when the exploitation period was only four months, the Takomsky bridge was destroyed. On this day, the wind speed reached 65 km / h. Given the fact that on this day the traffic on the bridge was minimal, this made it possible to avoid human casualties.
The very fact of destruction in dynamics was captured on film. This subsequently made it possible to thoroughly study and investigate this process. Newsreel and photos of the Tacoma-Narrows bridge in the process of its destruction are really very impressive.
Based on the film, the world-famous documentary The Tacoma Narrows Bridge Collapse was created.
Causes of destruction
According to the results of studies, studying documentary materials, it was established that the main factor that led to the accident was the outrageous dynamic torsional vibrations caused by a strong wind. It turned out that the design of the Takomsky Bridge was calculated and designed taking into account only statistical and wind loads. However, the possible impact of aerodynamic factors on its design has not been studied.
Oscillation of the bridge web occurred due to wind load. It began to intensify due to vertical vibration of the cables. The weakening of the cable on one side of the bridge and the tension on the other generated torsion phenomena, led to the tilting of the pylons and, as a result, to the breakage of the suspensions of the central span. The bridge turned out to be structurally overly flexible, with little resistance to the absorption of dynamic forces.
Filming recorded that the bridge began to swing when the wind speed was about 19 meters per second. Although in the project its resistance to winds was calculated based on 50 meters per second.
conclusions
The destruction of the Takomsky bridge forced the bridge constructors (and not only) to begin research in the field of aerodynamics, aerodynamic stability of structures and structures. This has led to a change in views on the design of bridges with large spans.
In theory, the cause of the destruction of the bridge began to denote the phenomenon of forced mechanical resonance. However, in practice it is believed that the so-called aeroelastic flutter (torsional vibrations) due to insufficient calculations of wind loads at the design stage.
New bridge
Analysis of the collapsed structure began immediately after the accident. The dismantling of pylons and side spans was carried out. This process lasted until 1943, when they began to build a new bridge. The foundations of the pylons, anchor abutments, and some other parts have found application from the old structure. The recreated bridge was commissioned in October 1950. At that time, it became the third suspension bridge in the world (based on a length of 1822 meters).
In order to impart stability to the structure and to reduce aerodynamic loads, open-type trusses were introduced into its elements. Installed additional stiffness racks. It is equipped with expansion joints and vibration damping systems. The bridge could pass up to 60 thousand cars per day.
In 2007, another bridge was built in parallel with the existing one. The purpose of construction is to increase highway throughput. Its length is 1645.9 m and its width is 853.4 m. The height of the pylons is 155.4 meters.