Tacoma Narrows: Failure of a Bridge

By Kim Smiley

The power of performing a root cause analysis of a problem can be demonstrated by working through well-known engineering disasters.  For example, creating a cause map for the failure of the Tacoma Narrows Bridge helps explain why the bridge collapsed and illustrates some of the lessons that can be learned.

The original Tacoma Narrows Bridge was opened for traffic on July 1, 1940.  A little more than four months later, the bridge violently failed and a 600 foot span of roadbed fell into the river below.  Why did the bridge tear itself apart?  What made the bridge collapse on November 7th and not some previous day?  One of the first questions asked when performing a root cause analysis is, “What is different about this issue?”   The first difference to consider was that November 7th was a windy fall day.  Construction of the bridge ended in the summer so this was the first fall the new bridge had experienced.  On the day the bridge failed, the wind was blowing across the roadbed at 42 mph.  This was the strongest the wind had blown since the bridge was constructed.  The second difference was the design of the bridge itself.  The Tacoma Narrows Bridge was particularly narrow relative to its length, making the roadbed more flexible than other suspension bridges.  Additionally, the bridge had shallow girders and was relatively weak in torsion compared to other suspension bridges built around the same time.  The combination of fall winds and the slender bridge design resulted in the collapse of the bridge.

High Level Cause Map

As the wind impacted the bridge, the force twisted the roadbed until it hit a point where it was constrained by the suspender cables, and then it twisted back in the other direction.  Other suspension bridges of the time experienced similar twisting motions, but what made this bridge different was that the amplitude of the motion increased with each cycle, rather than dying out.  The bridge was unable to dissipate the wind energy, and the motion of the bridge continued to grow until the twisting motion increased to the point where the suspender cables snapped and the roadbed was dropped into the river below.  The mathematical explanation of why the bridge collapsed is fairly complex, but simply put: the bridge was underdamped causing the twisting oscillations to increase rather than decrease with each twisting cycle.

Learn more in Part 2 of the blog.