Click “F” for Failure

by James Herzing18 October 2016

Next year (2017), the American Society of Civil Engineers will release a Report Card for America’s Infrastructure that rates the condition and performance of the nation’s 3,980,817 miles of roadway.  In 2013, (the most recent report) our bridges, roads and other critical infrastructure components got a D+. The Society’s 2009’s report card also issued the same near-failing grade. The only difference between 2009 and 2013 was that the estimated price tag to make it right jumped from 2.2 to 3.6 trillion dollars. IHS Automotive Research says that there are 235 million passenger vehicles on U.S. roads. The question is if all these vehicles are driving on all those D+ roads and bridges, what does “F” look like?

Answer—something like this. In 2007, Bridge 9340, an eight-lane, steel truss arch bridge that carried Interstate 35W across the Mississippi River in Minneapolis, Minnesota collapsed during the evening rush hour on August 1, 2007, killing 13 people and injuring 145. The bridge carried 140,000 vehicles daily. The National Transportation Safety Board (NTSB) suggested that the collapse was due to a design flaw, when a too-thin gusset plate ripped along a line of rivets, and additional weight on the bridge at the time of the collapse contributed to the catastrophic failure.

Collapse for the History Books

From its opening in July of 1940, up until November 7, 1940, “Galloping Gertie,” aka The Tacoma Narrows Bridge, was a long, elegant ribbon stretching across Washington's Puget Sound.  The bridge’s lightweight steel girders and thin concrete layering permitted unusual flexibility (not to mention the unusual nickname). That gusty November morning, the bridge began doing her final interpretation of the sine wave dance. The main suspension cables heaved violently side-to-side, snapped, frayed, and were tossed 100 feet into the air. The main towers and their bracing struts twisted and bent. Torsion stress beyond the elastic limit of the metal buckled and permanent warped the deck floor system. By day’s end, Gertie was a total write-off.

The Tacoma Narrows had been one of this nation’s finest examples of the "deflection theory" of suspension bridge design in the United States.  The "deflection theory" had been formulated in Austria for concrete arch bridges. Mathematician and brilliant bridge engineer Leon Moisseiff further developed and applied the theory, building several U.S. suspension bridges, including Gertie.

Deflection theory’smath suggested that stiffening trusses or cable-stays was not necessary and that the deck, main cables, and suspender cables "dead weight" elements provided sufficient structural strength against the effects of wind and traffic. Supposedly, aerodynamic force was only a stress that could move the bridge laterally. So, if you accounted for that stress by stiffening the cables or other parts, the bridge could be made both light and strong. It sounded good at the time.

Gertie’s collapse was first blamed on shoddy workmanship, but a later 1941 Federal Works Administration (FWA) report pointed to "Random action of turbulent wind." This half-explanation launched decades of ongoing investigation into simple harmonic motion and design errors which ignored the complex phenomenon of wind-induced aerodynamics in suspension bridges.

The Tacoma Narrows Bridge’s spectacular collapse shattered deflection theory, ended the career of her famous designer and disgraced an entire school of structural engineers who valued aesthetics over clunky bulk and rigidity.  In fairness, aerodynamics was poorly understood in the 1940’s. For seven more decades, experts would debate the exact nature of Gertie failure.  For some she still remains a mystery.  The fact that they still argue about it is proof of the mathematical complexity of natural phenomena.  

More Modern Collapse

In the wake of the 2013 collapse of another Washington state bridge, this one the I5 bridge north of Seattle , the National Transportation Safety Board (NTSB) labeled thousands of bridges around the U.S. as “fracture critical,” meaning they are one freak accident or mistake away from collapse, even if the spans are deemed structurally sound—which many, due to age, stress and material fatigue, are not.

Critical infrastructures (like our nation’s bridges and highways) are becoming increasingly interdependent and vulnerable to cascading failures—all the more reason to simulate failure in the design phase, rather than experiencing it on the Nightly News. Twenty-first century bridge builders have the advantage of comprehensive modeling tools like Autodesk Structural Bridge Design which offers engineers the ability to look deep into their new designs for the most reliable and economic material solutions. But failure simulation can also be a powerful persuasive tool for predictive maintenance and prioritized repair of existing and rapidly aging infrastructure.  

Since 2008, transportation spending got a temporary boost from economic stimulus funds. Of the S27 billion earmarked for highway projects, about $3 billion went to bridge projects. Obviously $27 billion is not $3 trillion and the political will to spend big on costly repairs oscillates much like Galloping Gertie in a stiff windstorm.  A compelling simulation may not have the same oomph that live video footage does, but it sure beats losing lives and millions of dollars to all-too-real and preventable catastrophic failure.