Tacoma Narrows Number Three
The replacement bridge, opening in 1950 as the third-largest suspension bridge in the world, substituted deep open-truss steel frames for the shallow 8-foot (2.4-meter) wind-catching steel girders of the delicate original. With its successfully stiff construction, this second span became known as Sturdy Gertie. The engineering design is credited to Charles E. Andrew and Dexter R. Smith, with construction by the Bethlehem Pacific Coast Steel Corp. and John A. Roebling's Sons Company.
Even today, resonance continues to be a tricky factor in engineering design. The Millennium Bridge in London, designed by Norman Foster and engineered by Arup, had a similar class of problem — resonant vibrations, though not wind-aided — but at a more modest level of discomfort rather than danger. Those problems were diagnosed and addressed, and the bridge was safely reopened. The recent devastating collapse of a bridge undergoing maintenance in Minneapolis, Minnesota, is another reminder that errors in engineering and construction remain a terribly present threat.
Side by Side
As a sculptural object, the original Galloping Gertie was quite beautiful. The same slender, cost-effective solid plate girders that doomed the span gave it a rather beautiful simplicity of line and a delicate elegance — rather too delicate, as it turned out. Vertical ribbing and reduced expression of joints and bracketing on the Art Deco-influenced towers helped to emphasize their height and slimness, and to connect with the vertical stiffening ribs on the solid plate girders of the deck frame.
The Tacoma Narrows Bridge of 1950, still long and lean as one of the world's longest suspension spans, also has a grace of proportions, and the visual unity of all-steel construction throughout the primary structure. The deeper, open trusses of the deck framework set a different motif. Seeking a more directly utilitarian expression, the towers express open cross-bracing to match, with three sets above the bridge deck dividing the height of 507 feet (154.5 meters) above the water into proportionally adjusting thirds — except for one western side span support, beneath the bridge deck, which was undamaged in the 1940 collapse and still remains in use today, ribbing intact.
In some ways, the Tacoma Narrows Bridge of 2007, designed by HNTB and the Parsons Transportation Group, and built by Bechtel Corporation and Kiewit Pacific Company, pushes farther into the utilitarian, following the trends in construction, material, and maintenance costs to switch to reinforced concrete for the towers. The towers of 2007 are only about three feet (one meter) taller, while accommodating a roadway specification about 20 percent wider gives them a slightly squatter proportion overall. The thicker concrete cross-sections, compared to all-steel sections, plus the choice of two cross-members instead of three, contribute to the blockier impression of the 2007 towers.
The superficial embossing of X-braces onto the crossmembers of the new concrete towers is clearly well-intentioned as a historic reference. But it has no chance at the authenticity of the actually cross-braced corresponding members on the 1950 towers, and ends up as a literally superficial, metaphorically hollow gesture. Surely there is a better way to express the positive relationship between these two great bridges, side by side.
But the overall grandeur of the great suspension spans, crossing together through the spectacular natural setting of the Narrows, frank and elegant in their matching catenary curvatures, is beautiful enough to supersede some fine-grain shortcomings.
There's a special beauty to a great structure under construction. Hasn't it felt like a special privilege of the building set to take pause in the rugged working beauty of a harsh construction site, to gaze upon a structural frame silhouetted at sunset as work settles down? Even in a prosaic site, such can be a reverent moment. How much more ready the reverence in a site and during a construction project as poetically beautiful as at the challenging Tacoma Narrows.
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