How does a Whitestone Bridge work

Rehabilitation for crumbling bridges

As a traffic planner for the city of New York City, Robert Moses was commissioned in April 1937 to build a car bridge over the East River between the boroughs of Bronx and Queens. Just two years later, on April 29, 1939, he cut the ribbon and opened the Bronx-Whitestone Bridge. It is 1149 meters long. Between the two 41-meter-high bridge piers, the four-lane carriageway extends over 701 meters, suspended from two 1208-meter-long, tree-thick cables. In keeping with the spirit of the times, the Swiss engineer, Othmar Ammann, who was commissioned with the construction, made the entire suspension bridge - pillars, ropes and supporting structure - out of steel.

Architects from all over the world praised the efficient, filigree and elegant construction in Art Deco style. But in November 1940, a similar bridge in Washington State, the Tacoma Narrows Bridge, rocked so much in a storm that it collapsed. It later turned out that the width of the bridge deck was too small in relation to the span. That didn't bode well for the Bronx-Whitestone Bridge: in 1943, after a storm, it swayed so violently that Ammann had to mount a massive and not at all elegant steel girder truss on each side along the roadway for stiffening.

For bridge builders, the story of the Bronx-Whitestone Bridge is a prime example when they talk about the inadequacies and need for optimization of many of today's bridges. It is not so much the wind that is bothering them, but above all the rapidly growing traffic. Take Germany as an example: in 1970 there were around 17 million vehicles on German roads; today there are over 50 million. The Federal Ministry of Transport predicts that truck traffic alone will increase by 85 percent by 2025. There are exactly 38 525 bridges (as of May 2010) on German motorways and federal highways. Many of them urgently need to be renovated. "Even today, 15 percent of these bridges are in a critical or unsatisfactory condition," says Jürgen Berlitz, specialist advisor for road traffic planning at ADAC. A total of 46 percent of the buildings are so damaged due to their age and increased stress that maintenance measures are urgently required.

The Federal Roads Office (BASt) estimates the lifespan of bridges at 80 to 100 years, depending on the type of construction. Around half of the bridges on German highways are already over 30 years old. Your steel tires and rusts, weather and chemical processes cause the concrete to crumble. In 1999, according to ADAC, 35 percent of the bridges were in very good or good condition, in 2008 it was only 16 percent. In 2008, the preservation of these bridges cost 350 million euros. But that's just a drop in the ocean. In order to make the buildings resilient in the long term, 5 to 7 billion euros are required, says ADAC expert Berlitz.

60 tons is too much

And there is a risk of even higher costs across Europe: the result of an EU study on the approval of so-called gigaliners should be available this summer. According to a BASt report, if such articulated lorries with a maximum weight of 60 tonnes should also press on the bridges in the future, most of the bridges can experience “loads that are greater than the rated values”. That means: They are then probably not allowed to be driven on - unless an additional eleven billion euros are invested in upgrading them. Faced with these problems, transport planners sooner or later face a choice: renovation and reinforcement or demolition and new construction. The latter, however, is hardly possible in view of tight budgets. Help comes from research: New materials and construction methods should help make tired bridges fit again. These innovations are conceived and tested in Switzerland.

From Zurich main station there are only three stations with the S-Bahn to Dübendorf. At the train station in the quiet town, small signs point to the destination of the journey. It is only a few minutes' walk to a spacious area with factory-like buildings made of red brick. This is where Empa, the Federal Materials Testing and Research Institute, has its headquarters. In front of the entrance to the Empa administration building, a flat pedestrian bridge arches over a small pond. "We show that you can build safe bridges without steel and concrete," says Urs Meier, former director of Empa and globally recognized bridge expert, about his structure. Its wooden bridge deck is stretched on the underside by parallel strips made of carbon fiber reinforced plastic (CFRP). The construction is reminiscent of an arrow bow directed skywards. The arrow arch bridge is Meier's sure-footed example of what the composite material can achieve in bridge construction.

Powerful flyweight

Since the 1980s, the civil engineer has been working with materials made from carbon fibers, which at that time were mainly used in the aerospace industry. “CFRP is the ideal material for building bridges,” says Meier. The fine black fibers can be used flexibly. They can be shaped into mats, beams or cables. Above all, a carbon fiber embedded in plastic is up to four times as strong as steel, but weighs only a fifth. In the 1990s, under Meier's leadership, Empa began winding CFRP tapes around weak bridge piers and gluing them under congested roadways. "This increases the load-bearing capacity of a bridge enormously," says the now 67-year-old civil engineer.

Today, specialized construction companies around the world use CFRP materials to reinforce bridges, also because the strips and mats, which are often only a few millimeters thick, hardly change the appearance of the structures. Around a quarter of the world's annual carbon fiber production is already used in construction. In view of the renovation emergency, the demand is expected to increase in the future. Then the high-tech material should also become cheaper - it currently costs around four times as much as steel. Another obstacle: There are no uniform standards for many carbon and fiberglass composites for building. This means a greater bureaucratic effort, because an individual permit must be obtained for each planned use. This is a deterrent for many builders.

Muscle training for the ropes

Urs Meier had the brilliant idea of ​​what could be done with the high-tech material when he was looking at a basic problem in bridge construction: In suspension and cable-stayed bridges, the tonnes of steel cable bundles have to withstand their own weight, the weight of the structure, the roadway and the traffic. The builders always practice the balancing act between minimum material expenditure and maximum load-bearing capacity. So why, so Meier thought, not replace the steel cables with lighter and at the same time stronger cables made of CFRP? That would significantly reduce the dead weight of the bridge, and it could also carry more traffic.

This opens up new horizons for architects: Bridges could achieve significantly larger free spans than a steel or reinforced concrete structure with the same load-bearing capacity, which in turn would open up new traffic routes. "The combination of low dead weight and high load-bearing capacity makes it possible to build a bridge with a span beyond the 3500 meter mark," says bridge expert Meier.

3500 meters - that is currently the largest span, i.e. the distance between two pillars of a bridge that can be built with conventional building materials. This limit is reached by the suspension bridge over the Strait of Messina, which has been under construction since 2009 and which is to connect the Italian mainland with Sicily from 2016: the gigantic structure is to bridge 3300 meters of the Mediterranean Sea, and the two bridge piers are each to rise 383 meters high into the sky . If the span were even larger, the bridge would collapse under its own weight (see box “Gibraltar Bridge Project”).

Empa's test laboratory is a large hall. Forklift trucks remove destroyed concrete blocks in a controlled manner, computers and measuring devices are everywhere. The large space is dominated by a fatigue testing machine as high as a house, riveted together from solid steel girders. Since the 1990s, the colossus has been trying to wear down CFRP bridge ropes through up to ten million load cycles. A typical carbon fiber rope consists of 241 parallel laid fiber strands with a diameter of five millimeters each. In the test, they show a fatigue strength three times as high as comparable steel ropes in the longitudinal direction. Strong deformations, for example due to wind, hardly affect these cables.

In 1996, for the first time, two 35-meter-long CFRP ropes were used alongside 22 steel ropes in the 124-meter-long Storchenbrücke in Winterthur. Since then, specially developed radio sensors have been sending data on temperature, cable forces and humidity from the cable-stayed bridge to the Empa laboratory via the Internet. The result of the long-term test confirmed the researchers' assumptions: unlike steel cables, CFRP cables do not wear out under load and are insensitive to temperature in the longitudinal direction. Unlike steel, they do not expand in sunlight and do not shrink in frost. CFRP ropes are now being used for test purposes on five other bridges.

Rust protection by buckets

“Plastics reinforced with carbon fibers have many advantages,” says Meier, “above all: They don't rust.” Corrosion is a huge problem in bridge construction. Over time, water penetrates into reinforced concrete structures, additives in the concrete react with the metal, the rust causes the concrete to crumble. The steel ropes and structure have to be coated with hundreds of liters of protective paint every five to ten years. That drives up maintenance costs. "Cables made from carbon fiber strands are more expensive to buy, but more sustainable," says the retired Empa researcher. All the cables need is an insulating polyethylene sheath to protect them from lightning strikes. A thin coating softens the effects of harmful UV radiation. "Many suspension and cable-stayed bridges are at the age at which steel cables have to be replaced due to fatigue," says Meier. "Actually a good opportunity to switch to CFRP ropes."

But the Dübendorfer has to be patient. Because despite their convincing properties, carbon fiber-reinforced plastics have so far not played a role in construction. Silvio Weiland knows why: “Builders prefer to rely on familiar materials. In addition, the planning and approval effort is still too high. ”The 35-year-old civil engineer manages the business of the Tudalit association, which was founded in 2009. The amalgamation of companies in the construction industry and bridge researchers from the Technical University of Dresden aims to make textile-reinforced concrete the building material of the future under the brand name "Tudalit". "Textile concrete is a composite material that is similar to reinforced concrete," says Weiland, "except that it is reinforced with a perforated mat made of carbon or glass fibers instead of steel grids."

Bridge piers with plaster cast

The processing of the mats on the construction site is similar to the application of a plaster cast: the mats are cut to size, placed individually or in layers on the bridge deck or around the pillars and then covered with a millimeter-thin layer of shotcrete. The whole thing is so quick and uncomplicated that the flow of traffic is hardly disrupted. Silvio Weiland also praises the versatility of the material: "Textile concrete enables constructions and components that cannot be made with reinforced concrete." The thin concrete layer protects the mats from the weather and UV radiation. If a component is damaged, the concrete cracks first, and then the fiber matrix.

Textile concrete is not only used as a high-tech pavement: in 2006, at the State Garden Show in Oschatz, Saxony, the world's first textile concrete bridge was built. The nine meter long arched bridge resembles an elongated bathtub. The walls are only three centimeters thin, the weight is five tons - made of reinforced concrete, the bridge would weigh 25 tons. The project partners at RWTH Aachen are already working on a variant that is around 100 meters long. If a bridge is supposed to span more than 200 meters, the cable-stayed bridge has been the first choice since the 1960s. Due to its design, it requires less steel and concrete than a suspension bridge, for example. This lowers construction costs - and reduces weight. But cable-stayed bridges also have a problem: “Stay cables tend to vibrate violently at critical wind speeds,” says Empa researcher Felix Weber. With rope lengths of 150 to 450 meters, the middle of the rope can swing one to three meters. Stay cables made of steel are just as affected by this as those made of CFRP. To eliminate this problem, Weber and Maurer Söhne from Munich have developed an active damping system for cable-stayed bridges. Without damping, many bridges of this type would have to be closed in strong winds, because the ropes could notch at the anchorage points and be damaged. This is not the case with the active (“smart”) hydraulic damper that Weber installed on a small cable-stayed bridge in the Empa test laboratory: near the cable anchorage on the bridge deck, a cylinder as thick as an arm and a piston braces itself against the cable. The researcher sets it vibrating with his hand. A motion sensor measures the vibration 100 times per second, and a PC uses this to calculate the optimal damping force. The actual damping is done by an oil mixed with magnetizable particles. Depending on the required damping force, a coil generates a weaker or stronger magnetic field. Depending on the strength of the field, the magnetizable particles in the oil clump together more or less, which changes the viscosity of the mixture. The damper regulates the resistance force automatically and continuously. The technology is already in use: the 518-meter-long Franjo-Tudjman Bridge near Dubrovnik in Croatia, which opened in 2002, was retrofitted with active dampers on all 38 stay cables in 2005. A storm had caused the steel cables, which were up to 220 meters long, to swing out a meter and a half. "With the smart dampers, the amplitude is reduced to a tenth," says Felix Weber. A network of radio sensors continuously records the vibration data, and the damping system can be optimized by remote control via the Internet. In the future, lightweight bridges will probably be equipped with such active dampers during construction. 48 of these were built into the Sutong Bridge over the Yangtze, which opened at the end of 2008 and connects the two Chinese cities of Suzhou and Nantong. It is currently the cable-stayed bridge with the largest span: 1088 meters. Without a damper, their steel cables, which are up to 541 meters long, could not be tamed in a storm.

Compressed air against rocking

Scientists from the Solid Construction Department at the Technical University of Berlin have devised another damping method. They constructed an approximately 15-meter-long tension band bridge based on CFRP. Concrete slabs lay on narrow, stretched ribbons made of carbon fiber as a walkway. In order to protect the simple construction from vibrations, the Berlin researchers want to dampen it with a kind of artificial muscle. The idea: Acceleration and position sensors record the state of the bridge and send control signals to three plastic tubes that are mounted on each side in the handrail of the railing. The tubes can contract or expand using compressed air. The artificial muscles react sensitively to every movement of the CFRP and concrete framework. “The whole thing is still in the experimental stage,” says Achim Bleicher, who is researching in the Berlin team.

The Bronx-Whitestone Bridge in New York has been completely renovated since 2002. The enormously swollen traffic and the weight of the additional reinforcement from the 1940s had put too much strain on the main cables of the suspension bridge. In order to reduce the weight, construction workers also removed the steel framework construction that was so unpopular by Othmar Ammann. Instead, a feather-light aerodynamic cladding made of fiberglass-reinforced plastic along the roadway directs the wind around the bridge in such a way that it cannot cause any damage. The building, which is over 70 years old, will not only survive the coming decades. It also looks as light and slim as it did when it opened. ■

The technology journalist Martin Borré encountered this topic during renovation work on ailing Rhine bridges in his home town of Cologne.

by Martin Borré

The bridge master's bag of tricks

Sensors, measurement technology and new materials are intended to help monitor the safety of bridges, extend their service life and improve their construction. This is what ropes made of CFRP are used, for example: hundreds of bonded carbon fibers that are four times as strong and only a fifth as heavy as steel ropes.In the future, bridge piers will also be made from this material. With textile concrete, CFRP mats replace the steel mesh in the concrete. Dimples in the ropes soften the effects of the wind and prevent vibrations. “Wings” on the bridge deck direct the wind around the structure. If the bridge ropes still vibrate, they can be dampened with “smart” hydraulic elements. The dampers receive their control data from radio sensors. Temperature sensors are also located on the ropes in order to take into account the different vibration behavior in hot and cold conditions. Cameras, GPS devices, motion sensors and weather stations monitor the condition of the bridge.

Small bridge customer

Over the millennia, bridge builders have developed a whole range of bridge types. Today the art is to develop these basic constructions further and to adapt them to the local conditions. The bridges for road and rail traffic are mostly based on one of the following three types:

Suspension bridge

An old type of bridge, known from jungle films like "Indiana Jones" and "Pirates of the Caribbean": Modern suspension bridges consist of suspension ropes that are guided over towers and on which the roadway hangs. In theory, this means that spans of up to 3500 meters can be achieved. Disadvantage: The suspension bridge tends to swing in the wind and twist easily, which is why it is unsuitable for rail traffic.

Cable-stayed bridge

A modern variant of the suspension bridge: the roadway is supported by inclined ropes that are anchored to a pylon and in the bridge deck. The entire load is directed via the ropes into the pylon, which diverts it into the ground. Builders appreciate this type of bridge because it is quicker and therefore cheaper to build than a suspension bridge, for example. If you combine several cable-stayed bridges, spans beyond 3500 meters can also be achieved. And: The relatively stiff construction also enables rail transport.

Girder bridge

Above all, girder bridges made of steel and / or reinforced concrete span highways, federal highways and country roads. There are different variants, but the principle is always the same: the bridge deck, which looks like a beam, rests flexibly on a substructure, which for example consists of several supporting pillars. Girder bridges are relatively easy and therefore inexpensive to erect, but are only suitable for spans of up to 200 meters.

Gibraltar bridge project

Politicians in Spain and Morocco have been thinking about a transport link between their countries since 1979. The proposal of bridge builder Urs Meier, ex-director of Empa in Dübendorf, Switzerland: A gigantic bridge should connect Europe with Africa. Namely where the continents are closest: on the 14 kilometer wide waterway from Gibraltar. It is hardly possible to build a tunnel because of the water that is up to 900 meters deep.

However, bridge piers can only be erected at a maximum depth of 350 meters. A bridge must therefore span the entire strait. “You can't do that with steel and concrete,” says Meier. “The bridge would collapse under its own weight.” The solution: a 16.2-kilometer combination of two cable-stayed bridges with cables and a structure made of strong but light carbon fiber reinforced plastic (CFRP). “The two pylons would be 850 and 1250 meters high above water,” says Meier. (For comparison: the tallest building in the world, the Burj Chalifa Tower in Dubai, measures 828 meters.) There would be a free span of 8,400 meters between the pillars. This would mean that the bridge with the largest span of 1911 meters, the Akashi Kaiky Bridge in Japan, would be four times longer. It would take 150,000 tons of carbon fiber to build the bridge, twice the world's annual production. The costs are also a problem: CFRP is four times as expensive as steel.

Compact

· Plastics reinforced with carbon fibers can significantly increase the load-bearing capacity of bridges.

· Mats made of textile-reinforced concrete defy rain, frost and UV radiation.

· Active damping systems prevent dangerous rocking of bridges.

more on the subject

INTERNET

Alternatives to the planned bridge to cross the Strait of Gibraltar: www.gibraltarinformation.com/ gibraltar-bridge.html

September 21, 2010

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