Cătălina Cîrnațu, 9D
Collapse of the Tacoma Narrows Bridge, Washington State, 1940
Before we were so knowledgeable in physical engineering and constructional structures, there were many failed attempts to figure out how to keep bridges stable through strong winds. One of them is the infamous Tacoma Narrows Bridge, also nicknamed “Galloping Gertie”.
Built in 1940, the Tacoma Narrows Bridge was the first overpass in Tacoma. It was the world's third-longest suspension bridge. From the time the deck was built, it began to move vertically in windy conditions, so the construction workers nicknamed it “Galloping Gertie”. The motion continued after the bridge opened to the public but some brave people kept crossing it.
On November 7th, the same year, the wind was blowing constantly above 64km/h, which produced aeroelastic flutter (dynamic instability of an elastic structure in a fluid flow). The deck oscillated in an alternating twisting motion that gradually increased in amplitude until the deck tore apart. The Tacoma Narrows Bridge collapsed primarily due to the aeroelastic flutter. In ordinary bridge design, the wind is allowed to pass through the structure by incorporating trusses. In contrast, in the case of the Tacoma Narrows Bridge, it was forced to move above and below the structure, leading to flow separation.
Right before the bridge collapsed, a man was forced to leave his car in the middle of the bridge, abandoning Tubby, a terrified Coker Spaniel, in the car. A brave dog lover tried to rescue the dog, but returned back to safety with a bite on the knuckle. As the only victim of that great disaster, Tubby has earned a special place in the hearts of many. His death symbolizes the drama of that terrible day.
The bridge collapse has been described as “spectacular” and in subsequent decades has attracted the attention of physicists, mathematicians and engineers. In many Physics textbooks, the event is presented as an example of elementary forced mechanical resonance, even if the reality was a little more complicated. After the collapse, a three-dimensional scaled model of 1:200 scale was built for wind tunnel experiments and to explicitly understand the reason for failure. The experiments brought about a new theory: wind-induced oscillations.
The shape of the bridge was aerodynamically unstable along the transverse direction. The vertical girders of the H-shape allowed flow separation, thus leading to vortex generation that matched the phase of oscillation. These vortices generated enough energy to push the girders out of their position.
The problem that caused the Tacoma Narrows Bridge collapse was not a new problem, but one which had been unspecified. Due to wind action, increased stiffness can be seen through various design methods such as adding a greater dead load, adopting dampers, stiffening trusses or by guy cables. However, these factors were not originally considered and only became part of the later forensics.
After the Tacoma Narrows Bridge collapsed, the new bridge was redesigned (based on lessons learned) and rebuilt in 1950. The newly built bridge incorporated open trusses (triangular), stiffening struts and allowed the wind to flow freely through openings in the roadbeds. Compared to the previous design, the twisting that developed in the new bridge was considerably less severe.