Engineering Disasters: Tacoma Narrows Bridge Collapse (1940)

  • Underestimating the power of nature has been a common factor in many disasters of years gone by. Should freak weather patterns be a consideration when designing structures?
  • Design engineering and expenditure are two very different areas of business. Cost v Safety, the great debate.
  • Thankfully lessons were learned from the Tacoma Narrows Bridge collapse which helped the safety of future bridge designs.

The Tacoma Narrows Bridge opened to traffic on July 1, 1940 and collapsed into Puget Sound on November 7 of the same year. At the time it was the third longest suspension bridge in the world, just behind the Golden Gate Bridge and the George Washington Bridge. Fortunately no human life was lost during the collapse but it changed the way in which bridges were designed in years to come.

Were early warnings missed?

Construction workers gave the bridge the nickname “Galloping Gertie” as it began to have noticeable vertical oscillations in windy conditions. These episodes were first witnessed when the deck was built and the warning bells should have begun to ring. There were several measures incorporated to try and stop the motion of the deck but unfortunately they were ineffective. While at the time the mass of the bridge was considered sufficient to keep it structurally sound the dangerous motion was even observed on the day the bridge was opened to the public.

Disaster after four months

While championed by many as a great stride for engineering design, just four months after opening disaster struck. The main span of the bridge collapsed under a light 40 mile per hour (64 km/h) wind on the morning of November 7, 1940. It’s not uncommon to see winds gusting to 90 miles per hour (145 km/h) in the area, so the bridge fell far short of the required design for the location.

Budget restrictions

Construction on the bridge started with a much lower budget than originally planned. The original design was created by Clark Eldridge and would have cost $11 million to build. One of the new plans was made by Leon Moseff, who redesigned the bridge with drastic modifications reducing the required budget to a mere $7 million.

Records show that contractors complained they could not build the foundation piers according to the revised designs. As a consequence the piers were built according to the original design which again should have set alarm bells ringing. The contractors were forced to use many “creative techniques” which included packing the girders in dry ice to allow misfit beams to be installed. Even when taking into account all of the construction challenges the bridge itself was solidly built with girders of carbon steel anchored in huge blocks of concrete.

However, former designs had open lattice beam trusses underneath the roadbed. This bridge was the first of its type to employ plate girders (pairs of deep I-beams) to support the roadbed. The earlier designs were specifically structured so that any wind would simply pass through the truss. The revised design redirected the wind above and below the structure and would prove to be a major contributing factor to the eventual collapse.

Freak winds create aeroelastic fluttering

The failure of the Tacoma Narrows Bridge occurred when a never-before-seen twisting mode occurred from winds at a mild 40 miles per hour (64 km/h). This is a so-called torsional vibration mode (which is different from the transversal or longitudinal vibration mode), whereby when the left side of the roadway went down, the right side would rise, and vice versa, with the center line of the road remaining still. This vibration was caused by a newly coined phenomenon called “aeroelastic fluttering”.

Back to the drawing board

The Tacoma Narrows Bridge disaster not only ended Moisseiff’s professional career, it also ended the theory of a whole generation of bridge engineering and practice. The collapse had a lasting effect on science and engineering.

In many physics textbooks the event is presented as an example of elementary forced resonance, with the wind providing an external periodic frequency that matched the bridge’s natural structural frequency. Many experts believe otherwise suggesting the actual cause of the collapse was down to the aforementioned aeroelastic flutter. While the collapse was obviously a major blow for the engineering industry it did boost research in the field of bridge aerodynamics-aeroelastics.

Thankfully lessons were learnt and it is safe to say that the Tacoma Narrows Bridge disaster influenced the future design of all long-span bridges.

About: William Tyrell

Mr. Tyrrell has over 40 years of project management, design and engineering experience in industries as diverse as ports, bulk handling facilities, offshore production and drilling, petrochemical and refineries, bauxite and nickel mining, mineral concentrators, pressure vessel fabrication, wood products, pulp and paper, microelectronics, and food processing.

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