Civil Engineering Disasters – The Tacoma Narrows Bridge Collapse

July 23, 2012

The first Tacoma Narrows Bridge, completed in 1940, probably stands as the greatest civil engineering failure in history. When constructed, it was to be the third longest suspension bridge in the world. Narrow and graceful, the bridge would connect the Kitsap Peninsula to Tacoma, Washington.

Even before opening, it was clear that the bridge had problems. Workers had noticed undulations in the bridge and named it Galloping Gertie. Since suspension bridges are designed to be flexible, the problem was determined to be inconsequential, as the wave-like undulations would not cause any structural damage to the bridge. However, because of concern for the comfort of commuters on the bridge, a number of measures were designed by civil engineering consultants and implemented to try and reduce these undulations.

The bridge failed on November 7th of 1940, a mere four months after opening. Fortunately, the only life lost in its spectacular failure was that of a cocker spaniel named Tubby. Structural engineers were left with the question of analyzing how the bridge failed. As with any civil engineering failure, a thorough analysis is a necessary part of avoiding similar catastrophes in the future.

This bridge was a revolutionary design in suspension bridges, intended to reduce cost, while providing an aesthetically pleasing structure to grace the Tacoma Narrows. Two basic designs for the bridge deck had been proposed, a 25 foot tall traditional lattice-style truss, and 8 foot tall plate girders (which are essentially tall I-beams). Since the girders produced a slimmer, more elegant design at a much lower cost, they were the design decided upon.

In addition to this design innovation, there was another critical design feature which contributed to the ultimate demise of Girtie. Since traffic was expected to be fairly low, the bridge was only to be two lanes wide, resulting in an incredibly narrow 25 feet width. Between the narrow width and the low profile, it was the smallest cross section ever conceived for any lengthy suspension bridge.

As with any structure, reducing dimensions also reduces rigidity. This, along with the large sail area provided by the eight foot tall girders, gave even the lightest wind the ability to easily push the 2,800 foot long main span. It was not uncommon for the center of Girtie to be deflected as much as several feet to the side, in addition to the wave-like motion that she was well known for. However, neither of these problems created any structural risk, as the bridge and its roadbed had been designed with some flexibility.

On November 7th, 1940, amidst a 42 mile-per-hour wind, the Tacoma Narrows Bridge’s movement changed drastically. Instead of the well-known undulating motion, the roadbed of the bridge started twisting, with the opposite ends of the center span twisting out of sync with each other. This put the structure at risk, as the connection of the girders and roadbed had not been designed to withstand this sort of motion.

A famous film, taken by the owner of a photography shop near the Tacoma end of the bridge provides ample documentation of the bridge’s torsional twisting and ultimate self-destruction. The first failures the film shows are not the roadbed or supporting girders, but rather the suspender cables. These are the vertical cables running from the main cable across the top of the towers, down to the roadbed.

To understand the importance of these cables, one must understand the theory behind how a suspension bridge works. The roadbed is not intended to be self-supporting, merely to have enough stiffness to prevent winds and the load passing across it to cause excessive movement. The entire weight of the bridge is supported by the towers, which are firmly anchored in concrete piers. Two main cables are draped over the towers, and are attached to massive, heavy anchorages on both shores. From these main cables, suspender cables drop down to attach to the roadbed and support it. All these cables transfer the weight of the bridge roadbed and load to the towers.

What happened to the Tacoma Narrows Bridge is that as the wind torsionally twisted the bridge as much as 45 degrees, the weight of the bridge needed up being supported by the suspender cables on only one side. That proved to be too much for the cables, which snapped. The resulting transfer of weight to adjacent cables overloaded them, snapping additional cables in a domino effect. Without the cable system to support the roadbed, joints in the roadbed failed, causing it to collapse.

All of this started because of the natural resonance which is in any object. As the bridge was being designed, this resonance was not properly accounted for. When the wind hit 42 miles per hour, it caused the motion which ultimately led to failure.

One long-term result of the lessons which were learned from the Tacoma Narrows Bridge disaster is that modern structural engineers now use computer simulations of wind flow programs to better understand and design for the natural resonance of bridges, buildings and other structures. This helps reduce greatly the possibility of another disaster like this in the future.