In the Wake of Tacoma: Suspension Bridges and the Quest for Aerodynamic Stability (review)

On 7 November 1940 the world's third-longest suspension bridge, the newly completed Tacoma Narrows Bridge, collapsed during a gale. This calamity was at once the most widely noted bridge failure anywhere and a severe shock to the structural engineering profession. The Tacoma Narrows Bridge was the work of one of the most respected suspension bridge engineers of the first half of the twentieth century and represented the culmination of new theories of suspension bridge design that had been applied to a growing number of major spans. In this exceptionally thorough work, Richard Scott delves deeply into the complex origins of the failure and the flaws in the so-called deflection theory upon which the design was based. He then takes us on a comprehensive survey of the extraordinary progress in suspension bridge design and construction over the subsequent six decades that owe so much to the lessons of Tacoma Narrows.

Scott begins the story with a brief review of the origins of the modern suspension bridge, pointing out the similarities to Tacoma Narrows in some notable nineteenth-century failures whose lessons were either ignored or never fully understood. John Roebling's great Brooklyn Bridge of 1883 and the notable suspension spans of the first several decades of the twentieth century depended upon deep stiffening trusses to maintain their stability under wind and traffic loads. Under the deflection theory, developed early in the century, the principal requirement of the trusses was to distribute local live loads to the main cables and avoid local grade changes, permitting substantial reductions in their size. By the 1930s this design approach had evolved to the use of plate girders that were breathtakingly shallow by comparison to the trusses of earlier practice.

Indeed, the shortcomings of plate-girder suspension spans that would bring down the Tacoma Narrows Bridge had begun to show themselves in aerodynamic problems with David Steinman's Thousand Islands Bridge and Othmar Ammann's Bronx-Whitestone Bridge of 1939. Under wind loads, both structures developed pronounced vertical oscillations, but these were brought under control with cable ties and stays, and buffer blocks. So it was that Leon Moisseiff proceeded with his design for a Tacoma Narrows Bridge that would span 853 meters (2,800 feet)—exceeded only by the George Washington and Golden Gate bridges—with plate-girder stiffeners only 2.4 meters deep. The depth-to-span ratio of 1:350 was unprecedented; New York's Williamsburg Bridge had a depth-to-span ratio of 1:40. While contemporary practice suggested a width-to-span ratio of 1:30, the Tacoma Narrows Bridge was designed with a narrow roadway that more than doubled this ratio to 1:72.

Even before completion the structure had begun to exhibit a rhythmic vertical motion under wind loads that earned it the nickname "Galloping Gertie," and diagonal cable ties and hydraulic buffers were installed to dampen the motion. But that was not enough. In the face of gale-force winds the bridge began vertical oscillations that then shifted to a torsional oscillation that soon tore the structure apart—all of it recorded on unforgettable still and motion picture film. In its 1941 report, the board that investigated the collapse concluded that the bridge's fundamental weakness lay in its great flexibility, both vertically and in torsion, and its relatively small capacity to absorb dynamic forces. It was clear that there were fundamental questions about the aerodynamics of suspension bridges that remained unanswered.

Scott gives us a good account of the long period of research into the aerodynamic characteristics of suspension bridges that followed the collapse, as well as the new era of spectacular achievement by bridge engineers that it helped to make possible. This post-Tacoma era is covered in chapters that range across such topics...