Whether it is a new crossing or the replacement of an existing deficient structure, bridge construction is playing a key role in the North American economic recovery program, the growth of business development within key cities, and the rejuvenation of urban and suburban areas.
In the modern era, cable-stayed bridges often are selected for their long-term durability performance, high fatigue behavior, and aesthetics that can complement their existing surroundings. The collaboration of technology and construction method is ideal for spans longer than typically expected on standard cantilever bridges, but shorter than those requiring a suspension bridge. Owner budget constraints, accelerated construction time frames, and opportunities to simplify construction methods also give a cable-stayed bridge design advantages compared with other medium- to long-span bridge types.
Currently, the longest span cable-stayed bridge in North America is the John James Audubon Bridge spanning the Mississippi River in Louisiana, with a main span of 1,581 feet. The Sutong Bridge in the Peoples Republic of China, at 3,570 feet, currently is the world’s longest span cable-stayed bridge.
A significant benefit of the cable-stayed bridge is the ease of maintenance and extended inspection program throughout its design life. Even within the most aggressive environments, cable-stayed structures can have a design life as long as 100 years. This feature is achieved by detailing components to meet the highest durability criteria and carefully selecting and testing materials to ensure their compatibility. Strands form the main tensile component of the cable stays; these are designed with a multi-layer protection system to match the performance of the other system components. Leak tightness tests, accelerated aging tests, and full-scale fatigue performance tests set new parameters that each cable-stay system must demonstrate. System technologies allow strands forming the cable bundle to be monitored individually, inspected, and replaced with compact equipment while the bridge remains in service.
Modern engineering advancements are pushing the boundaries for longer spans in shorter construction time frames while increasing the stringent durability factors. This results in continued development of cable-stayed technologies. Patented solutions for the pylon strand-by-strand saddles facilitate simplified pylon layouts and increased structural efficiency. Large splitting forces within the pylon induced when plains of cables are anchored opposing one another can be eliminated with the use of a single saddle. Detailing obviously is simplified by the use of the single-saddle unit, and the previous concerns with regard to fatigue performance, fretting corrosion, and the inability to replace the single strands have all been addressed.
A key component in this advancement has been the independent guiding and encapsulation of strands, allowing single strand-by-strand installation, inspection, and replacement. Cable-stay specialist companies can now either inject the saddle with flexible gel filler or allow the strand corrosion protective systems to pass through the individual strand guides, thus eliminating any fretting corrosion and long-term fatigue issues.
Architects and designers are beginning to take advantage of the benefits that the new saddle designs can offer in terms of longevity and durability, as well as the compact and slender pylon details. The new Willamette River Crossing in Portland, Ore., will be the first bridge to incorporate such technology in North America.
Another significant improvement affecting durability and longevity is the result of standards put forth by the International Federation for Structural Concrete (fib), Post Tensioning Institute (PTI), and Commission Interministerielle de la Precontrainte (CIP) requiring individual bridge assessments of the structure’s dynamic effects. Various factors can lead to the excitation of the cables and, left uncontrolled, these forces can create instability. Aerodynamic forces generally act on the cable directly while dynamic forces, such as traffic, can create movement of the structure and lead to anchorage displacements.
Significant advancements in technology to assess and control the dynamic excitation have been developed in recent years. Minimizing the number of moving parts, today’s damping systems require minimal maintenance during their operating life while contributing significantly to the durability of the cable stay. With high efficiency, outstanding durability, and a compact size that allows integration into the existing systems, the damping system can be tuned to perform differently for each individual cable on the bridge. This feature of the damping system significantly improves the durability of the cable-stayed bridge.
Margaret Hunt Hill Bridge
As part of the Trinity River Project in Dallas, the Margaret Hunt Hill Bridge is a prime example of a signature cable-stayed structure being central to the rejuvenation of a city suburb. With a main span of 1,197 feet, a bridge length of 1,957 feet, and a steel arch center support of 446 feet, the bridge will be the first major signature bridge to be constructed over the Trinity River Corridor. The bridge deck is 120 feet wide, carrying six lanes of traffic connecting downtown Dallas to the western suburbs of the city. Relieving traffic congestion and replacing an existing bridge on Interstate 30, the Margaret Hunt Hill bridge is about to become an iconic landmark on the Dallas skyline.
Because only lightweight equipment is required to install the cable stays, a more flexible overall construction schedule was possible. Without the use of any onsite cranes, the 58 cable stays were scheduled to be installed beginning in April 2011. The SSI2000 stay-cable system is designed to meet the requirements of fib, PTI, and CIP — all of which specify a design life of as long as 100 years with a defined adequate maintenance program. Such quality provisions allow for minimal maintenance costs and inspection disruptions.
Christopher S. Bond Bridge
Bridge inventory and inspection programs by the Missouri Department of Transportation in Kansas City highlighted that the Paseo Bridge, built between 1952 and 1954, would need replacement to maintain the average 102,000 vehicles per day traveling this corridor. As part of the kcICON project, the new Christopher S. Bond Bridge will span the Missouri River with a 550-foot main span and approach spans of 451 feet. The choice of a cable-stayed bridge was made to create a landmark structure across the Missouri River, as well as ensure a reasonable maintenance program during its long service life. The cable-stayed design also gives the bridge the capacity for change with the capability to increase the existing six-lane corridor to eight lanes plus a pedestrian/bicycle path should it be necessary in the future.
Replacement of cable stays on existing bridges is not a common practice; however, currently under way is the replacement of 72 stay cables on the Luling Bridge in St. Charles Parish, La. This project is the first complete cable-stay replacement project in North America. Because of the recent stringent testing requirements of all the cable system components by the fib, PTI, and CIP, the risks of failures or fatigue due to corrosion that were evident in earlier systems have been addressed.
A cost-effective solution was an obvious objective for the Louisiana Department of Transportation and Development, but replacement of the cables also had to minimize impact to the 45,000 vehicles per day crossing the Mississippi River. While the bridge remains in service, the old parallel wire cable system is being replaced successfully with a strand-by-strand system — the durability, tensile capacity, and fatigue resistance of this technology increasing the longevity of the bridge.
Justin Campbell, Business Development – Civil Projects, is with VSL, which provides cable-stay systems. He can be contacted at email@example.com.
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