Bridges under troubled surfaces
Vellore Gopalaratnam’s status as an expert in orthotropic steel plate bridge surfacing has its roots in research he did for the Missouri Department of Transportation on the Poplar Street Bridge in St. Louis.
Built in 1967, the original surface of the Mississippi River span lasted 15 years, but by 1983, it had become so worn, it was entirely removed and replaced with identical materials consisting of a double epoxy tack coat, stone chips and rubberized asphalt concrete. The second surface showed signs of failure within three years.
The University of Missouri professor of civil engineering began working with MoDOT to develop a proposal to evaluate the problem. In the meantime, a third wearing surface was applied to the eastbound lanes that included fiberglass reinforcing, but it too quickly showed signs of deterioration. Gopalaratnam and his crew of research assistants and an MU Engineering data acquisition technician set to work.
“Very early on, they started building these relatively light bridges,” said Gopalaratnam. “It really picked up in the 1960s all over the world as this type of bridge became cost-effective.”
Precursors to current orthotropic bridge technology originated in the 1920s with the introduction of long span movable bridges constructed by welding steel plates to steel girders. A German engineer patented the orthotropic system in the 1940s and a design manual was produced in Germany in 1957. The American Institute of Steel Construct followed with its own design manual in 1963.
Orthotropic bridges have welded steel decks that serve as the top flange tying the stiffening ribs, supporting the deck and the top flange of both the floor beams and the principal steel girders within the span. The load is transferred into the longitudinal main girders, the bridge’s main support. This flexible system minimizes materials.
Moreover, bridge surfaces can be shop-fabricated, reducing field costs and cutting time from construction. In addition to being both inexpensive and efficient, orthotropic bridge design offers latitude in aerodynamic and aesthetic design elements.
This adaptability has resulted in orthotropic bridges as some of the longest spans in the world. China’s award-winning Sutong Bridge over the Yangtze River, the longest cable stay bridge in the world at nearly seven-tenths of a mile, is one example.
The United States has embraced the technology less enthusiastically than the rest of the world’s bridge-builders. Out of the roughly 650,000 bridges in this country, fewer than 100 are orthotropic. That number includes both the Poplar Street Bridge, where Gopalaratnam conducted his original research in orthotropic bridge surfacing, and the San Mateo-Hayward Bridge spanning the San Francisco Bay. Resurfacing options for the latter, by the California Department of Transportation (CalTrans), are a current project under investigation in Gopalaratnam’s lab. His research in this area also includes a resurfacing for the Bronx-Whitestone Bridge in New York.
St. Louis Poplar Street Bridge
“Anything you put on this type of steel deck is punished,” Gopalaratnam said. “St. Louis’ landmark Poplar Street Bridge handles traffic from four main highways: I-55, I-70, I-64 and Highway 40. The surface is being subjected to traffic well in excess of original projections — an approximately fivefold increase since 1967.”
Weather extremes in this part of the country play a role in the degradation of bridge surfaces.
“We first started the MoDOT Poplar Street project almost 25 years ago. Rich Oberto cut his teeth on that project,” Gopalaratnam said of the MU Engineering data acquisition technician. “Students have also been involved in these projects all along, including Dr. Brian Hartnagel, who worked on the project as a master’s student and now works with the MoDOT bridge division.
“We went inside the 6- by 20-foot girders, measuring the strains using foil gauges. We’d go out early in the morning and work until dark. We gathered lots of data necessary to simulate fatigue loads in our laboratory tests and to gain insight into this specialized area of research,” Golpalaratnam said.
Back in the lab, the group focused on surface failures and how they might be mitigated with newer materials not available when the original surfaces were replaced.
Golpalaratnam’s work resulted in funding from MoDOT in the early 1990s to test wearing material systems as potential replacements.
“There were two different kinds of polymer concrete surfaces, a urethane-based material, an epoxy-asphalt and a rubberized asphalt wearing surface system,” he said, adding that they also tested different application methods.
Based on the results of tests developed and recommendations by the MU researcher, MoDOT chose one of the polymers and resurfacing commenced in 1992.
The study received broad word-of-mouth attention. Several states approached Golpalaratnam to learn the details of his work.
“The type of testing we are doing is unique. The learning curve is very steep, and since we’ve been doing this for the last two decades, we are in a good position with our knowledge base and the testing infrastructure we have developed,” he said.
When the Federal Highway Administration began work on revamping its orthotropic bridge manual, Gopalaratnam was asked to write the chapter on wearing surfaces. It is the first major overhaul of the manual since 1963, slated for completion in the next few months. More than a dozen leading experts in various aspects of bridge engineering from academia, as well as practicing structural engineers, have been involved in updating the manual.
San Mateo-Hayward Bridge in San Francisco
While working on the manual, he was in direct contact with others in the field, and his work came to the attention of CalTrans, a connection that ultimately aided in his selection to test materials for the resurfacing of the San Mateo-Hayward Bridge. Also built in 1967, it has undergone major retrofits in 1975 and 2005. It retains its original surface, perhaps because of less adverse weather conditions than those in the Midwest.
“We liked his approach to the project and knew he had the appropriate expertise,” said Ric Maggenti, the CalTrans project manager for the San-Mateo Bridge project. “I find Dr. Gopal very thorough and knowledgeable.”
“There are five bridges that connect San Francisco to the mainland, but in terms of traffic, it may be among the heavily used with an excess of 100,000 vehicles passing over it every day,” Gopalaratnam said.
San Mateo Bridge is the longest of the five and the 25th longest in the world with a total length of seven miles. The orthotropic section of this bridge is elevated and is approximately two miles long. This facilitates navigation clearances of 135 feet. Its construction was the first significant application of an orthotropic steel bridge.
“We are testing two materials: an epoxy asphalt and a polyester concrete. The aggregates in the latter are rounded river gravel and pea gravel, and those in the epoxy asphalt are crushed angular aggregates, all from California,” Gopalaratnam said. “The test program is more sophisticated in the level of instrumentation and automated control. We also have incorporated an automated digital imaging system to visually monitor cracking on the surface of the materials during the temperature-controlled fatigue tests.
“We are evaluating load, temperature effects, how surfaces fail, when they fail and how the wearing surface materials can decrease steel stress on the bridge itself. Experience has shown us there are benefits to reducing steel stress, thus extending the fatigue life of the bridge deck,” he added.
To prepare the fatigue specimens, the steel plates to which surfacing materials are bonded have to be cleaned to white metal standard by sandblasting them to remove all of the impurities. The clean surface is then treated with a corrosion-resistant primer coat. In a real world setting, a bridge is shot-blasted with steel pellets in a closed system with magnets to retrieve the shot, an Environmental Protection Agency requirement.
Civil engineering seniors Eric Schrader from St. Genevieve and Nathan Volkers from Blue Springs, along with graduate student Ravi Sankar Charmarthi from Mumbai, India, constitute the research team in Gopalaratnam’s lab. And as the group readied the lab and prepared samples last summer, Chris Novesel, a junior civil engineering student from Kansas State University, worked to set up the cameras for the tests.
The testing chamber will hold four samples that will be tested simultaneously with each specimen instrumented, monitored and photographed individually.
“We are looking at performance and failure,” said Chamarthi. “Our tests simulate fatigue loads on the bridge. The load and displacement are recorded in a data acquisition system. It can measure deflection, cracking of the top surface, and the cracks can be measured in different ways. There are continuity detection wires on the top surface and there are cameras for visual records.”
Deflection is the trampoline-like movement that results from a vehicle moving quickly across the flexible surfaces of a bridge. The orthotropic bridge deck is significantly more flexible than other types of bridges.
Novesel said he worked to install four cameras, each focused on a single sample in the temperature-controlled fatigue test chamber. The cameras are remotely controlled using two different software programs, PC Remote and LabVIEW.
“LabVIEW will do all of the timing and powering of the cameras,” said Novesel.
Schrader began by calibrating clip gauges to test the deflection of the samples and then worked on the construction of the heating and cooling chamber to control temperatures during the test, setting the unit to the prescribed temperatures.
The samples will be tested at room temperature, at freezing and at 50 degrees Celsius (122 degrees Fahrenheit) with many redundancies in the tests to confirm results. Static tests conducted by Chamarthi showed that there are significant differences in the stiffnesses between the two surfacing materials being tested, that they will not be tested together into the chamber.
“Samples will be subjected to 10 million cycles of loading or until failure, simulating approximately 50 years of peak strain events,” said Gopalaratnam. “Laboratory testing conditions are more aggressive than those which the wearing surfaces are likely to see under normal service.”
Visual images can be matched to data sets and the researchers can toggle back and forth between them for a visual history of the recorded data.
“Fatigue testing takes time. We are monitoring the tests, varying the temperature and the loading rate. The theory and ideas behind surfacing is a whole world I wasn’t aware of,” said Volkers of his first research experience.
The research investigation also includes a “tensile pullout test” to test the bonding strength of the surface to the steel deck — a quality control measure to check the performance of deck preparation in regard to delamination.
Samples are cored to the steel plate and a threaded pipe cap is glued to the top of the cylindrical wearing surface cores. The pipe cap is attached to a strain gauge and as force is exerted to pull the core loose, the bond strength in psi is measured.
“It was tough finding a bond stronger than roadway meeting steel plates, but we have had five successful pulls, pretty consistently over 650 psi, which is very acceptable,” said Schrader.
Testing is expected to continue through to next spring at which time, Gopalaratnam will submit a final report with recommendations to CalTrans.
“There are advantages to both of the materials being tested,” said Maggenti. “We want to make sure what we put down works, and will do whatever it takes to make sure.”
“We are using the project to look further than ever before,” Gopalaratnam said. “This broader picture we are looking at will help fill gaps in the knowledge base that were identified while preparing the FHWA Manual on Orthotropic Steel Bridges.”
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