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|Federal Highway Administration > Publications > Public Roads > Vol. 69 · No. 5 > Preservation Act|
Publication Number: FHWA-HRT-2006-003
by Earl E. Dubin
Whether they swing, retract, or are raised, New York City's movable bridges are receiving a much-needed dose of care.
Of the nearly 600,000 bridges listed in the Federal Highway Administration's (FHWA) 2004 National Bridge Inventory, close to 1,000 fall into the category of movable spans. As the name suggests, movables can rise, pivot to the side, or even slide away to accommodate boat traffic on the rivers below.
Movable bridges are common sights in rural areas such as upstate New York, where a dozen or so cross the Erie Canal. Likewise, they are familiar fixtures along the Nation's shorelines, spanning the Intracoastal Waterway. And they are workhorses in metropolises such as Chicago and New York City. In short, movable bridges are small in number compared to their more numerous fixed cousins, but they fulfill the important role of spanning navigable waterways in areas where development or other constraints preclude the construction of a fixed high-level bridge.
Of course, all bridges must be properly maintained to withstand the forces of heavy traffic, weather, aging, and other factors that can weaken their structural integrity. Movable bridges, however, must endure the additional stresses of motion. New York City has found an effective way to address this challenge.
"An essential component of keeping movables in good working condition is a multidisciplinary team of engineers and maintenance personnel knowledgeable in structural, mechanical, and electrical engineering," says Dave Hart, senior area engineer with FHWA's New York Division. "New York City has found that balanced approach."
Movables in New York City
Both types of bridges—fixed and movable—helped to create New York City's reputation as one of the world's foremost urban centers. Engineering marvels such as the Brooklyn Bridge, George Washington Bridge, Verrazano-Narrows Bridge, and others join New York's many boroughs and suburbs, enabling motorized vehicles, trains, bicycles, and even pedestrians to travel safely each day. The bridges connect the city, as a thriving center of commerce, with the broader U.S. economy.
Within the New York City Department of Transportation's (NYCDOT) inventory of more than 2,000 bridges are 25 movable spans that serve a critical role, accommodating more than 1 million daily vehicle crossings. To ensure the safe passage of traffic, both on streets and waterways, the movables must be maintained in prime working condition.
A Few Tons of Prevention
To address the structural, mechanical, and electrical needs of its movable bridges, New York City has implemented an aggressive reconstruction and rehabilitation program. During the next 10 years, it will spend an estimated $1 billion in city, State, and Federal funds, based on estimates from NYCDOT, to rehabilitate and reconstruct most of the movable bridge inventory.
Federal highway bridge funds are important to the successful rehabilitation and replacement of the city's movable bridges. In cooperation with FHWA and the New York State Department of Transportation (NYSDOT), the city implemented an aggressive preventive maintenance program for four fixed East River Bridges (see "Protecting New York City's Bridge Assets," Public Roads, May/June 2005). As part of the next phase of the program, a contract will be granted to perform preventive maintenance on the city's movable bridges, including lubrication and maintenance of mechanical gears and components, cleaning and repainting of structural members, and upkeep of vital electrical motors, control panels, and other electrical components.
"The extension of the city's East River Bridges preventive maintenance plans to the movable bridges was a logical next step," says FHWA's Hart. "The city and FHWA are spending a lot of money to rehabilitate and replace these bridges. It's important to protect our investment, and the development and implementation of a well-thought-out preventive maintenance plan is the best way to do that."
Beyond a Fixed Inspection
The inspection and maintenance of mechanical and electrical components provide two of the more significant challenges in any movable bridge rehabilitation or replacement plan. The National Bridge Inspection Standards (NBIS) inspection procedures were developed when the failure of critical structural components led to significant and often tragic events. For example, overall requirements for bridge inspection programs came into existence after the December 1967 collapse of the Silver Bridge between Gallipolis, OH, and Point Pleasant, WV, which killed 46 people. Likewise, the June 1983 Mianus River Bridge collapse in Connecticut resulted in inventory and inspection guidelines related to fracture-critical requirements. And the April 1987 collapse of the Schoharie Creek Bridge in New York State gave rise to new guidelines for underwater inspection.
In the past, the NBIS did not contain specific requirements for the inspection of mechanical and electrical components. Recent NBIS revisions now require the development of a comprehensive plan for the inspection of complex bridges (including movables). These revisions suggest that mechanical and electrical components should be inspected as part of a regular bridge inspection program.
According to the first edition (1998) of the American Association of State Highway and Transportation Officials' (AASHTO) Movable Bridge Inspection, Evaluation, and Maintenance Manual, inspection frequency requirements for structural components of fixed bridges also should apply to mechanical and electrical components of movable bridges. But in many cases, team leaders, who are structural engineers, carry out inspections of the mechanical and electrical components as well. These inspectors receive little instruction in how to determine whether mechanical and electrical components are functioning properly. Typical inspections involve nothing more than performing a bridge opening and listening for unusual noises such as grinding or banging.
The AASHTO manual encourages a visual check of mechanical and electrical components during routine inspections without major disassembly, depending on the condition of the components. The guidelines do encourage, however, a more indepth inspection of those components approximately on a 6-year cycle. According to AASHTO, this inspection should be more extensive and involve disassembling mechanical components, measuring shaft and gear clearances, testing electrical components, and conducting other tests using nondestructive testing equipment.
Of equal importance is interpretation of inspection data by a multidisciplinary team that includes structural, mechanical, and electrical engineers. In addition, as more and more drive systems are computerized, engineers who understand those types of systems should be an integral part of the team.
Currently, the attention given to the inspection of mechanical and electrical components varies from State to State and from owner to owner. According to Dan Byer, bridge engineer in the FHWA New York Division, results of a recent survey by the division show that "we have a long way to go in developing and implementing consistent inspection practices for these types of bridges."
He adds, "Recent NBIS revisions require the development of inspection procedures for complex bridges, including movables. However, it remains to be seen how movable bridge owners will respond to these revisions and if more consistent inspections of mechanical and electrical components occur nationwide."
Regardless of the current state of practice, a viable inspection program must work hand-in-hand with the owner's maintenance activities. For NYCDOT, these activities are closely interwoven.
Maintenance, Rehabilitation, And Replacement Responsibilities
In addition to meeting the demands of the traveling public, NYCDOT must carry out U.S. Coast Guard regulations that require movable bridges to be opened to navigation either on demand or according to an agreed-upon schedule. These regulations require NYCDOT to maintain an aggressive maintenance, rehabilitation, and replacement schedule to ensure that all bridge components function reliably.
The responsibility for maintenance, rehabilitation, and replacement of movable bridges rests with NYCDOT's Bureau of East River Bridges/Movable Bridges/Tunnels. Within the bureau's Movable Bridge Design section, approximately 25 engineers, technicians, and support staff work out of two floors of a lower Manhattan high-rise office complex. The skills needed for the job cover a wide range of expertise and experience.
"Within my design staff," says section director Balram Chandiramani, "I look for engineers with training in civil, mechanical, and electrical fields as it pertains to design of structures, power transmission, and modern electrical controls. My team consists of engineers and technicians at various levels of experience and education, from entry level up to project managers."
Working closely with Chandiramani's staff are maintenance engineers and personnel from the Bridge Maintenance, Inspections, and Operations division. "This division offers a multitalented resource that adds a wealth of global know-how to the program," says Russ Holcomb, NYCDOT deputy chief engineer for bridge maintenance, inspection, and operations.
"The New York City Department of Transportation is fortunate to have an in-house force of professional engineers, ironworkers, electricians, and other tradesmen," Holcomb adds. "Many were trained or practiced overseas, making the bureau a truly world-class organization. Specialized training, new hires, and outreach to recent graduates provide a proper mix of established and innovative practices to NYCDOT's knowledge base. The specialized knowledge and skills help keep the bridges operational, ensuring compliance with Coast Guard regulations with minimal impact on marine traffic."
New York City tries to address component deficiencies in the movable bridges with a program of scheduled electrical and mechanical maintenance tasks. The current maintenance program is run by NYCDOT staff but will be supplemented with contract personnel in the future. In addition to addressing existing maintenance of components, the program provides an opportunity for maintenance engineers and staff to observe that these elements are functioning properly.
This program supplies detailed information on the condition of a host of mechanical and electrical components. This information, coupled with the work of the city's bridge inspection and management unit, provides Chandiramani's staff with detailed data for assessing the existing needs of the city's bridges. These inspections then can be used to identify components in need of future repair or attention.
According to the AASHTO movable bridge maintenance manual, the balance system is one of the most important, yet often overlooked, design features of movable bridges. A poorly balanced movable span puts excessive loads on the machinery components of the bridge, reducing the life of the components and jeopardizing the proper functioning of the structure.
Balancing the movable spans is therefore a key maintenance and operational component. "In addition to performing routine electrical and mechanical maintenance tasks, our engineers use electronic instrumentation, such as strain gauges, to monitor the balance of the movable bridges," says Holcomb. These gauges are installed on drive shafts and other mechanical components to determine if they are being overstressed due to load imbalance. If a bridge is determined to need balance adjustment, this can be accomplished by the addition or removal of concrete, steel, or lead blocks from hollow chambers within the counterweights.
"The leaves of the bascule bridges [drawbridges] are very sensitive to changes in the movable span's dead load [that is, the bridge's own weight]," Holcomb says. "Structural repairs, system upgrades, even accumulations of roadway debris can alter the balance of the bridge, putting additional stress on the machinery [that moves the bridge] and reducing the service life of those components. NYCDOT's balance monitoring program reduces that risk by identifying bridges in need of balance adjustment."
In the case of vertical lift bridges, steel cables anchored to the lift span extend up the lift towers and over large gear wheels, called sheaves. The other end of the cables is attached to large concrete counterweights that assist the drive system when the bridge is raised or lowered by counterbalancing the weight of the lift span. However, unlike bascule bridges, the weight of the lift span of a vertical lift bridge changes as the bridge is raised or lowered. As the lift span is moved and the cables connecting it to the counterweight pass over the sheaves, auxiliary weight is needed to compensate for the weight of the cable now on the counterweight side of the sheave. On large vertical lift bridges, these lifting cables can weigh many tons, thus creating potential imbalance during the lifting process. Balance chains therefore are installed to provide auxiliary weight to compensate for the weight of the lifting cables. Proper balance of vertical lift bridges is critical to ensure that the bridge remains level as it is raised and lowered. If the lift span tilts (skews), it could become jammed, a complex and difficult condition to resolve.
Third Avenue Swing Shift
Sometimes maintenance and rehabilitation are not enough, and replacement becomes necessary. One of the NYCDOT projects, the replacement of the Third Avenue Bridge, used fast-tracked construction techniques to place an innovative new swing span over the Harlem River. Floated into place in October 2004, the new structure replaces a span built in 1898.
Each day, the five-lane Third Avenue Bridge carries more than 70,000 vehicles across the Harlem River from the Bronx into Manhattan. Measuring 107 meters (350 feet) long and 27 meters (88 feet) across, the main movable span pivots on bearings to provide two channels for river traffic.
In the past, most swing bridges of this size employed a rim-bearing type of construction that traveled on a large number of tapered rollers or wheels that rested on a curved or circular rack positioned around the center of the swing span. With this method, the bridge's dead load is transferred through the rollers into the rack and then into the pivot pier as the bridge opens.
The Third Avenue swing span, however, relies on a center pivot-bearing assembly that transfers the entire dead load of the span directly to the pivot pier. This assembly houses a spherical roller thrust bearing that supports the 2,721-metric-ton (3,000-ton) swing span and can resist the impact forces associated with the operation of the bridge plus horizontal forces due to a seismic event. The bearing assembly is large, with an overall height of nearly 1.5 meters (5 feet) and a base diameter of about 3 meters (10 feet).
"A spherical roller thrust bearing was selected because it provides several significant advantages," says Sean Bluni, structural designer at Hardesty & Hanover, LLP, who designed the structure and provided construction support. "Its low coefficient of friction decreases the power required to open and close the swing span, which results in smaller, less expensive drivetrain components. In addition, the physical layout of the bearing with a hollow cavity through the middle allows for the main electrical cables to pass through its center, which protects and consolidates the power and control wiring. The load-carrying capability of the bearing allows for high thrust loads and moderate radial loads, which simplified the design details, allowing the compact assembly to take periodic horizontal seismic loads that would otherwise need to be resisted by special seismic restraining fixtures. The spherical roller thrust bearing designed for this project is believed to be the largest, in terms of load-carrying capacity, ever used for a center bearing swing span."
The removal and installation process is known as float-in and float-out, which is being used more and more in accelerated bridge construction. The old structure is removed by cutting it into sections and setting it on barges for removal from the site. The new swing span is then floated into place, thus accelerating the construction schedule over traditional disassembly and assembly techniques.
With the Third Avenue Bridge, the innovative center pivot-bearing assembly allowed for a simpler float-in of the swing span structure, which was constructed in Alabama and shipped by oceangoing barge to New York City. Upon arrival, the 15,419-plus metric ton (17,000-plus ton) replacement was transferred from the ocean barge to two smaller barges before being placed onto the pivot pier. While docked at staging areas adjacent to the bridge, the new main span was secured on four multitracked, self-propelled transport trailers. Then the ocean barge was moved to the middle of the navigation channel and secured to the north and south banks. The two smaller transfer barges were brought alongside the ocean barge and secured. The transport units then turned the swing span 90 degrees. Finally, river water was pumped into the main barge, thus lowering the swing span onto support systems on the smaller barges.
According to Sam Scozzari, a senior engineering manager at the construction firm of Parsons Brinckerhoff, the entire placement process took approximately 6 hours. "The project team was confident of the engineering that went into the transfer process," says Scozzari. "It was a team effort that involved the contractor's engineers and the NYCDOT design team. Float-ins of these types of spans are occurring more and more in urban areas as space becomes limited and as urban traffic demands become more pronounced. However, this isn't done every day and was very interesting to witness."
Moving into the Future
"Although maintenance of the Nation's bridges, including movable bridges, was given low priority during the 1970s and 1980s," says FHWA's Byer, "a greater understanding of and dedication to the task of maintaining highway structures has been gained over the past 15 years."
New York City's preventive maintenance program will ensure that the time and money invested in rehabilitating and replacing these bridges is well spent and that the bridges will continue to provide viable service for highway and waterway users well into the future.
According to NYCDOT, the United States has only three nonfloating retractable bridges, and two of them are owned by New York City. Those two bridges, which were built in the late 1800s and early 1900s, are still in operation. One of them, Brooklyn's 50-meter (165-foot)-long Carroll Street Bridge, carries one 5-meter (17-foot) roadway. NYCDOT maintenance crews recently rehabilitated the bridge, which was designated as a New York City Historic Landmark in 1987.
Earl E. Dubin is a structural engineer with the FHWA New York Division Office. He earned his bachelor of science in civil engineering from the University of Buffalo. During his career he has served as bridge engineer for the city of Buffalo, traffic engineer for Erie County, and a team leader for the NYSDOT local bridge inspection program. Dubin is FHWA's ex-officio member of AASHTO's Technical Committee for Movable Bridges (T-8). He is a licensed professional engineer in New York State.
For more information, contact Earl Dubin at 518-431-4125, ext. 229, or firstname.lastname@example.org.
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