In December, the world’s widest cable-stay bridge, weighing more than 72,000 tons and measuring more than 213 feet wide,
opened to traffic. Port Mann Bridge, outside Vancouver, British Columbia, is the second-longest cable-stay span in North
America at 2800 feet, but this is no suspension bridge as is the Brooklyn Bridge or the Golden Gate. Port Mann is the
evidence of cable-stay bridges taking over—and doing the job for less—where once only suspension bridges came to play.
Twenty years ago in the Normandy region of France the world watched in amazement as the then-longest cable-stay bridge
reached a span of 2800 feet. But that record wouldn’t last. The Stonecutters Bridge in Hong Kong opened in 2009 at nearly
3400 feet and 2012 saw the opening of the Russky Bridge in Russia at more than 3600 feet across. "Twenty years ago the
Stonecutters or the Russian bridge would have likely been a suspension bridge," says Allan Brayley, chief engineer for mega-
contractor Flatiron’s heavy civic division. "Cable-stays are getting longer and longer and being built where suspension
bridges were once built."
To differentiate between the two, think about Golden Gate’s long, drooping cables, which connect the tops of the towers.
Suspension bridges feature three spans—one that connects the two towers, and one that runs from each tower to the shore.
Smaller vertical cables descend from those main cables, holding up the deck and giving the suspension bridge its name. In a
cable-stay bridge, by contrast, multiple load-bearing cables run off the tower straight to the roadway rather than descending
from the main cables.
The landmark splendor of a suspension bridge comes at a price—it requires two anchorage rooms and possibly an extra tower,
which can cost extra millions. So conventional wisdom had builders opting for the cable-stay bridges on spans shorter than
2000 feet and suspension bridges on anything longer than 3500 feet. The problem is that gray area, where building a
suspension bridge is too expensive but building a cable-stay bridge is impractical. Brayley, who worked on the Port Mann
Bridge in Canada, said stretching a cable-stay bridge to longer lengths meant making the towers taller and taller, because
"there is a sweet spot in the angle of the stays." But, of course, making taller towers increases the bill. Now, though,
cable-stay bridges have started to fill the gray area and even push against ground traditionally hallowed for suspension
bridges.
How? More durable concrete, the addition of stainless-steel rebar to reduce cracking and buckling, molten-zinc spray-on
coatings for steel, and the inclusion of corrosion-monitoring sensors have helped extend the life of bridges. The increased
strength of newer steel helps as well. Where 3600 pounds per square inch of capability was once the norm, now you can expect
nearly double that: 7000 psi. That means engineers can achieve the same strength with less material, reducing cost and weight
and optimizing bridge performance.
Each 0.6-inch-diameter post-tensioned steel strand gets strung and stressed individually now, giving each strand a similar
strength of about 1000 psi. "You are ending with equal tension and that makes a huge difference," Brayley says—it allows
engineers to use fewer strands per bridge. "And the understanding of how cable-stay bridges perform has been a real driving
force in being able to go longer and longer."
Patrick Cassity is vice president of Parsons, the designer of some of America’s largest suspension bridges (the New Tacoma
Narrows Bridge in Tacoma, Wash.) and cable-stay bridges (the John James Audubon Bridge in Louisiana). He says that advanced
carbon-composite technology that is now making supercars lighter and faster could allow bridges to keep getting longer—if it
becomes affordable enough, that is. "The strength-to-weight ratio is significant," Cassity says. "I would bet we are one day
making more use of advanced materials now used in the aerospace industry to build rockets and space shuttles."
Load-carrying abilities and material costs aren’t the only factors in the rise of cable-stayed bridges, either. "These days
aesthetics can be a primary driver," Cassity says. "You are dealing with a very large structure that becomes part of our
urban fabric and has a visual impact on the community or natural environment."
Simply, people want a nice-looking bridge, and there are two main places for the designer to be creative: towers and color.
(What would the Golden Gate be without constant touchups to its International Orange paint?) The pylons allow the most room
for character, especially on a cable stay. See the pointed nature of the Millau Bridge in France, or the crazy outward-facing
geometry of the Zolotoy Bridge in Russia. Bridge designers can choose to use steel or concrete and then get funky with shapes
and colors.
However, Cassity cautions, you can’t get too cute. "When you are dealing with such significant loading, if you start making
a pylon into a shape it doesn’t want to be, structurally you will pay a significant financial premium," he says.
The bridge deck can get creative too. Cassity says the most recent trend in bridge design is the inclusion of aesthetic
lighting technology that uses LED panels mounted to bridge girders to light up the sides and towers. Preprogrammed light
events create patterns for specific events or times of the year.
Lights and colors are fine. But what still gets people going about bridges is their ever-expanding reach—and the drive to
build bigger and bigger. "As man has more experience, accumulates more research data and better analytical tools, the spans
keep going higher and higher," Cassity says. "It feeds into man’s natural ego to build the longest spans ever."
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