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Working Work of Art
Posted By admin On May 6, 2013 @ 2:15 pm In Better Bridges,In the Magazine,Technology | No Comments
At 130, the Brooklyn Bridge still does the job it was built for.
by Doug Hecox and Josh Guterman, FHWA
“Many believe the Brooklyn Bridge will last forever, but it won’t last without people continuing to keep it well maintained.”
– NYCDOT’s Joannene Kidder
Even before it opened to traffic on May 24, 1883, the Brooklyn Bridge was an icon. It has stood protectively between New York City and Brooklyn ever since.
It is still a working work of art. And it is still doing the job it was built for thanks to both the genius of its builders and the people who maintain it.
A triumph of design and innovation, the bridge is the crowning achievement of legendary steel-suspension bridge pioneer John A. Roebling and remains a masterwork of American engineering and ingenuity.
At the time of its construction in the 1870s, the Brooklyn Bridge was more than simply the largest manmade structure since the pyramids. It was a bold statement of possibility, an imaginative use of masonry, steel and engineering which defied nature and opened the door to America’s Industrial Age. Even today, the bridge remains a powerful symbol that – through mathematics, ingenuity and perseverance – no dream is too big to achieve.
“It looks like a motionless mass of masonry and metal but, as a matter of fact, it is instinct with motion,” said Abram Hewitt at the bridge’s dedication in 1883. “It is an aggregation of unstable elements, changing with every change in the temperature, and every movement of the heavenly bodies. The problem was, out of these unstable elements, to produce absolute stability; and it was this problem which the engineers…had to solve, or confess to inglorious failure.”
Nearly a century and a half later, the problem remains. The bridge and its thousands of component parts are under constant attack by the elements – the unrelenting freeze-thaw process in which warming and cooling temperatures cause metal parts to expand and contract, and cracks to develop in masonry and pavement. Damage is further exacerbated by vibration from traffic and wind, and even the acidic content of accumulated bird droppings.
The armies of nature and time constantly attack, but the bridge is hardly defenseless. Legions of maintenance and repair workers are fighting back.
In 2009, New York City DOT (NYCDOT) workers began the bridge’s first major rehabilitation in more than a decade. The $508 million project, relying on $222 million in federal funds – including $30 million from the American Recovery and Reinvestment Act – is replacing deteriorating pavement and doubling the capacity of two ramps.
In addition, for the first time since 1976, workers are repainting the bridge’s exposed steel to prevent corrosion from rain, humidity and other environmental moistures. They are painting it “Brooklyn Bridge Brown,” the unofficial nickname of the federal paint color Medium Brown. The cable spans – the large cables which suspend all the little cables attached to the bridge deck – will be painted Light Buff.
Each piece of the Brooklyn Bridge, from the timber in its foundation to the cable straddles atop its towers, is a testament to superior design and workmanship. The bridge is so well-built that it needed no major rehabilitation for more than a century.
The Brooklyn Bridge is significantly larger than any of its contemporaries and remains an engineering marvel even by modern standards. When it opened to traffic, it was the world’s longest steel-wire suspension bridge and remained so for 20 years until the opening of the nearby Williamsburg Bridge.
The Brooklyn Bridge’s center span is significantly longer than earlier bridges of the day. It is 538 feet – or more than 50 percent – longer than Roebling’s Suspension Bridge completed in 1866 between Covington, Ky., and Cincinnati, Ohio.
The deck of the Brooklyn Bridge, which has supported pedestrians, trolley cars, trains and – once – a parade of elephants, looms about 12 stories above the water line. It is just 10 feet shorter than the Equitable Life Building that, though built in 1870, many consider to be the world’s first skyscraper.
The bridge is just as impressive underwater as it is above. Deep under the murky East River, its two towers dig deep into the riverbed. The base of the Brooklyn tower is 45 feet below water level while the Manhattan tower’s base nearly 79 feet below water level.
Not including its masonry, the bridge weighs nearly 15,000 tons and relies on more than 6.8 million pounds of steel wire. However, its size and historical significance are overshadowed by its enduring structural strength. In 2008, the NYCDOT funded a study of the bridge’s structural integrity. The research revealed that not only is the Brooklyn Bridge well-built and structurally sound but it remains so strong that – even after more than a century – the Brooklyn Bridge can withstand a 2,500-year earthquake event without failing.
Maintaining the aging Brooklyn Bridge is a full-time job for the NYCDOT’s Division of Bridges, which Commissioner Janette Sadik-Khan knows all too well.
“The Brooklyn Bridge is a beloved landmark and a destination for millions of annual visitors but, for New Yorkers, it is a basic transportation lifeline,” she says. “It takes a daily commitment to the basics, an army of engineering professionals and a long-term investment strategy of maintenance and rehabilitation to keep this bridge healthy for the next 130 years. With this approach, we are lifting the last of the city’s 788 bridges into a state of good repair.”
The city spends an estimated $2 million each year on electricians, masons, ironworkers, welders and oilers. Workers are on call 365 days a year to ensure that the bridge safely carries its daily load of more than 120,000 vehicles, 4,000 pedestrians and 3,100 bicyclists.
“We basically take care of everything from the smallest bolt to the biggest cable on the bridge,” says Sunil Desai, preventive maintenance director for NYCDOT’s East River Bridges. “From the footing down in the ground to the tower top, we’re out there day and night.”
Since the city started the East River Bridges rehabilitation program in 1980, more than $657 million has been devoted to the care and maintenance of the Brooklyn Bridge. According to Desai, the pavement milling, repaving, power cleaning and other repair work done by city workers and contractors is valued at more than $10 million each year.
In other words, NYCDOT spends nearly as much each year to maintain the bridge as was needed to build it.
During off-peak traffic hours, workers busily inspect the Brooklyn Bridge’s expansion joints and drainage system, patch pavement, lubricate cables and replace light bulbs. The bridge is also thoroughly pressure-washed – from 10 feet above the bridge deck to its foundation – particularly after snowstorms to remove potentially corrosive deicing agents, using environmentally friendly techniques.
Towers and Foundation
The Brooklyn and Manhattan towers serve as the major load-bearing structures of the historic bridge and are a continuing example of Roebling’s mastery of engineering and design. Dr. Mishac Yegian, P.E., and Dr. Serafim Arzoumanidis, P.E., who led the bridge’s 2008 structural integrity study for Parsons Corp., say that what they found surprised them.
“It’s amazing that, without having the benefit of advanced analytical tools to model this unusual structure, they accurately predicted the settlement,” says Yegian, the distinguished professor at Northeastern University’s College of Engineering, “and it was very small.”
Heavy structures are typically built on bedrock, to prevent settling – which can compromise structural integrity over time. For various reasons, getting to bedrock wasn’t possible on either side of the East River, making settling a concern for Col. Washington Roebling, who managed the project after his father’s death in 1870.
Yegian’s and Arzoumanidis’ analysis confirmed that the elder Roebling’s estimate of the towers’ settlements were less than an inch.
“Roebling was a pioneer. He was driven a lot by his intuition,” says Yegian. “He didn’t have the analytical tools to exactly determine all the forces as accurately as we would today. He made it work.
“If you ask me, it’s the most fascinating structure in the United States,” Yegian adds. “It’s an architectural marvel. I am in love with this bridge!”
The towers, which artfully support the bridge, seem to have drawn inspiration from European cathedrals “There are records that he replicated church architecture from Europe,” says Yegian. “[Roebling] knew that these structures had a lasting survival rate, and he used it.”
Using laboratory testing and “crosshole tomography”– one of the most sophisticated analysis techniques available – to study the condition of the foundations, Yegian and Arzoumanidis say they realized the timber used in the bridge’s foundations, coated with creosote as an early form of waterproofing, has not weakened since it was first put in place more than 130 years ago.
“The bridge’s foundations use a timber caisson, made of Southern Pine – which is very high-quality, dense wood,” says Yegian. “This timber grillage is outstanding quality, and has done extremely well. I see no reason why it couldn’t last another hundred years. We didn’t see any deterioration.”
During its construction, the bridge was the cutting edge of bridge technology – from the cables above to the concrete used in the foundations below. Even the construction methods used to build the bridge, including the large pneumatic caissons deeply embedded in the riverbed, were revolutionary for the time.
“Roebling was definitely a genius because he had the vision to not only design the structure but to develop materials and construction methods,” says Arzoumanidis. “There was no other bridge before like this.”
Yegian and Arzoumanidis agree that the modern engineering world needs more people like Roebling.
“We have lost our creativity and ingenuity because so much of what we teach is based on using computers, and Roebling was not a slave to computer models,” says Yegian. “He knew what was going to work well using only the basic fundamentals of engineering.”
Road Deck and Cables
Suspension cables are one of the most critical components of the bridge. With 14,000 miles of metal wire, the suspension cables used to carry the weight of the bridge could stretch more than halfway across the globe – from New York City to Beijing and back – and need to be lubricated. At the anchorages and on top of the towers, the cables’ lubrication is critical because, if unchecked, moisture can cause corrosion which, in turn, could weaken them and put the bridge as a whole at risk.
“I have a crew who goes out and lubricates the cables, to protect them from moisture in the air, which is a critical function,” Desai says. “The lubricant we use is Vitalife 400 – an oil-based wire rope lubricant – which works best for us. We use about 300 gallons each year to protect the cables.”
Even the Vitalife lubricant has a history that can be traced back to John A. Roebling’s Sons Co., which developed the product in the 1890’s with American Oil Supply International.
The spider web of steel wires supporting the bridge is an engineering feat in its own right. Each strand, about the width of a garden hose, consists of seven or eight shoelace-sized steel wires bundled together. Each cable consists of 15-20 wire strands.
“Each and every activity is uniquely challenging, but cable lubrication and cleaning is the hardest part,” Desai says. “The confined spaces of the anchorages, and the hatches on the tower tops, make lubricating the cables especially difficult – but they carry the weight of the bridge, so lubricating them to guard against corrosion is very important.”
Each winter, city salt trucks keep the bridge’s deck clear of ice and snow. In warmer months, the bridge’s drainage system is monitored to ensure there is no ponding or other potentially damaging water conditions after it rains.
Maintaining the traveler – a moving platform under the bridge – is also critical, he says, because it is needed for workers to grease bearings at the end of each suspender cable. Keeping them well-lubricated is necessary to allow the cables to flex, swivel and accommodate the bridge’s slight but ever-present vibrations.
Most of the great bridge’s recognizable features are original. The two cathedral-like towers facing off across the East River are original, as is the masonry, the timber grillage and the caissons deep underground upon which they stand. The main cables, with their distinctive catenary curves, are also original. Many of the handrails along the bridge’s promenade and other decorative elements – like the “harp lights” – are just as workers made them in the early 1880s. The arch-block structures on either end of the bridge, which are like sidespans and help connect the bridge to the streets in Manhattan and Brooklyn, are also original, though their asbestos lining has since been removed.
By all accounts, the Brooklyn Bridge is aging exceptionally well. Yegian’s and Arzoumanidis’ work in 2008 suggests that the bridge – if properly maintained – is sturdy enough to remain in use for another century – or more.
The Key to Longevity
The most important element of the bridge’s longevity isn’t wire, masonry or pavement. According to the NYCDOT’s Joannene Kidder, it’s people with a passion for engineering.
“We need more dedicated public servants and people who want to go into bridge engineering,” she says. “We have a generation of young people who aren’t going into engineering. It’s not seen as sexy, but America is woefully short of young engineers coming into this line of work. We need new young kids with better computer skills and new, forward-thinking ideas about using new materials and innovative construction techniques. We always need more money, but the real need is people.
“Many believe the Brooklyn Bridge will last forever,” she adds, “but it won’t last without people continuing to keep it well maintained.”
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