Toledo's New Signature Structure
A new $220 million bridge in Toledo, OH, features a one-of-a-kind design. Once completed, the Maumee River Crossing will be the second tallest structure in Toledo and a stunning addition to the city's skyline. What's more, the bridge's design integrates several unique new features of cable-stayed technology.
The bridge will span 373 meters (1,225 feet) with a single soaring 123-meter (403-foot) pylon inlaid with shatter-resistant glass on each of its four sides. During the day, the glass will reflect the skyline of Toledo. At night, the reflective surface will be backlit with dramatic lighting arrays capable of changing color displays.
Groundbreaking for the span took place on June 17, 2002, and the Ohio Department of Transportation (ODOT)the owner of the project expects construction to be completed by June 2006. The concrete superstructure's distinctive appearance is already developing community pride in Toledo, but it's the stay cable technology advances that are catching the attention of engineers.
"The [Maumee River Crossing's] unique stay cables serve as a notable structural design milestone," says Richard Martinko, district deputy director of ODOT. "This is the most expensive project ever undertaken by ODOT, and we are very pleased that testing has been so successful."
Since the mid 1980s, when the construction of cable-stayed bridges in North America accelerated, owners, designers, contractors, and the post-tensioning industry have focused on design improvements in stay cables. Among the innovations were non-bond socket, wedge-only anchorages; two epoxy coating variants for seven wire strand; continuous saddle-over-pylon cables; elimination of cementitious grout protection; incorporation of greased and sheathed strand; individual strand-based cable stressing procedures, and end-to-end multi-barrier corrosion protection systems.
All the innovations were put through full-scale fatigue tests between 1991 and 2000. The testing generated concerns about two of the innovations. First, concern about several premature, corrosion fatigue ruptures in tests of epoxy coated strand stays cast doubt on the advantages of epoxy coating corrosion protection. Second, fretting fatigue failures of grouted, uncoated strand saddle-type stay tests threatened the potential for owner cost savings of continuous-over-pylon cables.
Considering these findings and the needs of the Toledo community, the Ohio DOT's bridge engineers—Figg Bridge Engineers—proposed and designed a stay cable system for the Maumee River Crossing featuring:
- Cables 50 percent greater in diameter and strength than on any other bridge
- An unbonded saddle (or cradle) design over the pylon
- Configuration of a 373-meter (1,225-foot) span superstructure incorporating just 20 continuous-over-pylon cables and 40 anchorages
- Nested corrosion protection systems using epoxy coated strand and continuous external stainless steel sheathing over the saddle
- A novel cradle system that precludes strand-to-strand contact and wear, using individual stainless steel tubes installed in the grout-filled cradle pipe
In September 2000, the design engineers and ODOT program management consultants HNTB/Parsons Brinckerhoff contacted Construction Technology Laboratories, Inc., (CTL) to provide input on validating the Maumee River Crossing stay cable design. The goals were to diminish ODOT's risk of adapting unproven techniques, satisfy Federal Highway Administration (FHWA) acceptance criteria, and resolve testing and constructability challenges. The testing laboratory agreed to conclude testing within 11 months, by the end of November 2001.
On a parallel path, ODOT selected and contracted DSI-USA, Inc., to provide the stay cable system components for the bridge and the prototype cables for the testing program. The unusually complex and comprehensive design prototype validation program encompassed four full-scale tests of stay cables, three of which were larger than any others tested in the world, and two of which required timely design and construction of major new test fixtures.
The test program goals included:
- Verifying that the critical load transfer from stay cable to the pylon will in fact occur during construction and over the bridge's service life, as assumed in the engineers' design
- Affirming that the 82-, 119-, and 156-strand Maumee River Crossing stays can provide appropriate fatigue endurance and strength
- Confirming that after simulated damage resulting from a two-million-cycle cable fatigue test, the corrosion protection system will still function
- Assuring the full-scale fatigue performance and durability of the 119-strand cradle design
The unique design, extraordinary testing requirements, and tight schedule demanded constant interaction among representatives of the funding agencies and design, manufacturing, testing, and project management firms. The importance of questions about the unprecedented design necessitated new procedures. Challenges in the test setups and the manufacture of test specimens were substantial, requiring constant interaction and coordination.
Full-scale testing was to represent actual construction as closely as possible. The test program included material, production, and acceptance testing for stay cable components, plus fatigue and strength testing of full-scale cable assemblies. To better describe the performance of the cables and cable components, the program incorporated single-strand axial, flexural, and friction testing; cable anchorage leak-tightness testing; and strand-coating and cable-sheathing abrasion tests.
The new cable system design relies on the friction between the strands and the stainless steel sleeves within each cradle to transfer differential forces in two legs of each cable during and after construction. A minimum coefficient of friction of 0.5 was needed to satisfy this condition. Uncertainty on this issue was considerable, and the whole project would have been in jeopardy if this design parameter could not be verified successfully. Therefore, rigorous measurement of the coefficient of friction between epoxy coated strands and stainless steel sleeves was essential.
To that end, the laboratory designed a new strand-friction testing procedure and a new test setup. A minicradle-based test system was constructed using a concrete block and five stainless steel sleeves formed to specific radii and exit angles. Friction of the tensioned strands with respect to the sleeves was measured before, during, and after two million cycles of stress variation. Once the tests verified the existence of an adequate coefficient of friction, the four full-scale tests commenced.
New Axial Fixture
The Post-Tensioning Institute's (PTI) Recommendation for Stay Cable Design, Installation, and Testing, 2000 edition, served as the basis for the full-scale tests. Three full-size cables with 82, 119, and 156 strands needed to be tested in axial tension for fatigue and maximum-strength requirements. Acceptance criteria encompassed limiting wire breakage to 2 percent of the wire in the cable at the culmination of fatigue testing, followed by the cable successfully withstanding a proof load of 95 percent of nominal strength. One of the cables also needed to undergo water leakage or dunk testing after completion of the fatigue cycling to prove the water tightness of the anchorage zone. A full-scale, axial-flexure (cradle) test also had to be conducted on one of the cable sizes.
The testing laboratory's existing axial test fixture was used for fatigue and strength testing of an 82-strand cable and fatigue testing of a 119-strand cable. These tests met acceptance criteria set forth by PTI.
The 119-strand cable underwent leakage testing. The procedure was first introduced in the PTI Recommendations in the year 2000. The testing laboratory's team designed and assembled a test setup consisting of a standing tank and accessories for applying a 3-meter (9.8-foot) head of red-dye solution on an anchorage zone of the 119-strand cable after it had gone through the fatigue testing cycles. The leakage test indicated the satisfactory performance of the sealing system used in the cable anchorage zones.
The demands of the axial testing for the 156-strand cable were, however, far above and beyond the existing testing resources. Strength testing this cable required a loading capacity of 40.5 mega Newtons (9.1 million pounds). The project team designed and manufactured a new testing fixture capable of applying repeated (fatigue) loads of up to 22 mega Newtons (4.9 million pounds) and static loads of 49 mega Newtons (11 million pounds). The custom-made, center-hole hydraulic ram for this new fixture alone weighed 125 kN (28,000 pounds). The extraordinary hydraulic demand of this system was met by an independent dual pump with a maximum flow rating of 530 liters (140 gallons) per minute combined with an oil accumulation system.
The new test fixture was supplemented by another testing frame for applying lateral contact force between a stainless steel cover (sheathing) pipe and the perimeter strands in the cable. This procedure was intended to assess the potential occurrence of epoxy coating damage in the perimeter strands where they come into contact with weld beads joining stainless steel cover pipe segments. The 156-strand cable prototype also successfully met the fatigue and static test criteria of the PTI Recommendations as well as the State's requirements for epoxy coating abrasion.
Full-Size Cradle Test Frame
The biggest challenge in the testing program was the axial-flexural testing of a 119-strand cable, incorporating a full-scale prototype of a new cradle design, encased in a post-tensioned concrete segment representing the bridge pylon. The outcome of this test will contribute the most to future advances in stay cable design.
The test is to simulate the fatigue performance of the curved portion of a cable in the pylon. Major concerns about the new cradle design included fatigue due to the bending stress induced in the strands near the exit point from the cradle and potential damage to strand epoxy coating due to fretting action.
A new self-supporting testing frame capable of transferring about 13 mega Newtons (2.9 million pounds) of force to the cable specimen during two million cycles of loading was needed. The new frame was designed of steel sections in a double-truss structure. This frame supported two ends of the cable specimen. It was used as a reaction for hydraulic rams that push vertically against a 45-ton concrete block. The block represents a segment of the pylon structure at the cradle pipe on the bridge. Fifty tons of steel and 100 tons of concrete were used to construct the test fixture. The steel truss frame was fabricated in segments and shipped to the testing laboratory for assembly and erection. Two load cells at the ends of the cable specimen and several displacement transducers were used to control loading and collect data.
The test cable assembly and installation provided insights into cable erection for the bridge construction. In particular, the stay cable supplier's proposed individual strand stressing procedure was evaluated. The process encompasses simultaneous stressing at both ends hydraulically, using coupled "monostrand" jack arrays. The procedure was evaluated and compared against the testing laboratory's load cells located at the opposing anchorages of the 17-meter (56-foot)-long stay prototype.
Once stay installation was completed, the fatigue test began and lasted for 2 months. At its conclusion, the strands were extracted from the cradle and inspected thoroughly. No wire breaks were observed, and no major damage to the strand and the cradle pipe was detected.
In a footnote to the lessons from the 119-strand full-scale cradle test, the results of the strand-by-strand cable stressing in the laboratory prompted enhancements in the construction techniques for the Maumee River Crossing. Also, ODOT selected a new technique for measuring cable force to monitor stay cables tensioning on site to supplement the strand-by-strand cable stressing technique and assure the proper development of forces in stay cables. For the first time in the United States, a new laser technique for cable force measurement was written into bridge construction specifications, offering an alternative to monitoring hydraulic pressures or the lift-off method.
A non-contacting laser-based method developed with FHWA Broad Agency Announcement funding in 1997 will be used during construction in lieu of liftoff tests using large, heavy, center-hole jacks. Utilization of the lower-cost and more accurate method of force measurement will not only reduce the cost and duration of construction, but also will provide accurate and timely measurements.
Milestones and Achievements
The contract bidding schedule for stay cable delivery and the bridge construction was contingent on the progress and success of the test program. Delays created by a need to repeat tests, shortages in materials for the stay cable vendor, or difficulties with the laboratory equipment would directly and substantially impair schedules for contract letting. In fact, the tests were completed December 2, 2001.
In January 2002, bids were accepted for the bridge construction and soon thereafter FruCon Construction Company was awarded the contract to construct the bridge.
Because of the testing, ODOT and FHWA lessened their level of functional, public safety, and contractual uncertainty related to using an innovative, first-of-its-kind stay cable system. In addition, the bridge designer, project management consultant, and stay cable supplier derived practical insights from the full-scale cable fabrication process in the laboratory, influencing their preparedness for managing stay cable supply and bridge construction well before construction commenced.
Furthermore, the bridge builder and stay cable erection subcontractor gleaned lessons from the cable fabrication associated with the cradle pipe fabrication, grouting, cable installation, and stressing systems. They also reaped the benefits of a new cost-saving tool for cable stay alignment and load monitoring. The stay cable testing consultant enhanced its technical resources in the area of cable stayed design and construction, improving its ability to assist future clients and the industry.
Adrian Ciolko is vice president of the Structural Laboratory Division of Construction Technology Laboratories, Inc., in Skokie, IL. Among the many professional achievements of his 25-year technical and management career, he directed the design and development of the CTL center for studying and improving the fatigue performance of large-diameter bridge cables. Since 1991, the center has validated design of stay cables for 20 bridges constructed around the world. Additionally, he conceived and oversaw adaptation of advanced laser-based nondestructive test systems for structural evaluation of cable stayed and suspension bridges. Ciolko collaborated with the FHWA and ODOT Maumee River Crossing Bridge team leaders to conceive suitable validation criteria and methods for the innovative cable stayed structure.
Armin Mehrabi is a senior principal engineer with CTL and was the project principal for the Maumee River Crossing Bridge Cable Testing Program. He leads the company's long-span bridge engineering activities with a focus on the use of innovative techniques for bridge evaluation and inspection. In 1997, Mehrabi was chosen as one of Engineering News-Record magazine's Top 25 Newsmakers for his contribution to the development of nondestructive techniques for cable stayed bridge evaluation. He received his master's degree and doctorate in civil engineering from the University of Tehran and the University of Colorado at Boulder, respectively.